,,transcriber',s ,,note3 ,in transcribing this book1 the proofreaders found and corrected several minor typographical errors which did not affect the sense of the text4 ,in the caption to ,figure #eda1 the equation for the voltage of a ,weston cell at different temperatures was missing a digit #a and this has been corrected4 ,there is a reference to a ,figure #fai but no such figure exists in the original text4 ,there are references to a ,figure #aai and a ,figure #ddc2 these presumably exist in one of the preceding volumes of the series4 ,throughout1 the use of italic type is indicated by underscores4 ,in equations1 superscripts are indicated by a caret and braces1 as in ,x#b for ,x squared4 ,subscripts are indicated by underscore and braces1 as in ,et to mean ,e sub t4 ,,the ,,thought ,,is ,,in ,,the ,,question ,,the ,,information ,,is ,,in ,,the ,,answer ,,hawkins ,,electrical ,,guide ,,number ,,three ,,questions ,,answers ,,illustrations ,a ,,progressive ,,course ,,of ,,study ,,for ,,engineers1 ,,electricians1 ,,students ,,and ,,those ,,desiring ,,to ,,acquire ,a ,,working ,,knowledge ,,of ,,electricity ,,and ,,its ,,applications ,a ,,practical ,,treatise by ,,hawkins ,,and ,,staff ,,theo4 ,,audel ,,co4 #gb ,,fifth ,,ave4 ,,new ,,york ,,copyrighted1 #aiad1 by ,,theo4 ,,audel ,,co41 ,new ,york4 ,printed in the ,united ,states4 ,,table ,,of ,,contents ,,guide ,,no4 #c4 ,,galvanometers #dca to #dfd ,action of compass needle--simple galvanometer--difference between galvanoscope and galvanometer--sensibility--action of short and long coil galvanometers--classes of galvanometer--astatic galvanometer--tangent galvanometer--graduation of tangent galvanometer scale--table of galvanometer constants--mechanical explanation of tangent law--sine galvanometer--table of natural sines and tangents--comparison of sine and tangent galvanometers--differential galvanometer--ballistic galvanometer--kick--damping effect--use of mirrors in galvanometers--lamp and scale--damping--,d',arsonval galvanometer3 construction1 operation2 uses--galvanometer constant or figure of merit--shunts4 ,,testing ,,and ,,testing ,,apparatus #dfe to #ecf ,pressure measurement--,clark cell--,weston cadmium cell--pressure measurement error with ordinary voltmeter--,international volt--hydraulic analogy of amperes--coulombs--current measurement--,international ampere--voltameters--,ohm's law and the ohm--,international ohm--ohm table--practical standards of resistance--various methods of resistance measurement--direct deflection method--method of substitution--resistance box--fall of potential method--differential galvanometer method--drop method--voltmeter method--,wheatstone bridge--usual arrangement of resistances of ,wheatstone bridge--ratio coils of ,wheatstone bridge--the decade plan--two plug arrangement--plug out and plug in type of resistance box--testing sets--direct deflection method with ,queen ,acme set--ohmmeter--fall of potential method with ,queen ,acme set--apparatus for measuring low resistances--how to check a voltmeter--,kelvin wire bridge--internal resistance measurement--,evershed portable ohmmeter set--,l and ,n fault finder--ammeter test--diagram of ,queen standard potentiometer--diagrams illustrating loop testing--the ,murray loop--the ,varley loop--special loop--the potentiometer--location of opens--to pick out faulty wires in a cable--voltage of cell measurement with potentiometer--care of potentiometer--location of faults where the loop is composed of cables of different cross sections4 ,,ammeters1 ,,voltmeters1 ,,and ,,wattmeters #ecg to #egb ,definition of ammeter--classification of ammeter and voltmeters--moving iron type instrument--,keystone voltmeter--winding in ammeters and volts--connections for series and shunt ammeters--voltmeter connections--,westinghouse ammeter shunts--various types of instrument--plunger type instrument--magnetic vane instrument--inclined coil instrument--,whitney hot wire instruments--principle of electrostatic instruments--multipliers--portable shunts--,siemens electro-dynamometer--station instruments--,thompson watt hour meter--how to read a meter--installation of wattmeters--,westinghouse watt hour meter--,thompson prepayment watt hour meter--how to test a meter--,sangamo watt hour meter--,columbia watt hour meter--,duncan watt hour meter4 ,,operation ,,of ,,dynamos #egc to #eif ,before starting a dynamo--adjusting the brushes--brush position--how to set the brushes--method of soldering cable to carbon brush--brush contact pressure--direction of rotation--method of winding cables with marlin--method of assembling core discs--starting a dynamo--tinning block for electric soldering tool--shunt dynamos in parallel--shunt dynamos on three wire system--how to start a series machine--the term build up--how to start a shunt or compound machine--picking up--indication of reversed connections--how to correct reversed polarity--finding the reversed coil--loss of residual magnetism--remedy for reversed dynamo--attention while running--lead of brushes--method of taking temperature--lubrication--oils--allowabl e degree of heating--attention to brushes and brush gear4 ,,coupling ,,of ,,dynamos #eig to #faj ,series and parallel connections--coupling series dynamos in series2 in parallel--equalizer--shunt dynamos in series2 in parallel--switching dynamo into and out of parallel--to cut out a machine--dividing the lead--compound dynamos in series2 in parallel--equalizer connection--switching a compound dynamo into and out of parallel--equalizing the load--shunt and compound dynamos in parallel4 ,,dynamo ,,fails ,,to ,,excite #faa to #fbb ,various causes--brushes not properly adjusted--defective contacts--incorrect adjustment of regulators--speed too low--testing for break--insufficient residual magnetism2 remedy--open circuits--test for field circuit breakers--probable location of breaks--,watson armature discs--,fort ,wayne commutator truing device--short circuits--,watson armature--wrong connections--reversed field magnetism4 ,,armature ,,troubles #fbc to #fcd ,causes--how avoided--various faults--short circuit in individual coils--location of faulty coil--test for break in armature lead--bar to bar test for open or short circuit in coil or between segments--short circuits between adjacent coils--alternate bar test for short circuits between sections--short circuits between sections through frame or core of armature2 between sections through binding wires--partial short circuits in armatures--method of testing for breaks--burning of armature coils--,watson field coils--grounds in armatures--method of locating grounded armature coil--magneto test for grounded armatures--method of binding armature winding--breaks in armature circuit4 ,,care ,,of ,,the ,,commutator ,,and ,,brushes #fce to #feb ,conditions for satisfactory operation--oil for commutator--attention to brushes--,bissell brush gear--two kinds of sparking--commutator clamp--causes of sparking--bad adjustment of brushes--rocking--bad condition of brushes--brushes making bad contact--bad condition of commutator--detection of untrue commutator--high segments--flats--causes of flats2 remedy--method of repairing broken joint between commutator segment and lug--segments loose or knocked in--how to re-turn a commutator--,bissell commutators--overload of dynamo--method of repairing large hole burned in two adjacent bars of a commutator--operating dynamos with metal brushes--indication of excessive voltage--method of smoothing commutator with a stone--causes of excessive voltage--loose connections1 terminals1 etc41--breaks in armature circuit--sandpaper holder for commutator--short circuits1 in armature circuits2 in field--breaks in field--sandpaper block--short circuits in commutator4 ,,heating #fec to #ffb ,various causes--how detected--procedure--heating1 of connections2 of brushes1 commutator and armature--excessive heating--ventilated commutator--self-oiling bearing--some causes of hot bearing--effect of hot bearings--points relating to hot bearings--operation above rated voltage and below normal speed--forced system of lubrication--heating of field magnets--causes of eddy currents in pole pieces--detection of moisture in field coils--indication of short circuits in field coils4 ,,operation ,,of ,,motors #ffc to #fif ,before starting a motor--starting a motor--various starting resistances--starting boxes--speed regulators--,cutler ,hammer starter--time required to start motor--how to start--sliding contact starters--series motors on battery circuits--starting a shunt motor--multiple switch starters--effect of reverse voltage--rheostat with no voltage and overload release--failure to start--starting panel--,cutler ,hammer starting rheostats--,allen ,bradley automatic starter--,monitor starter with relay for push button control--a remote control of shunt motors--regulation of motor speed2 various methods--,monitor printing press controller--speed regulation of series motor1 by short circuiting sections of the field winding--varying the speed of shunt and compound motors--,cutler ,hammer multiple switch starter--regulation by armature resistance--,compound starter--regulation by shunt field resistance--,holzer ,cabot instructions for shunt wound motor--,reliance adjustable speed motor--,cutler ,hammer reversible starter--combined armature and shunt field control--selection of starters and regulators--,watson commutators--organ blower speed regulator--,general ,electric controller--speed regulation of traction motors--controller of the ,rauch and ,lang electric vehicles--two motor regulation--controller connection diagrams--stopping a motor4 ,,chapter ,,xxvi ,,galvanometers ,if a compass needle be allowed to come to rest in its natural position1 and a current of electricity be passed through a wire just over it from north to south1 the north seeking end of the needle will be deflected toward the east4 ,if the wire be placed under the needle and the current continued from north to south the needle will be deflected toward the west4 ,again1 if the current be passed from north to south over the needle1 and back from south to north under the needle1 as shown in fig4 #ejd1 the magnetic effect will be doubled1 and the needle deflected proportionately4 ,upon these phenomena depend the working of galvanometers4 ,illustration3 ,fig4 #ejc4--;,effect of neighboring current upon a magnetic needle4 ,above the needle and parallel to it is a conductor carrying an electric current1 the current flowing in the direction indicated by the arrow4 ,this causes the north pole of the needle to turn toward the east4 ,if the conductor be held below the needle1 its north pole will turn in the opposite direction or toward the west4 ,these movements are easily determined by ,ampere's rule as follows3 ,if a man could swim in the conductor with the current1 and turn to face the needle1 then the north pole of the needle will be deflected toward his left hand4 ,ques4 ,describe a simple galvanometer4 ,ans4 ,it consists essentially of a magnetic needle suspended within a coil of wire1 and free to swing over the face of a graduated dial4 ,ques4 ,what is a galvanoscope and how does it differ from a galvanometer8 ,ans4 ,a galvanoscope1 as shown in fig4 #ejd1 serves merely to indicate the presence of an electric current without measuring its strength4 ,it is an indicator of currents where the movement of the needle shows the direction of the current1 and indicates whether it is a strong or a weak one4 ,when the value of the readings has been determined by experiment or calculation any galvanoscope becomes a galvanometer4 ,illustration3 ,fig4 #ejd4--;,effect upon a magnetic needle of a neighboring current in a loop4 ,in this arrangement the same conductor is simply carried back beneath the needle and hence both the upper and lower portions tend to turn it in the same direction1 while the side branch or vertical section is ineffective4 ,in accordance with ,ampere's swimming rule1 the upper wire causes the ,n pole of the needle to turn to the left1 while if a man can imagine himself swimming in the lower wire in the direction of the current1 and facing the needle 7that is1 swimming on his back71 the ,n pole of the needle will turn to his left--that is to the east4 ,the effect of the loop then has double the effect of the single wire in fig4 #ejc4 ,ques4 ,for what use are galvanometers employed8 ,ans4 ,they are used for detecting the presence of an electric current1 and for determining its direction and strength4 ,ques4 ,how is the direction and strength of the current indicated8 ,ans4 ,when a galvanometer is connected in a circuit1 the direction of the current is indicated by the side towards which the north pole of the needle moves1 and the current strength by the extent of the needle's deflection4 ,illustration3 ,fig4 #eje4--;,effect upon a magnetic needle of a neighboring current in a coil4 ,the coil as shown1 is equivalent to several loops1 that is1 the force tending to deflect the needle is equal to that of a single loop multiplied by the number of turns4 ,hence1 by using a coil with a large number of turns1 a galvanometer may be made very sensitive so that the needle will be perceptibly deflected by very feeble currents4 ,an instrument1 as shown in the figure is called a galvanoscope4 ,when it is accurately constructed1 and supplied with a scale showing how many degrees the needle is deflected it is then called a galvanometer4 ,ques4 ,how should a galvanometer be set up before using8 ,ans4 ,when no current is flowing1 the coil should be parallel to the magnetic needle when at rest4 ,ques4 ,what is a sensitive galvanometer8 ,ans4 ,one which requires a very small current or pressure to produce a stated deflection4 ,it does not follow that a galvanometer which is sensitive for current measurement will also be sensitive for pressure measurement4 ,illustration3 ,fig4 #ejf4--;,bunnell simple detector galvanometer4 ,it has middle clamps and scale divided into degrees4 ,ques4 ,define the term sensibility4 ,ans4 ,with reference to mirror reflecting galvanometers it may be defined in three ways4 ,first1 in megohms1 the sensibility being the number of megohms through which one volt will produce a deflection of one millimeter with the scale at one meter distance4 ,second1 in micro-volts1 the sensibility being the number of micro-volts which applied directly to the terminals of the galvanometer will produce a deflection of one millimeter on a scale one meter from mirror4 ,the sensibility is best stated in megohms for high resistance galvanometers and in micro-volts for low resistance galvanometers1 and is frequently given both for galvanometers for intermediate resistance4 ,third1 in micro-amperes1 the sensibility being the number of micro-amperes that will give one millimeter deflection with scale at a distance of one meter4 ,ques4 ,upon what does the sensibility depend8 ,ans4 #a1 ,upon the number of times the current circulates around the coil1 #b1 the distance of the needle from the coil1 #c1 the weight of the needle1 #d1 the current strength1 and #e1 the amount of friction produced by its movement4 ,illustration3 ,fig4 #ejg4--;,breguet upright galvanometer with glass shade4 ,illustration3 ,fig4 #ejh4--;,bunnell horizontal galvanometer4 ,it has two coils1 one of which is of zero resistance and one of fifty ohms resistance adapting it to a variety of test4 ,the needle is usually quite small1 and often a compound one4 ,in very sensitive galvanometers1 the coils are wound with thousands of turns of very fine wire1 and shunts are generally used in connection with them4 ,,note4--,strong currents must not be passed through very sensitive galvanometers1 for even if they be not ruined1 the deflections of the needle will be too large to give accurate measurements4 ,in such cases the galvanometer is used with a shunt1 or coil of wire arranged so that the greater part of the current will flow through it1 and only a small portion through the galvanometer4 ,ques4 ,what two kinds of coil are used8 ,ans4 ,the short coil and the long coil4 ,ques4 ,what is the difference between a short coil and a long coil galvanometer8 ,ans4 ,a short coil galvanometer has a coil consisting of a few turns of heavy wire2 a long coil galvanometer is wound with a large number of turns of fine wire4 ,illustration3 ,fig4 #eji4--;,bunnell galvanometer for measurements of instruments1 lines1 batteries1 wires and any object from #a_/ajj to #aj1jjj ohms or more4 ,ques4 ,what is the action of short and long coil galvanometers8 ,ans4 ,with a given current1 the total magnetizing force which deflects the needle is the same1 but with a short coil1 it is produced by a large current circulating around a few turns1 instead of a small current circulating around thousands of turns as in the long coil4 ,the short coil being of low resistance is used to measure the current1 and the long coil with high resistance1 is suitable for measuring the pressure4 ,hence1 a short coil instrument with its scale directly graduated in amperes is an ammeter1 and the long coil type with graduation in volts is a voltmeter4 ,classes of ,galvanometer4--,there are numerous kinds of galvanometer designed to meet the varied requirements4 ,according to construction1 galvanometers may be divided into two classes1 as those having3 #a4 ,movable magnet and stationary coil2 #b4 ,stationary magnet and movable coil4 ,illustration3 ,fig4 #eaj4--;,astatic needles4 ,two magnetic needles of equal moment are mounted in opposition on a light support4 ,the whole system is suspended by a delicate fibre1 and when placed in a uniform magnetic field such as that of the earth1 there will be no tendency to assume any fixed direction1 the only restraining influence on the needles being that due to torsion in the suspension fibre4 ,either type may be constructed with short or long coil1 and there are several ways in which the deflections are indicated4 ,the principal forms of galvanometer are as follows3 #a4 ,astatic2 #b4 ,tangent2 #c4 ,sine2 #d4 ,differential2 #e4 ,ballistic2 #f4 ,d',arsonval4 ,astatic ,galvanometer4--,it has been pointed out how a compass needle is affected when a wire carrying a current is held over or under it1 the needle being turned in one direction in the first instance1 and in the opposite direction for the second position of the wire4 ,illustration3 ,fig4 #eaa4--;,connections of single coil astatic needles4 ,the coil surrounds the lower needle and the direction of the current between the two needles tends to turn them the same way4 ,the earth's magnetism naturally holds the compass needle north and south4 ,the magnetic field encircling the wire1 being at right angles to the needle 7when the wire itself is parallel therewith71 operates to turn it from its normal position1 north and south1 so as to set it partially east and west4 ,however1 on account of the fact that the earth's magnetism does exert some force tending to hold the needle north and south1 it is evident that no matter how strong the current1 the latter can never succeed in turning the needle entirely east and west4 ,the accomplishment of this is further prevented by the reason of the points of the needle1 where the magnetic effect is greatest1 quickly passing out of the reach of the magnetic field1 where it is now practically operated on only in a slight degree4 ,thus it would take quite a powerful current to hold the needle deflected any appreciable distance4 ,the use of a shorter needle is1 therefore1 more desirable4 ,it is evident in this style of instrument that the effect of the current cannot be accurately measured1 because it acts in opposition to the earth's magnetism1 and as this is constantly varying1 some method must be employed which will either destroy the earth's magnetism or else neutralize it4 ,in the astatic galvanometer1 the earth's magnetism is neutralized by means of astatic needles4 ,these consist of a combination of two magnetic needles of equal size and strength1 connected rigidly together with their poles pointing in opposite and parallel directions1 as shown in fig4 #eaj4 ,as the north pole of the earth attracts the south pole of one of the needles1 it repels with equal strength the north pole of the other needle1 hence1 the combination is independent of the earth's magnetism and will remain at rest in any position4 ,illustration3 ,fig4 #eab4--;,connections of double coil astatic needles4 ,with this arrangement1 the direction of current in both coils will tend to turn the system in the same direction1 making the needles more sensitive than with a single coil as in fig4 #eaa4 ,if one of the needles be surrounded by a coil1 as shown in fig4 #eaa1 the magnetic effect of the current will be correctly indicated by the deflection of the needle4 ,sometimes each needle is surrounded by a coil1 as in fig4 #eab1 the coils being so connected that the direction of current in each will tend to deflect the needles in the same direction4 ,ques4 ,for what use is the astatic galvanometer adapted8 ,ans4 ,for the detection of small currents4 ,it is used in the nil or zero methods1 in which the current between the points to which the galvanometer is connected is reduced to zero4 ,illustration3 ,fig4 #eac4--,queen reflecting astatic galvanometer4 ,it is mounted on a mahogany base with levelling screws4 ,a plain mirror is attached above the upper needle4 ,the entire combination of mirror and needles is suspended by unspun silk from the interior of a brass tube1 which also carries a weak controlling magnet4 ,a dial #d inches in diameter and graduated in degrees1 enables the deflections of the needle to be accurately read4 ,the mirror can be used with a reading telescope and scale1 or by means of a lantern1 the image of a slit may be reflected from the mirror to a screen4 ,resistance1 4#e to #a1jjj ohms4 ,ques4 ,upon what does the movement of the needles depend8 ,ans4 ,upon the combined effect of the magnetic attraction of the current which tends to deflect the needles1 and the torsion in the suspension fibre which tends to keep the needle at the zero position4 ,ques4 ,does the astatic galvanometer give correct readings for different values of the current8 ,ans4 ,when the deflections are small 7that is1 less than #aj or #ae71 they are very nearly proportional to the strength of the currents that produce them4 ,thus1 if a current produce a deflection of #f it is known to be approximately three times as strong as a current which only turns the needle through #b4 ,but this approximate proportion ceases to be true if the deflection be more than #ae or #bj4 ,illustration3 ,fig4 #ead4--;,central ,scientific ,co4 tangent galvanometer4 ,a #i inch brass ring is mounted on a mahogany base which rotates on a tripod provided with levelling screws4 ,the needle has an aluminum pointer and jewelled bearing4 ,the winding consists of #cjj turns of magnet wire so connected to the plugs in front that #bj1 #dj1 #hj1 or #afj turns or any combination of these numbers may be used4 ,for heavy currents a band of copper is used by connecting to the extra pair of binding posts in the rear of the instrument4 ,ques4 ,why does the instrument not give accurate readings for large deflections8 ,ans4 ,the needles are not so advantageously acted upon by the current1 since the poles are no longer within the coils1 but protrude at the side4 ,moreover1 the needles being oblique to the force acting on them1 part only of the force is turning them against the directive force of the fibre2 the other part is uselessly pulling or pushing them along their length4 ,illustration3 ,fig4 #eae4--;,bunnell tangent galvanometer4 ,this instrument is mounted on a circular hard rubber base1 #g-c_/h inches diameter1 provided with levelling screws and anchoring points4 ,the galvanometer consists of a magnetized needle #g_/h inch in length1 suspended at the center of a rubber ring six inches in diameter1 containing the coils4 ,there are five coils of #j1 #a1 #aj1 #ej and #aej ohms resistance4 ,the first is a stout copper band of inappreciable resistance2 the others are of different sized copper wires1 carefully insulated4 ,five terminals are provided1 marked1 respectively1 #j1 #a1 #aj1 #ej and #aej4 ,the ends of the coils are so arranged that the plug inserted at the terminal marked #ej puts in circuit all the coils2 marked at the terminal #ej--;all except the #aej ohm coil2 and so on1 till at the zero terminal only the copper band is in circuit4 ,fixed to the needle1 which is balanced on jewel and point1 is an aluminum pointer at right angles1 extending across a five inch dial immediately beneath4 ,one side of the dial is divided into degrees2 on the other side1 the graduations correspond to the tangent of the angles of deflection4 ,ques4 ,how may correct readings be obtained8 ,ans4 ,the instrument may be calibrated1 that is1 it may be ascertained by special measurements1 or by comparison with a standard instrument1 the amounts of deflection corresponding to particular current strengths4 ,thus1 if it be once known that a deflection of #cb on a particular galvanometer is produced by a current of #a_/ajj of an ampere1 then a current of that strength will always produce on that instrument the same deflection1 unless from any accident the torsion force or the intensity of the magnetic field be altered4 ,illustration3 ,fig4 #eaf4--,tangent galvanometer4 ,it consists of a short magnetic needle suspended at the center of a coil of large diameter and small cross section4 ,in practice1 the diameter of the coil is about #ag times the length of the needle4 ,if the instrument be so placed that1 when there is no current in the coil1 the suspended magnet lies in the plane of the coil1 that is1 if the plane of the coil be set in the magnetic meridian1 then the current passing through the coil is proportional to the tangent of the angle by which the magnet is deflected from the plane of the coil1 or zero position--hence the name3 tangent galvanometer4 ,the ,tangent ,galvanometer4--,it is not possible to construct a galvanometer in which the angle 7as measured in degrees of arc7 through which the needle is deflected is proportional throughout its whole range to the strength of the current4 ,but it is possible to construct a very simple galvanometer in which the tangent of the angle of deflection shall be accurately proportional to the strength of the current4 ,illustration3 ,fig4 #eag4--;,horizontal section through middle of tangent galvanometer1 showing magnetic whirls around the coil and corresponding deflection of needle4 ,illustration3 ,fig4 #eah4--;,diagram of forces acting on the needle of a tangent galvanometer4 ,a simple form of tangent galvanometer is shown in fig4 #eaf4 ,the coil of this instrument consists of a simple circle of stout copper wire from ten to fifteen inches in diameter4 ,at the center is delicately suspended a magnetized steel needle not exceeding one inch in length1 and usually furnished with a light index of aluminum4 ,when the galvanometer is in use1 the plane of the ring must be vertical and in the magnetic meridian4 ,a horizontal section through the middle of the instrument is shown in fig4 #eag4 ,for simplicity1 the coil is supposed to have but a single turn of wire1 the circles surrounding the wire representing the magnetic lines of force4 ,by extending the lines of force until they reach the needle1 it will be seen that with a short needle1 the deflecting force acts in an east and west direction when the galvanometer is placed with its coil in the magnetic meridian4 ,if1 in fig4 #eah1 ab represent the deflecting force acting on the ,n end of the needle1 the component of this force that acts at a right angle to the needle will be ab cos x in which1 x is the angle of the deflection4 ,the controlling force is ad ,h and when the needle is in equilibrium1 the component ae ,h sin x is equal and opposite to ac1 hence ab cos x ,h sin x from which ab ,h7sin x _/ cos x7 ,h tan x ,since ab is proportional to the current1 ab k ,c ,h tan x in which k is a constant depending upon the instrument4 ,for any other current ,c'1 k ,c' ,h tan x' hence ,c3 ,c' tan x 3 tan x' ,this means that the currents passing through the coil of a tangent galvanometer are proportional1 not to the angle of deflection1 but to the tangent of that angle4 ,illustration3 ,fig4 #eai4--;,diagram illustrating the tangent law4 ,this is the law of the combined action of two magnetic fields upon a magnetic needle4 ,if two magnetic fields be at right angles in direction as indicated in the figure1 the resultant field is obtained by the parallelogram of forces and it makes an angle theta with one of the component fields such that tan theta ,m ,h where ,m and ,h are the strengths of the component fields4 ,in the tangent galvanometer this principle is employed in the measurement of currents4 ,a magnetic needle is pivoted in a field of known strength4 ,the current to be measured is passed round a coil 7or coils7 which generates a field at right angles to the original field4 ,the needle then lies along the direction of the resultant field1 and by finding the tangent of its angle of deflection1 and knowing the field strength produced by unit current in the coil1 the current strength can be found4 ,illustration3 ,fig4 #ebj4--;,graduation of tangent galvanometer scale with divisions representing tangent values4 ,in the figure let a tangent ,,ot be drawn to the circle1 and along this line let any number of equal divisions be set off1 beginning at ,o4 ,from these points draw lines back to the center4 ,the circle will thus be divided into a number of spaces1 of which those near ,o are nearly equal1 but which get smaller and smaller as they recede from ,o4 ,these unequal spaces correspond to equal increments of the tangent4 ,if the scale were divided thus1 the readings would be proportional to the tangents4 ,ques4 ,upon what does the sensibility of a tangent galvanometer depend8 ,ans4 ,it is directly proportional to the number of turns of the coil and inversely proportional to the diameter of the coil4 ,ques4 ,how may the tangent galvanometer be used as an ammeter8 ,ans4 ,the strength of the current may be calculated in amperes by the formula given below when the dimensions of the instrument are known4 ,the needle is supposed to be subject to only the earth's magnetism and to move in a horizontal plane4 ,the current is calculated as follows3 7#a7 amperes 77,h r7_/,n7 tan x in which ,h constant from table below2 r radius of coil2 ,n number of turns of coil2 x angle of deflection of needle4 ,the constant ,h1 given in the following table represents the horizontal force of the earth's magnetism for the place where the galvanometer is used4 ,each value has been multiplied by 7#bpi7_/#aj so that the formula 7#a7 for amperes is correct as given4 ,table of ,galvanometer ,constants4--,values of ,h4 ,boston 4#fii ,chicago 4#gei ,denver 4#iai ,jacksonville #a4jid ,london 4#gde ,minneapolis 4#fha ,new ,york 4#gdd ,new ,haven 4#gca ,philadelphia 4#ghc ,portland1 ,me4 4#fgd ,san ,francisco #a4jba ,st4 ,louis 4#hga ,washington 4#haj ,illustration3 ,fig4 #eba4--,mechanical explanation of the tangent law4 ,construct an apparatus as shown in the figure4 ,the short wooden block1 ,,ns1 represents the magnetic needle4 ,this piece of wood turns around its center1 ,c1 which may be an ordinary nail4 ,it will now be seen that two different forces act upon ,n2 namely1 the weight1 ,g 7one or two ounces71 and the changeable weights which are placed in the scoop1 ,w 7made of cardboard74 ,the height of the roll1 or wheel1 ,r1 is such that the cord1 ,,rn1 runs horizontally1 when ,,ns stands vertically1 i4e41 when there is no weight in the little scoop4 ,if the wheel1 ,r1 be placed sufficiently far from ,,ns1 the string ,,rn1 will always remain almost horizontal1 even if ,,ns be deviated4 ,the thin hand on ,,ns moves over a vertical scale1 which is divided into equal parts1 as shown4 ,this scale may be made of cardboard4 ,if the hand point to division #a when one ounce is placed in the scoop1 it will point to #b for two ounces1 to #c for three ounces1 etc4 ,at #de the needle is deviated at its greatest angle1 and this is1 therefore1 the sensitivity angle of the tangent galvanometer4 ,the deviating values are1 therefore1 proportionate to the scale parts #ja1 #jb1 and #jc1 and so on2 and1 inasmuch as these themselves are tangents1 the tangent law will hold good4 ,ques4 ,how is the tangent galvanometer constructed to give direct readings8 ,ans4 ,to obviate reference to a table1 the circular scale of the instrument is sometimes graduated into tangent values1 as in fig4 #ebj1 instead of being divided into equal degrees4 ,illustration3 ,fig4 #ebb4--,queen tangent and sine galvanometer4 ,this instrument properly adjusted can be used as a standard instrument for laboratory work4 ,the brass ring is #ab inches in diameter1 and the grooves in which the wire is wound are carefully turned so as to be of true rectangular cross section1 thus allowing the constant of the instrument to be accurately calculated and compared with the constant as obtained by other methods4 ,the compass box is #e inches in diameter and is so held in position that it may be raised or lowered1 rotated on its vertical axis1 shifted out of the plane of the coil1 etc41 thus enabling the operator to acquire proficiency with the instrument and to meet all cases of derangement possible4 ,the dial is graduated to single degrees1 and the needle is suspended by a very light cocoon fibre4 ,the whole instrument can be turned about its vertical axis1 and a quadrant graduated in degrees upon the base allows the amount of rotation to be accurately measured1 and the laws of the sine galvanometer investigated4 ,the instrument is wound to measure 4#be ampere to #h amperes4 ,ques4 ,what is the objection to the scale with tangent values8 ,ans4 ,it is more difficult to divide an arc into tangent lines with accuracy than into equal degrees4 ,ques4 ,what disadvantage has the tangent galvanometer8 ,ans4 ,the coil being much larger than the needle1 and hence far away from it1 reduces the sensitiveness of the instrument4 ,the ,sine ,galvanometer4--,this type of instrument has a vertical coil which may be rotated around a vertical axis1 so that it can be made to follow the magnetic needle in its deflections4 ,in the sine galvanometer1 the coil is moved so as to follow the needle until it is parallel with the coil4 ,under these circumstances1 the strength of the deflecting current is proportional to sine of angle of deflection4 ,illustration3 ,fig4 #ebc4--;,central ,scientific ,co4 universal tangent galvanometer4 ,this instrument may be used as a tangent1 ,gaugain1 ,helmholtz-,gaugain1 sine1 cosine1 ,wiedemann or detector galvanometer4 ,the coils1 which slide on a beam parallel to the one carrying the needle box1 are wound on brass rings #ab inches in diameter4 ,on each ring are wound two coils of #dh turns each1 connected to separate binding posts1 and double wound so as to be of equal resistance4 ,the coils and needle box are each provided with an indicator for reading their position on the scale4 ,the needle box is swivelled and removable and one coil may be rotated about its vertical axis and its position read on a disc graduated in degrees4 ,currents may be measured ranging from 4#jjjjjb ampere to #ajj amperes4 ,ques4 ,describe the construction of a sine galvanometer4 ,ans4 ,a form of sine galvanometer is shown in fig4 #ebd4 ,the vertical wire coil is seen at ,m4 ,a needle of any length less than the diameter of the coil ,m1 moves over the graduated circle ,n4 ,the coil ,m1 and graduated circle ,n may be rotated on a vertical axis1 and the amount of angular movement necessary to bring the needle to zero1 measured on the graduated circle ,h4 ,ques4 ,how is the current strength measured8 ,ans4 ,it is proportional to the sine of the angle measured on the horizontal circle ,h1 through which it is necessary to turn the coil ,m1 from the plane of the earth's magnetic meridian to the plane of the needle when it is not further deflected by the current4 ,illustration3 ,fig4 #ebd4--,sine galvanometer4 ,it differs from the tangent galvanometer in that the vertical coil and magnetic needle are mounted upon a standard free to revolve around a vertical axis1 with provision for determining the angular position of the coil4 ,the needle may be of any length shorter than the diameter of the coil4 ,in the figure the parts are3 ,m1 coil2 ,n1 graduated dial of magnetic needle2 ,h1 graduated dial by which the amount of rotation necessary to bring the needle to zero is measured2 ,e1 terminals of the coil2 ,o1 upright standard carrying coil and graduated dial of magnetic needle2 ,c1 base with levelling screws4 ,ques4 ,how is the sine galvanometer operated8 ,ans4 ,in using the instrument1 after the needle has been set to zero1 the current is sent through the coil1 producing a deflection of the needle4 ,the coil is then rotated to follow the motion of the needle1 the current being kept constant1 the rotation being continued until the zero on the upper dial again registers with the needle4 ,the current then is proportional to the sine of the angle through which the coil has been turned1 as determined by the lower dial4 ,ques4 ,has the sine galvanometer a large range8 ,ans4 ,for a given controlling field1 it does not admit of a very large range of current measurement1 since1 for large deflection1 on rotating the coil the position of instability is soon reached4 ,,table ,,of ,,natural ,,sines ,,and ,,tangents ,angle ,sin4 ,tan4 #j 4#jjjj 4#jjjj #a 4#jage 4#jage #b 4#jcdi 4#jcdi #c 4#jebc 4#jebd #d 4#jfih 4#jfii #e 4#jhga 4#jhge #f 4#ajde 4#ajea #g 4#abai 4#abbh #h 4#acib 4#adje #i 4#aefd 4#aefd #aj 4#agcf 4#agfc #aa 4#aijh 4#aidd #ab 4#bjgi 4#babf #ac 4#bbej 4#bcji #ad 4#bdai 4#bdic #ae 4#behh 4#bfgi #af 4#bgef 4#bhfg #ag 4#bibd 4#cjeg #ah 4#cjij 4#cbdi #ai 4#cbef 4#cddc #bj 4#cdbj 4#cfdj #ba 4#cehd 4#chci #bb 4#cgdf 4#djdj #bc 4#cijg 4#dbde #bd 4#djfg 4#ddeb #be 4#dbbf 4#dffc #bf 4#dchd 4#dhgg #bg 4#dedj 4#ejie #bh 4#dfie 4#ecag #bi 4#dhdh 4#eedc #cj 4#ejjj 4#eggd #ca 4#eaej 4#fjji #cb 4#ebii 4#fbdi #cc 4#eddf 4#fdid #cd 4#eeib 4#fgde #ce 4#egcf 4#gjjb #cf 4#ehgh 4#gbfe #cg 4#fjah 4#gecf #ch 4#faeg 4#ghac #ci 4#fbic 4#hjih #dj 4#fdbh 4#hcia #da 4#fefa 4#hfic #db 4#ffia 4#ijjd #dc 4#fhbj 4#icbe #dd 4#fidg 4#ifeg #de 4#gjga #a4jjjj #df 4#gaic #a4jcee #dg 4#gcad #a4jgbd #dh 4#gdca #a4aajf #di 4#gedg #a4aejd #ej 4#gffj #a4aiah #ea 4#ggga #a4bcdi #eb 4#ghhj #a4bgii #ec 4#gihf #a4cbgj #ed 4#hjij #a4cgfd #ee 4#haib #a4dbha #ef 4#hbij #a4dhbf #eg 4#hchg #a4ecii #eh 4#hdhj #a4fjjc #ei 4#hegb #a4ffdc #fj 4#hffj #a4gcba #fa 4#hgdf #a4hjdj #fb 4#hhbi #a4hhjg #fc 4#hiaj #a4ifbf #fd 4#hihh #b4jejc #fe 4#ijfc #b4adde #ff 4#iace #b4bdfj #fg 4#ibje #b4ceei #fh 4#ibgb #b4dgea #fi 4#icci #b4fjea #gj 4#icig #b4gdge #ga 4#idee #b4ijdb #gb 4#ieaa #c4jggb #gc 4#iefc #c4bgji #gd 4#ifac #c4dhgd #ge 4#ifei #c4gcba #gf 4#igjc #d4jajh #gg 4#igdd #d4ccae #gh 4#igha #d4gjdf #gi 4#ihaf #e4addf #hj 4#ihdh #e4fgac #ha 4#ihgg #f4cach #hb 4#iijc #g4aaed #hc 4#iibe #h4addc #hd 4#iide #i4eadd #he 4#iifb #aa4dc #hf 4#iigf #ad4cj #hg 4#iihf #ai4jh #hh 4#iiid #bh4fd #hi 4#iiih #eg4bi ,ques4 ,what is the position of instability8 ,ans4 ,the position of the needle beyond which the rotation of the coil will cause it to turn all the way round4 ,ques4 ,how may the range be increased8 ,ans4 ,by an adjustable controlling field or a shunt4 ,ques4 ,what advantage has the sine galvanometer over the tangent instrument8 ,ans4 ,its advantage is in the case where the relative values of two or more currents are required to be measured1 or where the constant of the instrument is obtained by comparison with a standard measuring instrument and not calculated from the dimensions of the coil1 because all galvanometers thus used follow the sine law independently of the shape of the coil1 while only circular coils will follow the sine law4 ,illustration3 ,fig4 #ebe4--;,differential galvanometer4 ,it consists of two coils of wire1 so wound as to have opposite magnetic effects on a magnetic needle suspended centrally between them4 ,the needle of a differential galvanometer shows no deflection when two equal currents are sent through the coils in opposite directions1 since1 under these conditions1 each coil neutralizes the effect of the other4 ,sometimes the current is so sent through the two coils1 that each coil deflects the needle in the same direction4 ,in this case the instrument is no longer differential in action4 ,if1 when this condition obtains1 the magnetic needle be suspended at the exact center of the line which joins the centers of the coils1 the advantage is gained by obtaining a field of more nearly uniform intensity around the needle4 ,when the needle is suspended by a silk fibre1 a final and most delicate adjustment can be obtained by raising or lowering one of the levelling screws slightly1 so as to tilt the needle nearer to or farther from one of the coils4 ,the ,differential ,galvanometer4--,this is a form of galvanometer in which a magnetic needle is suspended between two coils of equal resistance so wound as to tend to deflect the needle in opposite directions4 ,the needle of a differential galvanometer shows no deflection when two equal currents are sent through the coils in opposite directions1 since under these conditions1 each coil neutralizes the other's effects4 ,such instruments may be used in comparing resistances1 although the ,wheatstone bridge1 in most cases1 affords a preferable method4 ,ques4 ,what is the special use of the differential galvanometer8 ,ans4 ,it is used for comparing two currents4 ,ques4 ,what is the method of comparing currents8 ,ans4 ,if two equal currents be sent in opposite directions through the coils of the galvanometer1 the needle will not move2 if the currents be unequal1 the needle will be deflected by the stronger of them with an intensity corresponding to the difference of the strengths of the two currents4 ,ques4 ,how are the coils adjusted8 ,ans4 ,this is done by coupling them in series in such a way that they tend to turn the needle in opposite directions1 and when a current is passing through them1 they are moved nearer to the needle or farther from it until the needle stands at zero with any current4 ,if the coils be not movable1 a turn or more can be unwound from the coil giving the greatest magnetic effect until a balance is obtained1 the wire so unwound can then be coiled in the base of the instrument4 ,ballistic ,galvanometer4--,this type of galvanometer is designed to measure the strength of momentary currents1 such for instance1 as the discharge of a condenser4 ,in construction the magnetic system is given considerable weight1 and arranged to give the least possible damping effect4 ,the term damping effect means the offering of a retarding force to control swinging vibrations1 such as the movements of a galvanometer needle1 and to bring them quickly to rest4 ,if a momentary current be passed through a ballistic galvanometer1 the impulse given to the needle does not cause appreciable movement to the magnetic system until the current ceases1 owing to the inertia of the heavy moving parts1 the result being a slow swing of the needle4 ,illustration3 ,fig4 #ebf4--,queen dead beat and ballistic reflecting galvanometer4 ,as illustrated1 the coils are easily removable and enclose a heavy block of copper fixed in a central fork4 ,in a cylindrical hole bored in this block hangs the bell magnet which with its mirror is suspended by a long cocoon fibre1 and the eddy currents induced in the copper bring the system quickly to rest after a deflection4 ,by lifting the copper block out of the frame the instrument is made ballistic4 ,the instrument is made with coils of any desired resistance up to #a1jjj ohms4 ,ques4 ,what name is given to the swing of a ballistic galvanometer needle8 ,ans4 ,it is called the kick4 ,ques4 ,how is the current measured8 ,ans4 ,as the needle swings slowly around it adds up1 as it were1 the varying impulses received during the passage of the momentary current1 and the quantity of electricity that has passed is proportional to the sine of half the angle of the first swing or kick4 ,if a reflecting method be used with a straight scale1 the observed deflection depends upon the tangent of twice the angle of movement of the needle4 ,for small deflections1 however1 the change of flux can be taken as directly proportional to the observed deflection4 ,illustration3 ,fig4 #ebg4--,thompson galvanometer with mirror reflecting system for reading the deflections of a galvanometer needle by the movements of a spot of light reflected from a mirror attached to the needle or movable magnetic system4 ,use of ,mirrors in ,galvanometers4--,in order that small currents may be measured accurately1 some means must be provided to easily read a small deflection of the needle4 ,accordingly1 it is desirable that the pointer be very long so that a large number of scale divisions may correspond to small deflections4 ,in construction1 since sensitive galvanometers must be made with the moving parts of little weight1 it would not do to use a long needle1 hence a ray of light is used instead1 which is reflected on a distant scale by a small mirror attached to the moving part4 ,in the ,thompson mirror reflecting galvanometer1 as shown in fig4 #ebh1 a small vertical slit is cut in the lamp screen below the scale1 and the ray of light from the lamp1 passing through the slit1 strikes the mirror which is about three feet distant1 and which reflects the beam back to the scale4 ,it should be noted that the angle between the original ray of light and the reflected ray is twice the angle of the deflection of the mirror2 the deflections of the ray of light on the scale1 however1 are practically proportional to the strength of currents through the instrument4 ,the mirror arrangement as shown in fig4 #ebh1 requires a darkened room for its operation1 but such is not necessary when a telescope is used as in fig4 #ebi4 ,here the scale readings are reflected in the mirror and their value observed by the telescope without artificial light4 ,illustration3 ,fig4 #ebh4--,telescope method of reading galvanometer deflections by reflection of scale reading in mirror4 ,here two mirrors are used1 but in most cases the telescope is pointed directly toward the mirror on galvanometer shown in fig4 #ebg1 because the two mirror system1 as illustrated in the figure1 is used on portable galvanometers since it is the more compact4 ,damping4--,this relates to the checking or reduction of oscillations4 ,thus1 a galvanometer is said to be damped when so constructed that any oscillations of the pointer which may be started1 rapidly die away4 ,galvanometers are frequently provided with damping devices for the purpose of annulling these oscillations1 thus causing the moving part to assume its final position as quickly as possible4 ,sometimes the instrument is fitted with a damping coil1 or closed coil so arranged with respect to the moving system that the oscillations of the latter give rise to electric currents in the closed coil1 whereby energy is dissipated4 ,again1 air vanes are employed1 but anything in the nature of solid friction cannot be used4 ,illustration3 ,figs4 #ebi and #ecj4--;,galvanometer lamp and scale for individual use4 ,the scale is etched on a ground glass strip #f centimeters wide by #fj centimeters long with long centimeter divisions and short millimeter divisions the entire length1 reading both ways from zero in the center4 ,it is mounted in an adjustable wooden frame4 ,a straight filament lamp 7#aaj volts7 is enclosed in a metal hood japanned black to cut out all reflected light4 ,this form of filament makes a single brilliant line on the scale1 enabling closer readings than the spot of light arrangement4 ,the lamp hood is adjustable to any desired height on the support rod4 ,d',arsonval ,galvanometer4--,this instrument has a movable coil in place of a needle1 and its operation depends upon the principle that if a flat coil of wire be suspended with its axis perpendicular to a strong magnetic field1 it will be deflected whenever a current of electricity passes through it4 ,ques4 ,describe the construction of a ,d',arsonval galvanometer4 ,ans4 ,the essential features are shown in figs4 #ecb and #ecc4 ,the coil1 which is rectangular in section is wound upon a copper form1 and suspended between a permanent magnet by fine wires to the points ,a and ,b4 ,the magnet has its poles at ,n and ,s4 ,it is a soft iron cylinder fixed between the poles in order to intensify the magnetic field across the air gaps in which the coil moves4 ,illustration3 ,fig4 #eca4--,queen reading telescope4 ,this arrangement is utilized to measure the deflections of a galvanometer having suspended mirror moving system4 ,it consists of a reading telescope mounted as illustrated with a millimeter scale1 having a length of #ej centimeters4 ,in use1 the image of the scale is seen in the galvanometer mirror through the telescope4 ,the eye piece of the telescope has a cross hair which acts as a reference line so that by noting the particular division on the scale when the galvanometer is at rest1 the amount of deflection can be readily observed when the galvanometer is deflected4 ,the instrument has all the necessary adjustments to set it up quickly and for bringing the cross hair and scale in focus4 ,it is generally placed at a distance of one meter from the galvanometer mirror4 ,ques4 ,explain its operation4 ,ans4 ,an enlarged horizontal cross section of the galvanometer on line ,,xy is shown in fig4 #ecc4 ,the current is flowing in the coil as in fig4 #ecb1 up on the left side and down on the right4 ,the position of the coil when no current is flowing is indicated by n' s'4 ,by applying the law of mutual action between magnetic poles1 it is seen that when the current is applied1 the poles developed at n' s' will move into the position n'' s''4 ,see fig4 #aai4 ,ques4 ,how is the coil affected by a change in the direction of the current8 ,ans4 ,the polarity of the coil is reversed and consequently the direction of the deflection4 ,illustration3 ,figs4 #ecb and #ecc4--;,diagrams showing essential features of construction and principle of operation of ,d',arsonval galvanometer4 ,ques4 ,upon what does the sensitiveness of the instrument depend8 ,ans4 ,upon the strength of the field of the permanent magnet1 the number of turns in the suspended coil1 and the torsion of the wires by which it is suspended4 ,ques4 ,when is this galvanometer called dead beat8 ,ans4 ,when the construction is such that the moving part comes quickly to rest without a series of diminishing vibrations4 ,illustration3 ,figs4 #ecd to #ecf4--,queen horizontal magnet ,d',arsonval galvanometer with telescope and scale4 ,it is very sensitive and is used in many electrical measurements1 including commercial testing1 such as measuring insulation of cables1 fault location1 etc4 ,it is not affected by surrounding magnetic disturbances1 and may1 therefore1 be used in proximity to dynamos and switchboards4 ,the instrument has a pair of binding post terminals1 one of which connects to a bottom spiral of the system and the other forms a junction with the top of the tube holding the system1 forming a complete circuit through the coil4 ,the tube containing the system may be readily removed from the magnet and another tube having a different system inserted as is required for various kinds of electrical measurement4 ,the entire system with its suspension may be inspected by the removal of a thumb screw4 ,to inspect interior of tube first be sure that the screw ,b is turned so that the coil is clamped4 ,entirely remove screw ,c1 and1 holding the outside tube near the window1 press firmly with the finger on the extreme top of the suspension support4 ,the inside rib1 with complete suspension1 will draw from the tube1 and the working parts can be fully inspected4 ,carefully return same to its original position in tube1 setting tight the screw ,c4 ,the galvanometer is designed so that the coil is clamped in position when the galvanometer must be transported4 ,the insulation of the galvanometer terminals and binding posts is such as to guard against any possible leakage4 ,as a further protection1 each levelling screw is provided with a hard rubber insulator4 ,this feature is essential since1 in making insulation measurements1 the operator wishes to be assured that the deflection being obtained is the result of leakage upon the cable or wire being measured and not leakage between the galvanometer terminals4 ,the galvanometer is provided with an attached telescope and scale for noting the deflections4 ,the deflections produced by this galvanometer are proportional to the current4 ,to facilitate quickly setting up the instrument1 two way levels are provided4 ,ques4 ,what causes this8 ,ans4 ,the instrument is made dead beat by winding the coil on a copper or aluminum frame1 so that when in operation1 currents are induced in the frame by the motion of the coil in the magnetic field2 these currents oppose the motion of the coil4 ,ques4 ,for what service is the ,d',arsonval galvanometer adapted8 ,ans4 ,it is desirable for general use as it is not much affected by changes in the magnetic field4 ,it may be made with high enough period and sensibility to be satisfactory as a ballistic instrument1 but for extreme sensibility an instrument of the astatic type is more generally used4 ,galvanometer ,constant or ,figure of ,merit4--,in order that a galvanometer shall be of value as a measuring instrument1 the relation between the current and the deflection produced by it must be known4 ,this may be obtained experimentally by determining the value of the current required to produce one scale division4 ,the galvanometer constant then may be defined as the resistance through which the galvanometer will give a deflection of one scale division when the current applied is at a pressure of one volt4 ,accordingly1 the deflection as indicated on the scale must be multiplied by its constant or figure of merit1 in order to obtain the correct reading4 ,if the scale readings be not directly proportional to the quantity to be measured1 the law of the instrument must also be considered4 ,thus in a tangent galvanometer as previously explained ,i ,k tan phi where ,i current1 phi the deflection or scale reading1 and ,k the galvanometer constant4 ,illustration3 ,fig4 #ecg4--;,diagram showing method of connecting galvanometer shunt4 ,by the use of a shunt the range of measurement of a galvanometer can be greatly increased4 ,illustration3 ,fig4 #ech4--;,diagram of a form of universal shunt box for use with galvanometers of widely different resistances4 ,the galvanometer1 as indicated at ,g1 is connected across the ends of a series of resistances ,,ab4 ,the main wires are connected1 one to end ,a of the series and the other to a travelling point whose position is varied by means of plugs or by a dial switch4 ,galvanometer ,shunts4--,the sensitiveness of a galvanometer used for measuring current may be reduced to any desired extent by connecting a resistance of known value in parallel with it4 ,thus1 if it be desired to measure a current greater than can be measured directly by the galvanometer1 a part of the current can be sent through the resistance or shunt1 and the total value of the current calculated4 ,a galvanometer shunt bears a definite ratio to the resistance of the galvanometer1 being usually adjusted so that only 4#a1 4#ja1 or 4#jja part of the current passes through the galvanometer4 ,the degree in which a shunt increases the range of deflection of a galvanometer is called its multiplying power4 ,illustration3 ,fig4 #eci4--;,ayrton-,mather universal shunt4 ,this shunt may be used with any galvanometer4 ,the total resistance is #aj1jjj ohms1 with shunt powers of #a1 #e1 #aj1 #ej1 #ajj1 #ejj1 and #a1jjj4 ,it is also fitted with positions in which the galvanometer is shorted and off4 ,the coils are of constantan wire4 ,if 4#a of the current flowing1 passed through the galvanometer and 4#i through the shunt1 then the current in the circuit would be ten times that through the galvanometer4 ,accordingly the current in the galvanometer must be multiplied by the multiplying power of the shunt to obtain the true value of the current in the circuit4 ,in order to determine the resistance necessary to be used with a certain galvanometer1 the resistance of the latter is to be divided by the multiplying power desired less one4 ,,example4--,what must be the resistance of a shunt for a galvanometer of #b1jjj ohms resistance where only one fifth of the current is to pass through the galvanometer8 ,the multiplying power less one is #e - #a #d and the required resistance is #b1jjj #d #ejj ohms4 ,when it is essential that the total resistance of the circuit should not be altered by an alternation of the galvanometer shunt1 a compensating box should be used which automatically inserts a resistance for each shunt in series with the shunted galvanometer to bring the total resistance up equal to the unshunted value4 ,thus the current in the main circuit is not altered4 ,,chapter ,,xxvii ,,testing ,,and ,,testing ,,apparatus ,the practical electrician frequently has to make tests of various kinds which require the rapid and accurate measurement of voltage1 current and resistance4 ,it is therefore essential that he understand the methods employed in testing and the operation of the instruments used4 ,most tests are made with a galvanometer1 and the devices1 such as resistances1 switches1 etc41 which are used in connection with the galvanometer may be obtained put up in a neat and substantial box together with the galvanometer1 the combination being called a testing set4 ,numerous forms of testing set are illustrated in this chapter4 ,the construction1 use1 and operation of the various types of galvanometer have been explained in chapter twenty-six4 ,ammeters and voltmeters1 which are simply special forms of galvanometer1 and which are largely used are fully described in the preceding chapter4 ,pressure ,measurement4--,an electromotive force has been defined as that which causes or tends to cause a current2 it is analogous to water pressure2 potential difference corresponds to difference of level4 ,the total electromotive force of a circuit is independent of resistance or current1 and cannot be limited to mean the fall of pressure between any two points1 as for instance the terminals of a battery4 ,if the pressure of a battery be two volts when measured on open circuit by a static voltmeter1 there will still be two volts on closed circuit1 but there will now be a loss of pressure through the internal resistance of the battery and the voltage across the terminals will be less than the total voltage4 ,the static voltmeter1 never closing the circuit1 actually measures the total voltage4 ,illustration3 ,fig4 #edj4--;,clark cell 7,kahle's modification of the ,rayleigh ,h form71 the standard for the ,international volt4 ,the cell has for its positive electrode1 mercury1 and for its negative electrode1 amalgamated zinc4 ,the electrolyte consists of a saturated solution of zinc sulphate and mercurous sulphate4 ,the pressure is #a4dcd volts at #ae;,c41 and between #aj;,c4 and #be;,c4 the pressure decreases 4#jjaae of a volt for each increase of #a;,c4 ,the containing glass vessel consists of two limbs1 closed at bottom and joined above to a common neck fitted with a ground glass stopper4 ,the diameter of the limbs should be at least #b cms41 and their length at least #c cms4 ,the neck should be not less than #a4e cms4 in diameter4 ,at the bottom of each limb a platinum wire of about 4#d mm4 in diameter is sealed through the glass4 ,to set up the cell1 place mercury in one limb1 and in the other hot liquid amalgam1 containing #ij parts mercury and #aj parts zinc4 ,the platinum wires at the bottom must be completely covered by the mercury and the amalgam1 respectively4 ,on the mercury1 place a layer #a cm4 thick of the zinc and mercurous sulphate paste4 ,both this paste and the zinc amalgam must be covered with a layer of the neutral zinc sulphate crystals #a cm4 thick4 ,the whole vessel must then be filled with the saturated zinc sulphate solution1 and the stopper inserted so that it shall just touch it1 leaving1 however1 a small bubble to guard against breakage when the temperature rises4 ,before finally inserting the glass stopper a strong alcoholic solution of shellac is applied to the upper edge1 after which the stopper is pressed firmly in place4 ,ques4 ,what error is introduced in measuring the pressure of a battery with an ordinary voltmeter8 ,ans4 ,since the measurement is made on closed circuit the reading does not give the total pressure of the battery4 ,the error is very slight because the resistance of the voltmeter is very high and the current so small that the loss of pressure in the battery can be neglected4 ,illustration3 ,fig4 #eda4--,weston ,cadmium ,cell4 ,it is made in two forms2 one known as the ,weston normal cell1 in which the solution of cadmium sulphate is saturated at all temperatures at which the cell may be used4 ,the other1 known as the ,weston standard cell1 in which the cadmium sulphate solution is unsaturated at all temperatures above #d;,c4 ,the ,weston normal cell1 or saturated form is slightly affected by changes in temperature1 but1 on account of the fact that it can be accurately reproduced1 it was adopted by the ,london ,conference in #aijh1 as a convenient voltage standard4 ,the value of its voltage suggested by the committee of the ,london ,conference on ,electrical ,units and ,standards1 and adopted by the ,bureau of ,standards at ,washington1 ,jan4 #ast1 #aiaa1 is #a4jahc ,international volts at #bj;,c4 ,at any other temperature its voltage is3 ,et ,e#bj - #j4jjjjdjf7t-#bj7 - #j4jjjjjjie7t-#bj7b #j4jjjjjjja7t-#bj7c ,the ,weston standard cell1 or unsaturated form is practically unaffected by changes in temperature and is the form most commonly used for laboratory work and general testing4 ,the average pressure of this form is #a4jahg ,int4 volts4 ,ques4 ,define the ,international volt4 ,ans4 ,it is the electromotive force that1 steadily applied to a conductor whose resistance is one ,international ohm1 will produce a current of one ,international ampere1 and which is represented sufficiently well for practical use by #a1jjj_/a1dcd of the voltage between the poles of the ,clark cell at a temperature of #ae ,c41 when prepared as in fig4 #edj4 ,the relation between the units volt1 ampere and ohm1 are shown graphically in figs4 #edb and #edc4 ,illustration3 ,figs4 #edb and #edc4--;,diagrams showing hydraulic analogy illustrating the difference between amperes and coulombs4 ,the rate of current flow of one ampere in fig4 #edc may be compared to the rate of discharge of a pump as in fig4 #edb4 ,assuming the pump to be of such size that it discharges a gallon per stroke and is making #fj strokes per minute1 the quantity of water discharged per hour 7coulombs in fig4 #edc7 is #a #fj #fj #c1fjj gallons4 ,following the analogy further 7in fig4 #edc71 the pressure of one volt is required to force the electricity through the resistance of one ohm between the terminals ,a and ,b4 ,in fig4 #edb1 the boiler must furnish steam pressure on the pump piston to overcome the friction 7resistance7 offered by the pipe and raise the water from the lower level ,a' to the higher level ,b'4 ,the difference of pressure between ,a and ,b in the electric circuit corresponds to the difference of pressure between ,a' and ,b'4 ,the cell furnishes the energy to move the current by maintaining a difference of pressure at its terminals ,c and ,d2 similarly1 the boiler furnishes energy to raise the water by maintaining a difference of pressure between the steam pipe ,c and exhaust pipe ,d'4 ,illustration3 ,fig4 #edc4 ,if the current strength in fig4 #edc be one ampere1 the quantity of electricity passing any point in the circuit per hour is #a #fj #fj #c1fjj coulombs4 ,current ,measurement4--,it is necessary to adopt some arbitrary standard in order to compare currents of different strengths4 ,the term strength of a current1 or current strength means the rate of flow past any point in the circuit in a given unit of time4 ,the unit of current1 called the ampere1 is defined as the unvarying current which1 when passed through a solution of nitrate of silver in water 7#ae per cent4 by weight of the nitrate7 deposits silver at the rate of 4#jjaaah gramme per second4 ,illustration3 ,fig4 #edd4--,queen weight voltameter for determining the strength of current by the weight of metal deposited in a given time4 ,the two outside plates form the anode and are joined together and to one binding post1 while the cathode is placed between them and connected to the other binding post4 ,the cathode thus receives a deposit on both sides4 ,an adjustable arm serves to lower the plates into the electrolyte4 ,to calculate the strength of an unknown current which has passed through a weight voltameter1 divide the gain in weight by the number of seconds the current flows through the instrument and by the weight deposited by one ampere in one second4 ,that is1 current strength in amperes gain in weight 7time in seconds 4#jjjcbhf74 ,ques4 ,how much copper or zinc will one ampere deposit in one second8 ,ans4 4#jjjcbhf gramme of copper in a copper voltameter1 or 4#jjjcchf gramme of zinc in a zinc voltameter4 ,ques4 ,what is the difference between an ampere and a coulomb8 ,ans4 ,an ampere is the unit rate of flow of the current1 and a coulomb is the unit quantity of electricity1 that is1 the ampere is the rate of current flow that will deposit 4#jjjcbhf grammes of copper in one second and a coulomb is the quantity of electricity that passes a given point in one second when the current strength is one ampere4 ,in other words a coulomb is one ampere second4 ,illustration3 ,fig4 #ede4--;,gas voltameter for determining the strength of current by the volume of gas evolved4 ,to use1 connect up as shown in the illustration4 ,adjust so that the zero position of the burette is about one-half inch below the level of the top of the ,u tube4 ,pour acidulated water into the mouth of the burette till the water in the ,u tube is about one-half inch from the top4 ,with the electrodes inserted through the corks1 carefully place each one in position by giving a slight twist to the right as the cork enters4 ,the water level in the ,u tube and burette should now be the same1 or further adjustment must be made to attain this result4 ,the level in the burette does not necessarily have to correspond with the zero graduation1 but must not be below it4 ,unclamp the burette and hold it nearly horizontal4 ,the liquid will not run out if the corks be tight1 so that this is the air leakage test4 ,attach the connectors and wires from the current source 7which should have a pressure of #b or more volts7 placing a switch in the circuit4 ,when the switch is closed1 bubbles of gas will rise in the ,u tube from both electrodes1 displacing the water and forcing it up the burette4 ,hydrogen will be liberated over the negative electrode1 and oxygen over the positive electrode in the proportion of twice as much hydrogen as oxygen4 ,to calculate the current strength1 divide the volume of gas liberated by the time in seconds1 and by the volume of gas liberated 7in cubic centimeters7 by one ampere in one second and by 4#agcc2 that is3 amperes volume of gas liberated 7time in seconds 4#agcc74 ,,example4--,if an arc lamp require a current of #h amperes1 how much electricity does it consume per hour8 ,since one coulomb one ampere second1 the quantity of electricity consumed per hour is #h amperes 7 #fj #fj 7 #bh1hjj coulombs4 ,voltameter4--,a voltameter is an electrolytic cell employed to measure an electric current by the amount of chemical decomposition the current causes in passing through the cell4 ,there are two classes of voltameter3 #a4 ,weight voltameters2 #b4 ,gas voltameters4 ,ques4 ,what is the difference between these two classes of voltameter8 ,ans4 ,in one1 the current strength is determined by the weight of metal deposited or weight of water decomposed1 and in the other by the volume of gas liberated4 ,fig4 #edd shows a weight voltameter and fig4 #ede a gas voltameter4 ,ques4 ,how should the plates of a weight voltameter be treated before use8 ,ans4 ,they must be thoroughly cleaned and polished with sandpaper1 the sand being afterwards removed by placing them in running water4 ,the fingers must not be placed on any part of the plate which is to receive the deposit4 ,ques4 ,what form of voltameter has been selected to measure the ,international ampere8 ,ans4 ,the silver voltameter arranged as here specified3 ,the cathode on which the silver is to be deposited shall take the form of a platinum bowl1 not less than #aj cms4 in diameter1 and from #d to #e cms4 in depth4 ,the anode shall be a disc or plate of pure silver some #cj sq4 cms4 in area1 and #b or #c cms4 in thickness4 ,this shall be supported horizontally in the liquid near the top of the solution by a silver rod riveted through its center4 ,to prevent the disintegrated silver which is formed on the anode falling upon the cathode1 the anode shall be wrapped around with pure filter paper1 secured at the back by suitable folding4 ,the liquid shall consist of a neutral solution of pure silver nitrate containing about #ae parts by weight of the nitrate to #he parts of water4 ,ques4 ,what is the value of the ,international ampere as measured with the silver voltameter8 ,ans4 ,the ,international ampere is represented sufficiently well for practical use by the unvarying current which1 when passed through a silver voltameter 7as described above7 deposits silver at the rate of 4#jjaaah gramme per second4 ,illustration3 ,fig4 #edf4--,single contact and short circuiting key4 ,this key is intended especially for use with ,d',arsonval galvanometers in zero deflection methods4 ,the key is connected in circuit with the galvanometer so that whenever the key is not depressed1 the galvanometer is short circuited1 and its oscillations quickly damped out by the currents induced in its coil4 ,the back end of the spring is held in a slot in a rubber block attached to the base4 ,ohm's ,law and the ,ohm4--,the various tests here described depend for their truth upon the definite relation existing between the electric current1 its pressure1 and the resistance which the circuit offers to its flow4 ,this relation was fully investigated by ,ohm in #ahbg4 ,using the same conductor1 he proved not only that the current varies with the pressure1 but that it varies in direct proportion4 ,ohm's law has already been discussed in a previous chapter and the several ways of expressing it are repeated here for convenience3 volts #a4 ,amperes -------2 ohms #b4 ,volts amperes ohms2 volts #c4 ,ohms ---------4 amperes ,various values have been assigned1 from time to time1 to the ohm or unit of resistance1 the unit in use at the present time being known as the ,international ohm4 ,this was recommended at the meeting of the ,british ,association in #ahib1 was adopted by the ,international ,electrical ,congress held in ,chicago in #ahic1 and was legalized for use in the ,united ,states by act of ,congress in #ahid4 ,the ,international ohm in graphically defined in fig4 #edh4 ,the previous values given to the ohm which were more or less generally accepted are as follows3 ,the ,siemens' ,ohm4--,a resistance due to a column of mercury #ajj cm4 long and #a sq4 mm4 in cross section at #j ,c4 ,b4 ,a4 7,british ,association7 ,ohm4--,a resistance due to a column of mercury approximately #ajd4i cm4 long and #a sq4 mm4 in cross section at #j ,c4 ,legal ,ohm4--,a resistance due to a column of mercury #ajf cm4 long and #a sq4 mm4 in cross section at #j ,c4 ,this unit was adopted by the ,paris conference of #ahhd4 ,,ohm ,,table,a ,inter- ,date national ,legal,b4 ,a4 ,siemens' ,ohm ,ohm,ohm ,ohm ---------------------------------------- ----------------------- ,international ,ohm #ahic-d #a4jjjj #a4jjbh #a4jacf #a4jfcj ,legal ,ohm #ahhd 4#iigb #a4jjjj #a4jajg #a4jfjj ,b4 ,a4 ,ohm #ahfd 4#ihff 4#ihid #a4jjjj #a4jdhh ,siemens' ,ohm 4#idjg 4#idcd 4#iece #a4jjjj ,a ,,note4--,in the above table to reduce1 for instance1 ,british ,association ohms to ,international ohms1 multiply by 4#ihff1 or divide by #a4jacf2 to reduce legal ohms to ,international ohms1 multiply by 4#iigb1 or divide by #a4jjbh1 etc4 ,illustration3 ,fig4 #edg4--;,double contact key4 ,it is of especial value in connection with a ,wheatstone bridge4 ,when used with the latter it forms a combination battery and galvanometer key4 ,the battery is wired to the top leaves of the key and the galvanometer to the lower leaves4 ,hence1 when operated1 the battery circuit will be closed before the galvanometer circuit1 as it is desirable to avoid undue disturbance of the needle4 ,illustration3 ,fig4 #edh4--,the international ohm4 ,it is defined as the resistance of #ad4deb grammes of mercury in the form of a column of uniform cross section #ajf4c centimeters in length1 at a temperature of #j ,c4 ,this is approximately equivalent to a column #ajf4c cm4 long1 having a uniform cross section of #a sq4 mm4 ,in the figure the international ohm is defined graphically4 ,the resistance of the external circuit and the standard one volt cell is assumed to be zero4 ,illustration3 ,fig4 #edi4--,leeds and ,northrup one ohm standard resistance 7,reichsanstalt form72 adjusted at #bj ,c4 ,low resistance standards may be properly divided into two classes3 #a4 those which are designed primarily as resistance standards1 and #b4 those designed as current carrying standards4 ,those of the first mentioned class are often used to measure currents up to their capacity4 ,the above standard has both pressure and current terminals4 ,the binding posts for the former are mounted on high posts so as to be easily accessible when the standard is immersed in oil4 ,when used as a resistance standard of precision1 it should not be subjected to a current of more than one ampere1 and when used as a current carrying standard of lesser accuracy1 a current of #b or #c amperes may be used4 ,practical ,standards of ,resistance4--,the column of mercury as shown in fig4 #edh1 is the recognized standard for resistance1 however1 in practice1 it is not convenient to compare resistances with such a piece of apparatus1 and therefore secondary standards are made up and standardized with a great degree of precision4 ,these secondary standards are made of wire4 ,the material generally used being manganin or platinoid4 ,illustration3 ,fig4 #eej4--;,direct deflection method of testing resistances2 a useful and simple method which may be used in numerous tests4 ,galvanometer readings are taken through the known1 and unknown resistances1 and the current being proportional to the deflections1 the value of the unknown resistance is easily calculated4 ,resistance ,measurement4--,resistance is that which offers opposition to the flow of electricity4 ,ohm's law shows that the strength of the current falls off in proportion as the resistance in the circuit increases4 ,this gives a basis for measuring resistance4 ,there are various methods by which an unknown resistance may be measured1 as by the3 #a4 ,direct deflection method2 #b4 ,method of substitution2 #c4 ,fall of potential method2 #d4 ,differential galvanometer method2 #e4 ,drop method2 #f4 ,voltmeter method2 #g4 ,wheatstone bridge method4 ,direct ,deflection ,method4--,this method is based on the fact that the greater the current through a galvanometer the greater the deflection of the needle2 it is a simple method and is capable of extended application4 ,the apparatus required consists of battery1 galvanometer1 known resistance1 and double contact key4 ,the connections are made as in fig4 #eej4 ,the known resistance is put in circuit with the galvanometer and after noting the deflection1 the key is moved so as to cut out the known resistance and throw into circuit the unknown resistance4 ,the deflection of the galvanometer is again noted and compared with the first deflection4 ,illustration3 ,fig4 #eea4--;,charge and discharge key1 adapted to condenser testing where the condenser is to be alternately charged and discharged4 ,the insulated handle enables the key to be used without insulating the body4 ,illustration3 ,fig4 #eeb4--,pohl commutator4 ,this is equivalent to a two pole double throw switch4 ,the depressions in the base are filled with mercury into which the contacts dip in closing the circuit4 ,if the deflections be proportional to the current1 the unknown resistance will be as many times the known resistance as the deflection with the known resistance is greater than the deflection with the unknown resistance4 ,method of ,substitution4--,this is the simplest method of measuring resistance4 ,the resistance to be measured is inserted in series with a galvanometer and some constant source of current1 and the galvanometer deflection noted4 ,a known adjustable resistance is then substituted for the unknown and adjusted till the same deflection is again obtained4 ,the value of the adjustable resistance thus obtained is equal to that of the resistance being tested4 ,illustration3 ,fig4 #eec4--,substitution method of testing resistances4 ,the connections and apparatus are the same as in fig4 #eej1 except that a resistance box is used in place of the known resistance4 ,in making the test1 first note deflection with unknown resistance in circuit1 then press key so that the current will pass through the resistance box1 and adjust the resistance in the box so that the deflection of the galvanometer is about the same as with the unknown4 ,now switch from one circuit to the other1 changing the resistance in the box until equal deflections are obtained4 ,when this obtains1 the resistance in the box is the same as the resistance being tested4 ,ques4 ,what kind of adjustable resistance is used in making the above test8 ,ans4 ,a resistance box4 ,ques4 ,describe a resistance box4 ,ans4 ,it consists of a box containing numerous resistance coils with their ends connected to terminals and provided with plugs so that they may be thrown into or out of circuit at will1 thus varying the resistance in the circuit4 ,illustration3 ,fig4 #eed4--,ordinary resistance box4 ,it contains sets of standard resistances consisting of coils of insulated wire having low conductivity and small temperature coefficient4 ,the ends of the coils are joined to the section of the bar between the plugs4 ,the insertion of a plug cuts out a coil4 ,in using1 care should be taken to put the plugs in with a slight twist so that there shall be no resistance introduced by poor contact4 ,fall of ,potential ,method4--,this is a very simple method of measuring resistances1 and one that is convenient for practical work in electrical stations because it requires only an ammeter1 voltmeter1 battery and switch--apparatus to be found in every station4 ,the connections are made as shown in fig4 #eee4 ,in making the test the ammeter and voltmeter readings are taken at the same time1 and the unknown resistance calculated from ,ohm's law4 ,accordingly1 since3 7#a7 amperes volts _/ ohms solving for the resistance1 7#b7 ohms volts _/ amperes ,illustration3 ,fig4 #eee4--;,fall of potential method of testing resistances2 a convenient method for testing at stations1 requiring only the usual instruments to be found at a station4 ,the resistance of the voltmeter must be very high1 otherwise the test must be made as in fig4 #eef4 ,,example4--,if in fig4 #eee the readings show #f volts and #b amperes how many ohms is the resistance being tested8 ,substituting in formula 7#b7 ohms #f_/b #c ,ques4 ,can this test be made with any kind of voltmeter8 ,ans4 ,its resistance must be very high to avoid error4 ,when a voltmeter having small resistance is used1 it should be connected so as to measure the fall of pressure across both ammeter and unknown resistance as shown in fig4 #eef4 ,illustration3 ,fig4 #eef4--;,fall of potential method of testing resistances2 diagram showing connections for testing with low resistance voltmeter4 ,the resistance measured with this connection will be the sum of the resistances of the coil and the ammeter4 ,the resistance of the ammeter is usually known and can be subtracted from the sum to obtain the required resistance4 ,differential ,galvanometer ,method4--,this is what is known as a nil or zero method1 that is1 a method of making electrical measurements in which comparison is made between two quantities by reducing one to equality with the other1 the absence of deflection from zero of the instrument scale showing that the equality has been obtained4 ,the test is made with a differential galvanometer1 and resistance box connected as in fig4 #eeg4 ,the current then will divide so that part of it flows through the resistance being tested and around one set of coils of the galvanometer while the other part will flow through the resistance box and the other set of coils as indicated4 ,when the resistance box has been so adjusted that its resistance is the same as the unknown resistance the current in the two branches will be equal1 and the needle of the galvanometer will show no deflection4 ,illustration3 ,fig4 #eeg4--;,differential galvanometer method of testing resistances4 ,in making the test1 the resistance box is adjusted till the galvanometer needle shows no deflection4 ,when this condition obtains1 the resistance in circuit in the resistance box is equal to the unknown resistance1 hence1 a reading of the box gives the value of the unknown resistance4 ,ques4 ,what name is given to this method of testing8 ,ans4 ,it is called a zero method1 distinguishing it from deflection methods4 ,ques4 ,for what kind of resistance is the method adapted8 ,ans4 ,since it is a nil or zero method1 it is better adapted to the measurement of non-inductive than of inductive resistances4 ,ques4 ,what precaution should be taken with inductive resistances8 ,ans4 ,the current must be allowed to flow until it becomes steady to overcome the influence of self-induction4 ,ques4 ,what may be said with respect to the differential galvanometer method8 ,ans4 ,with an accurate instrument it is very reliable4 ,illustration3 ,fig4 #eeh4--;,drop method of testing resistances4 ,the apparatus is connected as shown and readings taken with voltmeter across known and unknown resistance4 ,the unknown resistance is then easily calculated4 ,drop ,method4--,this is a convenient method1 and one which may be used for measuring either high or low resistances with precision4 ,it is used for many practical measurements1 and requires only a voltmeter1 battery1 known resistance and a two way switch4 ,the instruments are connected as in fig4 #eeh1 and in making the test1 the voltmeter is switched into circuit across the known resistance and then across the unknown resistance1 readings being taken in each case4 ,the value of the unknown resistance1 is then easily calculated from the following proportion3 drop across known resistance known resistance ------------------------------ ------------------ drop across unknown resistance unknown resistance from which unknown known resistance drop across unknown resistance resistance ---------------------------------------- ----------------- drop across known resistance ,illustration3 ,fig4 #eei4--,leeds and ,northrup portable galvanometer 7pointer type ,a74 ,the sensitiveness of this instrument is such that it may be substituted in numerous cases for the non-portable reflecting type of galvanometer2 as for instance1 in the checking of ammeters and voltmeters to an accuracy of 4#b by the potentiometer method1 and on almost all ,wheatstone bridge measurements to commercial accuracies4 ,a current of #b micro-amperes will cause the pointer to move #a mm4 over the scale1 that is1 it has a sensibility of #ejj1jjj ohms4 ,the method of suspending the moving system is such as to practically eliminate initial friction which is of value in all zero deflecting methods4 ,the suspensions and moving system are guarded by springs1 which together with the solid construction of the case render the instrument capable of withstanding rough usage4 ,overall dimensions are #e-a_/d #b-e_/h #c-a_/b2 weight about #c pounds4 ,ques4 ,what may be substituted for the voltmeter8 ,ans4 ,a high resistance galvanometer1 whose deflections are proportional to the current1 the value of the deflections being substituted in the formula4 ,ques4 ,what precaution should be taken in making the test8 ,ans4 ,the current used should not be strong enough to appreciably heat the resistance1 and if the current be not very steady1 several readings should be taken of each measurement and the average values used in the formula4 ,ques4 ,how are the most accurate results obtained8 ,ans4 ,by selecting the known resistance as near as possible to the supposed value of the unknown resistance4 ,illustration3 ,fig4 #efj4--,voltmeter method of testing resistances4 ,knowing the resistance of the voltmeter1 turn switch to the left and from reading calculate resistance corresponding to one division of the scale4 ,turn switch to right and multiply reading by resistance required for deflection of one division4 ,this gives resistance of voltmeter and unknown resistance2 subtracting from this the resistance of voltmeter gives value of the unknown resistance4 ,voltmeter ,method4--,this is a direct deflection method and consists in determining first the resistance that will deflect the needle through one division of the scale on a given battery current1 then with this as a basis for comparison the voltmeter is connected across the unknown resistance whose value is easily calculated from the reading4 ,in making the test1 the instruments are connected as in fig4 #efj4 ,the current from battery is first passed through the galvanometer by turning switch as shown4 ,illustration3 ,fig4 #efa4--,megohm box or set of standard high resistances4 ,the box contains five resistances of #bjj1jjj ohms each4 ,the six pillars are petticoat insulated1 the resistances being placed between each pair of pillars4 ,there is a double contact post on top of each pillar so that these can be connected together with copper links4 ,assuming that the resistance of the instrument is #h1jjj ohms and that the current deflects the needle through #aj divisions of the scale1 then for a deflection of one division the resistance is #h1jjj #aj #hj1jjj ohms4 ,accordingly1 if1 when the switch is moved to the right1 connecting the voltmeter across the unknown resistance1 the needle be moved through #f divisions of the scale1 the combined resistance of the voltmeter and unknown resistance is #hj1jjj #f #ac1ccc-a_/c ohms1 and subtracting the resistance of the voltmeter1 the value of the unknown resistance is #ac1ccc-a_/c - #h1jjj #e1ccc-a_/c ohms4 ,ques4 ,for what kinds of test is the voltmeter method best adapted8 ,ans4 ,for measuring high resistances1 as the insulation of wires1 etc4 ,ques4 ,what may be said with respect to the current used8 ,ans4 ,its voltage should be as high as possible within the limits of the voltmeter scale4 ,illustration3 ,fig4 #efc4--,standard resistance box3 #ajj1jjj ohms1 in four units of #aj1jjj1 #bj1jjj1 #cj1jjj1 and #dj1jjj ohms4 ,an infinity plug separates each coil from the ones adjacent4 ,segments are elevated from the hard rubber top by special washers in order to increase insulation4 ,binding posts are so arranged as not to be in the way when plugs are used4 ,illustration3 ,fig4 #efb4--,standard high resistance box3 #ajj1jjj ohms4 ,it is mounted in a brass box with a hard rubber top4 ,connections should be made to terminals marked #c and #d4 ,when the flexible cord is on plug #a1 the box is short circuited1 but when on plug #b1 the resistance of #ajj1jjj ohms is in series4 ,the box is especially suited to rapid cable testing4 ,ques4 ,in testing cable insulation what is desirable with respect to voltmeter and current8 ,ans4 ,a low reading voltmeter should be used in connection with a large battery4 ,illustration3 ,fig4 #efd4--;,diagram showing principle of ,wheatstone's bridge4 ,a1 ,b1 ,c1 and ,d1 are the four members which constitute the bridge4 ,the current from the battery divides at ,p1 part traversing ,,dc1 and part traversing ,,ba4 ,the galvanometer connected to ,m and ,n will indicate when the currents are equal in the two branches by giving no deflection4 ,this is then a zero or nil method of testing4 ,the resistances and keys required in testing are shown in fig4 #efe4 ,in the actual instrument1 the members ,a1 ,b1 ,c1 and ,d are known by the names given in the figure4 ,wheatstone ,bridge ,method4--,for accurate measurements of resistance this method is almost universally used4 ,the so-called ,wheatstone bridge was invented by ,christie1 and improperly credited to ,wheatstone1 who simply applied ,christie's invention to the measurement of resistances4 ,illustration3 ,fig4 #efe4--;,diagram showing arms of ,wheatstone bridge1 resistances and method of connecting galvanometer1 battery and unknown resistance4 ,the bridge consists of a system of conductors as shown in fig4 #efd4 ,the circuit of a constant battery is made to branch at ,p into two parts1 which re-unite at ,q1 so that part of the current flows through the point ,m1 the other part through the point ,n4 ,the four conductors ,a1 ,b1 ,c1 ,d1 are spoken of as the arms of the balance or bridge4 ,it is by the proportion existing between the resistances of these arms that the resistance of one of them can be calculated when the resistances of the other three are known4 ,when the current which starts from the battery arrives at ,p1 the pressure will have fallen to a certain value4 ,the pressure in the upper branch falls again to ,m1 and continues to fall to ,q4 ,the pressure of the lower branch falls to ,n1 and again falls till it reaches the value at ,q4 ,now if ,n be the same proportionate distance along the resistances between ,p and ,q1 as ,m is along the resistances of the upper line between ,p and ,q1 the pressure will have fallen at ,n to the same value as it has fallen to at ,m2 or1 in other words1 if the ratio of the resistance ,c to the resistance ,d be equal to the ratio between the resistance ,a and the resistance ,b1 then ,m and ,n will be at equal pressures4 ,to find out if this condition obtain1 a sensitive galvanometer is placed in a branch wire between ,m and ,n which will show no deflection when ,m and ,n are at equal pressure or when the four resistances of the arms balance one another by being in proportion1 thus3 7#a7 ,a3,c ,b3,d ,if1 then1 the value of ,a1 ,b1 and ,c be known1 ,d can be calculated4 ,the proportion 7#a7 is reduced to the following equation before substituting4 ,d ,,bc_/,a ,for instance1 if ,a and ,c be1 as in fig4 #efe1 #aj ohms and #ajj ohms respectively1 and ,b be #ae ohms1 ,d will be 7#ae #ajj7 #aj #aej ohms4 ,illustration3 ,fig4 #eff4--;,diagram showing usual arrangement of resistances in arms of ,wheatstone's bridge4 ,in practice the bridge is seldom or never made in the lozenge shape of the diagrams1 figs4 #efd and #efe1 these being given merely for clearness4 ,the resistance box of fig4 #eed is1 in itself1 a complete bridge1 the appropriate connections being made by screws at various points4 ,the letters in the above diagram correspond with those in figs4 #efd and #efe1 and the three figures should be carefully compared4 ,as constructed1 ,wheatstone bridges are provided with some resistance coils in the arms ,a and ,c1 as well as with a complete set in the arm ,b4 ,the advantage of this arrangement is that by adjusting ,a and ,c1 the proportionality between ,b and ,d can be determined1 and can1 in certain cases1 be measured to fractions of an ohm4 ,in fig4 #efe resistances of #aj1 #ajj1 and #a1jjj ohms are included in the arms ,a and ,c4 ,illustration3 ,fig4 #efg4--,standard resistance box and ,wheatstone bridge4 ,this pattern is a modification of the ,anthony form of bridge4 ,all the resistances are wound upon metal spools4 ,the bridge ratio coils are #a1 #aj1 #ajj1 #a1jjj1 #aj1jjj4 ,the rheostat coils are arranged in five rows1 of ten coils each4 ,the ordinary decade plan 7explained in fig4 #egj7 is followed4 ,the coils may be joined in series in multiple1 or in any combination of series and multiple4 ,the coils may thus be checked against each other in many combinations4 ,for instance1 all the ten ohm coils taken in parallel may be compared with any one ohm coil4 ,the precision of adjustment is said to be #a_/bjth of #a for the coils of the tenth ohm series1 and #a_/ejth of #a for the coils of the rheostat4 ,the ratio coils are certified to be like each other to within #a_/ajjth of #a4 ,the box is supplied with battery and galvanometer keys of substantial construction4 ,ques4 ,describe the method of testing with the bridge4 ,ans4 ,fig4 #efg illustrates the general arrangement of resistances to be found in an ordinary bridge4 ,the connections are made as shown4 ,in testing1 first depress the battery key1 then tap the galvanometer key4 ,this should be repeated adjusting the resistances till no deflection is obtained4 ,the resistance then in the arm ,b 7,c ,a7 will give the value of the unknown resistance4 ,illustration3 ,fig4 #efh4--,ratio coils of ,wheatstone bridge4 ,almost every box intended to serve as a ,wheatstone bridge is furnished with a set of coils which forms the arms of proportion or ratio arms of the bridge4 ,there is a choice of several different ways of arranging these coils4 ,the figure shows the simplest arrangement1 which is employed in boxes not intended for the highest accuracy4 ,the required ratio1 as for instance #a3ajj1 is obtained by withdrawing a plug from each arm ,a and ,b4 ,ratios #a_/a1 #a_/aj1 #a_/ajj1 #aj_/ajj1 etc41 or #a1jjj_/a1 #a1jjj_/aj1 #a1jjj_/ajj1 #ajj_/ajj1 etc41 are obtainable in this manner4 ,this simple arrangement is open not only to the objection that the contact resistance of the plugs which remain in is always included with the resistance unplugged1 but also to all other objections to be urged against the use of many plugs where a few will do4 ,the method has the limitation that it is not possible to reverse the arms of the bridge1 that is1 to transpose the arms ,a and ,b4 ,ques4 ,why should the battery key be depressed before the galvanometer key8 ,ans4 ,to avoid the sudden swing of the galvanometer needle1 which occurs on closing circuit in consequence of self-induction4 ,ques4 ,how is it known whether too much or too little resistance be unplugged8 ,ans4 ,the galvanometer needle will be deflected to one side for too much resistance1 and to the opposite side for too little resistance4 ,illustration3 ,fig4 #efi4--,method of reversing arms of ,wheatstone bridge with reversing blocks4 ,the arrangement shown in the figure is classical1 being that used in the ,english post office type of ,wheatstone bridge4 ,it is open to the objections which apply to the use of several plugs1 one of which is withdrawn to obtain the desired resistance4 ,ques4 ,what is the meaning of ,inf41 marked on the bridge8 ,ans4 ,it stands for infinity1 because the resistance coil at the point marked infinity is omitted so that adjacent sections of the arm are disconnected when the plug is taken out4 ,in fact1 the air gap interposed by the removal of the plug by no means provides an infinitely great resistance1 but is usually called such because it is vastly greater than any of the other resistances of the bridge4 ,illustration3 ,fig4 #egj4 ,illustration3 ,figs4 #egj and #ega4--;,diagrams illustrating the decade plan of combining resistance coils4 ,in this method the coils are connected in series and the arrangement avoids the disadvantage of the ordinary ,wheatstone bridge in that the latter requires a large number of plugs to short circuit the resistances not in use1 which introduces an element of uncertainty as to resistance of the plug contacts and the necessity of adding up the values of all the unplugged resistances in order to determine the total resistance in circuit4 ,the necessary regular succession of values in a rheostat built on the decade plan can be obtained with either nine or ten coils per decade4 ,the chief reason for using the latter number is found in the facility with which all the coils of one decade can be compared with one coil of the next higher decade1 thus permitting the coils of a rheostat to be checked among themselves4 ,thus1 the ten #a ohm coils can be checked with a #aj ohm1 the ten #aj's with a #ajj1 etc4 ,in some sets the ten coils of a decade can be connected in series or in parallel1 and it then becomes an advantage to have ten coils to a decade1 since the coils in one decade in parallel equal one of the coils of the next lower decade4 ,when these latter advantages are not required1 and especially when dials or sliding switches are used1 there is little or no advantage in using more than nine coils per decade1 as shown in fig4 #egj4 ,here all the coils of the set are connected in series so that the circuit is never open4 ,thus it is a slight advantage to have permanent connections a1 b1 and c1 because all the coils of a decade can be thrown in circuit by simply pulling out a plug1 it not being necessary to insert it again1 as would be the case if the a1 b1 and c connections were not used4 ,moreover1 if any plug make bad contact1 its effect is somewhat lessened by having this bad contact shunted by the remaining coils of the decade4 ,again1 there are occasions where violent deflections of a galvanometer are prevented by not having the circuit entirely open when a plug is taken out4 ,the ,decade ,plan4--,in this method of combining resistance coils1 there are #i or #aj one ohm coils for the units place1 #i or #aj ten ohm coils for the tens place1 #i or #aj one hundred ohm coils for the hundreds place and so on4 ,each series of coils of the same value is designated a decade4 ,the connections are usually made as shown in figs4 #egj and #ega4 ,it is apparent from the figure that any value in any one decade can be obtained by inserting between a bar and a block1 only one plug2 moreover if several decades be in series1 any value up to the limit of the set can be read off directly from the position of the plugs without having to add up the unplugged resistance as in the ordinary arrangement4 ,illustration3 ,fig4 #egb4--,two plug arrangement of ratio coils4 ,each of the ratio coils has one of its terminals connected to a common center which corresponds to the block marked ,c in the figure4 ,the other terminal of each coil is connected to an individual block1 there being one block for each coil4 ,the bar ,b on one side of these blocks is joined to the rheostat and the bar ,a on the other side to an ,x post4 ,in the ordinary use of this set of ratio coils two plugs only are used4 ,one plug is inserted between the bar ,a and one of the blocks1 #a1 #a'1 #aj1 #aj'1 etc41 of the central row of blocks4 ,the other plug is inserted between the bar ,b and any one of the other blocks of the central row4 ,there are two ratio coils of each value4 ,to obtain an even ratio as #a1jjj to #a1jjj'1 one plug is inserted between the block #a1jjj and the bar ,a1 and the other plug between the #a1jjj block and bar ,b1 the ratio arms are reversed2 that is1 the #a1jjj ohm coil is connected to the ,x post and the #a1jjj ohm coil to the end of the rheostat4 ,when uneven ratios are used1 the same ratio can be obtained by four different combinations4 ,to obtain the ratio one to ten1 insert a plug between ,a and #a1 and another between ,b and #aj1 or between ,a and #a'1 and ,b and #aj1 and get #a3aj1 or between ,a and #a1 and ,b and #aj'1 and get #a3aj'1 or again1 between ,a and #a' and ,b and #aj'1 and get #a' to #aj'4 ,other ratios are obtained in a similar manner4 ,by using more than two plugs and connecting certain of the coils in parallel combinations1 a large number of other ratios may be obtained4 ,this arrangement offers a convenient method of measuring the sensibility of a bridge and galvanometer combination that is frequently applicable4 ,if for instance the one ohm coil is used on either side after a balance has been obtained the one ohm may be shunted with the #a1jjj ohm on the same side4 ,this will make a variation of #a_/aj of #a and the galvanometer deflection may be noted for this variation4 ,similarly1 the #a ohm may be shunted with the #ajj for a variation of #a1 or with the #aj1jjj for a variation of #a_/ajj of #a4 ,the ten ohm coil may be shunted with the #a1jjj for a variation of #a and with the #aj1jjj for a variation of #a_/aj of #a4 ,in the arrangement of ratio coils1 errors due to plug contacts are negligible because only two plug contacts enter the circuit1 and with an even ratio1 it is only the difference in the resistances of the two plug contacts that can affect the result4 ,in measuring any of the ratio coils while in the box it is only necessary to connect to the bar ,c and to either the bar ,a or ,b and plug in the coils to be measured4 ,illustration3 ,fig4 #egc4 ,illustration3 ,figs4 #egc and #egd4--,the ,leeds and ,northrup decade4 ,the object of this arrangement is to reduce the number of coils required4 ,in fig4 #egc1 the #a1 #c'1 #c and #b are connected in series4 ,let the terminals of the #a ohm and #b ohm coils be numbered 7#a71 7#b71 7#c71 7#d7 and 7#e7 7fig4 #egc74 ,the current enters at point 7#a7 and leaves the coils at the point 7#e71 traversing #a1 #c'1 #c1 #b #i ohms in all4 ,if this series be multiplied by any factor n1 then n 7#a #c' #c #b7 n #i ohms4 ,it will be seen that if the points 7#a7 and 7#e7 be connected1 all the coils are short circuited1 and the current will traverse zero resistance4 ,if the points 7#b7 and 7#e7 be connected1 the #c'1 #c and #b ohm coils will be short circuited and the current will traverse #a ohm4 ,by extending the process so as to connect two and only two points at a time it is possible to obtain the regular succession of values n 7#j1 #a1 #b1 #c1 #d1 #e1 #f1 #g1 #h1 #i71 the last being obtained when no points are connected4 ,fig4 #egd shows ,leeds and ,northrup's method of connecting these points two at a time with the use of a single plug4 ,the circles in the diagram represent two rows of ten brass blocks each4 ,to the first two blocks at the top of the rows1 the points 7#e7 and 7#a71 fig4 #egc1 are connected2 to the second two1 the points 7#b7 and 7#e7 are connected1 etc41 no points being connected to the last pair of blocks4 ,hence1 if a plug be inserted between blocks #a and #e1 fig4 #ege1 the points 7#a7 and 7#e7 of diagram fig4 #egc are connected1 giving the value of #j1 if between the blocks #b and #e the points 7#b7 and 7#e7 are connected1 giving the value #a1 and so on4 ,the value #i is obtainable when the plug is in the last pair of blocks1 which have no connections4 ,fig4 #egb shows a top view of the blocks of a simple decade constructed upon this plan4 ,ques4 ,what other advantages are gained with the decade arrangement8 ,ans4 ,the single plug used with each decade is never out of use1 being either in the zero position or set on some value1 and is therefore not easily lost by being laid aside4 ,the use of only one plug in a decade makes it easy to ascertain that the plug is making good contact as only one block in a row is plugged at a time1 the other blocks are not kept under a strain by having plugs forced tightly between them4 ,this strain on the blocks1 which always exists in those sets in which a resistance is thrown in by removing a plug1 tends to separate or loosen them and often to warp the hard rubber upon which they are mounted4 ,another advantage of the decade plan is that it permits obtaining a succession of values by means of sliding contacts or dial switches1 a method which is becoming deservedly more appreciated4 ,illustration3 ,fig4 #ege4--,leeds and ,northrup dial ,wheatstone bridge4 ,rotating switches are used instead of plugs1 which permits quicker adjustment of the resistances1 adapting it to rapid working4 ,the ratio coils are arranged as in fig4 #efh4 ,there are four dials which form the rheostat4 ,the units dial contains #i one ohm coils2 the tens dial1 #i ten ohm coils2 the hundreds dial1 #i one hundred ohm coils1 and the thousands dial #i one thousand ohm coils4 ,the values of the ratio coils are #a1 #a1 #aj1 #aj1 #ajj1 #ajj1 #a1jjj1 #a1jjj1 #aj1jjj1 #aj1jjj4 ,ques4 ,what is the difference between plug out and plug in types of resistance box8 ,ans4 ,in the plug out type1 resistance is put in the circuit by removing plugs1 as in fig4 #efe2 in the plug in type1 resistance is put in the circuit by inserting plugs as in figs4 #egj and #ega4 ,illustration3 ,fig4 #egf4--,queen ,acme portable testing set4 ,it consists of a ,wheatstone bridge1 with reversible arms1 battery of four dry cells1 ,d',arsonval galvanometer1 battery and galvanometer keys4 ,there are sixteen resistance coils1 having a combined resistance of #aa1aaj ohms4 ,each bridge arm is provided with three coils of #a1 #aj1 #ajj ohms1 and #aj1 #ajj1 #a1jjj ohms respectively4 ,the commutator admits of a ratio of #a to #a1jjj on either bridge arm1 giving the set a theoretical range from 4#jja of an ohm to #aa1aaj1jjj ohms4 ,for resistances above #a1jjj1jjj ohms1 the normal battery power must be increased4 ,the contact keys are located as shown4 ,the battery key has single contact1 but the galvanometer key has double contact2 depressing it closes the galvanometer circuit1 and releasing it short circuits the galvanometer1 bringing the latter quickly to rest4 ,testing ,sets4--,for convenience in testing1 a combination of the instruments used is put up in a neat and substantial case1 and known as a testing set4 ,there are innumerable forms of testing set1 a few of which are shown in the accompanying illustrations4 ,the usual combination is a ,wheatstone bridge1 galvanometer1 battery and necessary keys and connections4 ,illustration3 ,fig4 #egg4--;,connections and circuits of ,queen acme portable testing set4 ,there are three rows of blocks1 ,,ll'1 ,,mm'1 ,,nn'4 ,,ll is connected to ,,nn' by means of a heavy copper bar1 joining ,l' and ,n'4 ,,ll' and ,,nn' constitute the rheostat1 from which any resistance from #a ohm to #aa1aaj ohms may be obtained by removing the proper plugs4 ,the block ,n of the rheostat is connected to the lower line post ,d4 ,the upper line post ,c is connected to the block ,x of the commutator4 ,the block ,c has no other permanent connection1 except key ,g4 ,the block ,r of the commutator is connected to the block ,l of the rheostat1 and has no other connection1 excepting by plugs4 ,each half of ,,mm' constitutes a bridge arm1 designated ,a and ,b respectively4 ,beginning at the lower line post ,d1 the connections form a continuous circuit through the rheostat1 thence through the bridge arm ,b1 thence through the bridge arm ,a1 thence to the upper line post ,c4 ,the commutator serves merely to reverse the bridge arms ,a and ,b4 ,the battery terminals are connected as shown3 the positive terminal directly to the common junction of the two bridge arms1 and the negative terminal through the battery key to the rheostat4 ,the positive terminal of the galvanometer is connected through the galvanometer key with the block ,x1 and the negative terminal with the block ,r of the commutator or what is equivalent1 with the block ,l of the rheostat4 ,the commutator blocks ,a1 ,b1 ,r and ,x1 are connected by plugs as shown4 ,when the commutator plugs are in the position ,,pq1 the bridge arm ,b is connected to the rheostat and the bridge arm ,a is connected to the line1 the ratio between the bridge arms ratio being ,a ,b ,x ,r but when the plugs are in the position ,,st1 the bridge arms are reversed in position ,a1 being connected with the rheostat and ,b1 with the line1 and the bridge arm ratio becomes ,a ,b ,r ,x4 ,the connections of the testing set may be more readily understood from the simplified diagram fig4 #egh4 ,illustration3 ,fig4 #egh4--,simplified diagram showing connections of ,queen ,acme portable testing set4 ,ques4 ,describe the operation of the ,queen ,acme testing set figs4 #egf and #egg1 in measuring resistance4 ,ans4 ,connect the terminals of the resistance to be measured to the line posts ,c and ,d4 ,place the battery connections on the two upper tips #j and #a1 thus throwing one end of the battery into circuit1 which is sufficient until an approximate balance is obtained4 ,employ the #ajj ohm coil in each bridge arm1 and place the commutator plugs in the position ,,pq1 or in the position ,,st4 ,then remove plugs from the rheostat until the value of total resistance employed1 or nearly as may be guessed is equal to that of the unknown resistance4 ,now press the battery key ,ba1 and holding it down momentarily1 press the galvanometer key ,ga4 ,if the galvanometer needle swing to the right toward the symbol the resistance employed in the rheostat is too high and must be reduced4 ,if the needle swing to the left toward -1 the resistance employed is too low and must be increased4 ,by altering the resistance of the rheostat accordingly1 a value will soon be found1 which when varied slightly either way1 will reverse the deflection of the galvanometer needle4 ,now remove the battery connection from tip #a1 and place it on the tip #d1 thus throwing the whole battery into circuit4 ,then press the keys again as before1 first the battery key1 then the galvanometer key4 ,this will increase the deflection of the galvanometer needle for the same variation in the rheostat1 thus enabling the making of a more accurate adjustment4 ,the measurement thus made will be the best result that can be obtained with bridge arms of equal value1 but by selecting more suitable values of the two arms from the following table of bridge ratios a much higher degree of accuracy may be obtained4 ,illustration3 ,fig4 #egi4--;,diagram of the ,queen dial decade portable testing set4 ,its dimensions are #i-a_/b long1 #g wide and #g deep1 and weighs #aa-a_/b pounds4 ,the resistances are arranged upon the dial decade plan1 being placed in circuit by means of a rotating switch contact4 ,the switches are so constructed that they may be turned in either direction1 thereby permitting them to be turned quickly from the highest resistance in any dial to the lowest resistance in the same dial4 ,this arrangement avoids the necessity of turning back through all the remaining resistances in any particular group of coils and is of value in locating swinging crosses or conditions of momentary balances4 ,the connections for the various tests are made by the manipulation of one small knife switch 7,w4,b4--,m4,l47 and the switch ,,ba42 these are plainly lettered1 thus avoiding the necessity of referring to a diagram of connections4 ,in construction1 the dial switches are made up of eight laminations of ,no4 #bh ,b4 ,s4 phosphor bronze1 and the form is such as to prevent wearing grooves on the top of the contact studs4 ,in this instrument the electrical circuits are soldered throughout excepting the switch contact whose resistance is negligible4 ,the resistances are wound with manganin4 ,the battery comprises six cells sub-divided which are easily replaceable4 ,the galvanometer is the same as in the ,queen acme set1 but has the addition of an ,ayrton shunt1 which is useful in making insulation measurements4 ,the necessary keys1 binding posts1 and switches are provided so as to facilitate the use of the instrument for the various measurements that can be made with it4 ,table ,showing the ,best ,values of ,bridge ,arms for ,measuring any ,desired ,resistance ,position of ,best ,values of ,commutator ,plugs as ,value of ,resistance being measured shown in ,a ,b fig4 #ehb ---------------------------------------- ------------------------ ,below #a4e ohms #a #a1jjj ,,pq ,between #a4e and #aa ohms #a #ajj ,,pq #aa and #gh ohms #aj #ajj ,,pq #gh and #a1ajj ohms #ajj #a1jjj ,,pq #a1ajj and #f1ajj ohms #ajj #ajj ,,pq or ,,st #f1ajj and #aaj1jjj ohms #a1jjj #ajj ,,st #aaj1jjj and #a1aaj1jjj ohms #a1jjj #aj ,,st #a1aaj1jjj and #aa1aaj1jjj ohms #a1jjj #a ,,st ,ques4 ,in testing with the ,queen ,acme set how should the plugs be placed in the commutator8 ,ans4 ,always make the arm ,a the smaller except when the two arms are of equal value4 ,ques4 ,if the resistance being measured is higher than #f1ajj ohms1 or lower than #a1ajj ohms1 how should the commutator plugs be placed8 ,ans4 ,if higher than #f1ajj ohms1 they should be placed in the position ,,st2 if lower than #a1ajj ohms1 in position ,,pq4 ,when the plugs are placed in the ,,st position1 the unknown resistance is found by dividing the value of the larger bridge arm by that of the smaller1 and multiplying the total employed resistance in the rheostat by the quotient4 ,when the plugs are placed in the ,,pq position1 the employed resistance in the rheostat is divided by the quotient4 ,illustration3 ,fig4 #ehj4--,queen portable silver chloride testing battery4 ,the silver chloride cell has the advantage of long life1 light weight1 and compactness4 ,the pressure of each cell when new is 4#h volt4 ,direct ,deflection ,method with ,queen ,acme ,set4--,to measure for instance1 insulation resistance by direct deflection connect a known high resistance1 say #ajj1jjj ohms between the line post ,c 7fig4 #egg71 and the positive battery post4 ,remove all plugs from the commutator1 and place all plugs in the rheostat1 as any employed resistance in the rheostat will be in circuit with the galvanometer and the battery4 ,place the battery connection so as to throw only one cell into circuit4 ,now press the keys and obtain a deflection of the galvanometer needle4 ,for example3 assume that the needle to be deflected about #h divisions of the scale4 ,since this deflection is due to the current from one cell passing through a resistance of #ajj1jjj ohms1 then #ajj1jjj #h 4#h megohms represents the resistance through which one cell will produce a deflection of one division on the scale4 ,hence1 4#h megohms is the constant of the galvanometer4 ,illustration3 ,fig4 #eha4--,ohmmeter4 ,it consists essentially of a slide wire ,wheatstone bridge1 with the scale divided to read either directly in ohms1 or in per cent4 of a fixed resistance value4 ,a galvanometer is mounted on the containing case of each1 and battery and galvanometer keys are provided4 ,in the direct reading type1 the scale is so cut that when the galvanometer is balanced1 the pointer of the instrument indicates the value of the resistance between the ,x posts4 ,the scale is calibrated for any desired range4 ,these ohmmeters being slide wire bridges1 the greatest accuracy is at the center of the scale1 hence one should be selected that will bring the part of the scale likely to be the most used at or near the center4 ,a convenient type is that in which the scale is cut in per cent41 #ajj per cent4 being at the center of the scale4 ,fixed coils of #a1 #aj1 #ajj1 #a1jjj and #aj1jjj ohms are contained in the instrument with a plugging arrangement allowing any one to be used4 ,when a balance is obtained1 the actual resistance is determined by multiplying the dial reading by the value of the fixed coil in use4 ,this amounts simply to shifting the decimal point4 ,for instance1 if the #ajj ohm coil were being used1 and the pointer were at 4#hge1 the resistance would be #hg4e ohms4 ,now1 replace the known high resistance 7#ajj1jjj ohms7 by the unknown resistance 7for instance such as a cable7 the value of which is to be determined4 ,add enough cells to produce as large a deflection of the needle as possible4 ,assume that #ge cells give a deflection of #a4e scale division4 ,then1 the galvanometer constant multiplied by the number of cells and the product divided by the deflection will give the insulation resistance of the cable2 or #j4h megohm #ge cells #fj4j2 and #fj4j #a4e #dj megohms as the resistance of the cable4 ,illustration3 ,fig4 #ehb4--;,commutator plug setting for comparing electromotive forces by the fall of potential method with ,queen acme set4 ,fall of ,potential ,method with ,queen ,acme ,set4--,to compare electromotive forces by this method1 place the battery connection 7fig4 #egg71 so as to throw into circuit all the cells1 taking care not to reverse them by crossing the battery cords4 ,plug the commutator as shown in fig4 #ehb1 and remove #a1jjj ohms from bridge arm ,b4 ,place all plugs in arm ,a4 ,from the rheostat unplug #e1jjj ohms4 ,then connect one of the cells being tested1 with its positive terminal to the battery post and its negative terminal to the line post ,c4 ,when the keys are pressed1 the galvanometer needle will swing either to the right or to the left4 ,if it swing toward 1 reduce the resistance in the rheostat2 if it swing toward -1 add resistance to the rheostat4 ,when a value is found wherein a variation of an ohm either way reverses the deflection1 add to this value the resistance unplugged in arm ,b1 and divide the sum by the resistance in arm ,b4 ,the result gives the ratio between the voltages of the testing set battery and cell being tested respectively4 ,the division is decimal and may be readily accomplished by merely pointing off as many places as there are ciphers in the resistance employed from arm ,b4 ,this operation repeated with any number of different cells1 will give their voltages in terms of the voltage of the testing set battery1 and from these ratios their relative values may be readily obtained4 ,illustration3 ,fig4 #ehc4--;,diagram of apparatus for measuring low resistances based on the principle of the ,kelvin double bridge4 ,in the diagram ,,ab represents a heavy piece of resistance metal of uniform cross section and uniform resistance per unit of length2 ,,cd is another piece of resistance metal of smaller cross section1 and the two are joined together by a heavy copper bar1 ,,ac1 into which both are silver soldered2 ,,ll are the current terminals and ,,pp are the pressure terminals4 ,the resistance of ,,ab between the marks #j and #ajj on the scale ,s is 4#jja ohm4 ,from the point #a on the resistance ,,cd to #j on ,,ab is also 4#jja ohm1 from #b to #j is 4#jjb and so on1 and from #i to #ajj is 4#ja ohm4 ,the slider ,m moves along the resistance ,,ab and its position is read on the scale ,s which is divided into #ajj equal parts and can be read by a vernier to thousandths4 ,subdivided in this way the resistance between the tap off points ,,pp may have any value from 4#jja to 4#ja ohms by steps of 4#jjjjja ohm4 ,if the testing set battery be replaced by a standard cell1 the first measurement gives at once the voltage of the cell tested4 ,if the voltage of the cell or battery being tested exceed that of the testing set battery1 reverse the position of the two batteries1 and the subsequent operations1 as outlined above1 will give the desired results4 ,how to check a ,voltmeter with the ,queen ,acme ,set4--,in using a set as in fig4 #egf1 first remove about #aj1jjj ohms from the rheostat1 plug the commutator as shown in fig4 #ehb1 remove #ajj ohms from the arm ,b1 of the bridge1 and connect a standard cell with the positive terminal to the battery post and the negative terminal to the line post ,c4 ,then1 connect the circuit to the battery posts of the testing set the positive lead to the post and the negative lead to the - post4 ,now1 press both keys and note the direction of the deflection of the galvanometer needle4 ,if it move toward 1 the rheostat resistance is too high2 if toward -1 too low4 ,illustration3 ,fig4 #ehd4--,kelvin bridge4 ,this includes a low resistance standard of 4#a ohm variable by steps of 4#jjjja ohm1 a set of ratio coils1 and a holder for rods or wires to be measured1 with a scale to measure their length4 ,it is also provided with heavy flexibles to be used in measuring the resistances of irregularly shaped pieces4 ,the connections are clearly shown in the diagram4 ,the range of measurements of this bridge is3 #a ohm to 4#a ohm by steps of 4#jja ohm readily estimated to 4#jjja2 4#a to 4#ja ohm by steps of 4#jjja ohm readily estimated to 4#jjjja2 4#ja ohm to 4#jja ohm by steps of 4#jjjja ohm1 readily estimated to 4#jjjjja2 4#jja ohm down by steps of 4#jjjja ohm1 readily estimated to 4#jjjjja ohm4 ,change the rheostat resistance accordingly until the balance attained is such that a very slight variation of the rheostat resistance one way or the other will reverse the galvanometer deflection4 ,to find the pressure on the circuit1 add #ajj to rheostat resistance and point off two places4 ,multiply this value by the voltage and the product will be the desired voltage4 ,if the voltage of the standard cell be exactly one volt1 the total employed resistance represents the voltage on the circuit4 ,illustration3 ,fig4 #ehe4--,queen slide wire bridge4 ,it consists of a portable slide wire1 ,wheatstone bridge arranged to read directly in ohms in addition to its use for locating crosses and grounds4 ,it is complete with battery1 galvanometer and telephone receiver4 ,the bridge is balanced by moving the hand stylus until the galvanometer shows no deflection or until there is no sound in the telephone receiver4 ,in order to provide a wide range of measurement and maximum accuracy1 ratio coils or multipliers having values of #a1 #aj1 #ajj1 #a1jjj and #aj1jjj are provided4 ,the scale of the instrument is arranged in two parts1 one of which indicates ohms and the other is divided into uniform divisions for use when locating crosses and grounds by the ,murray and ,varley loop methods4 ,a small induction coil is included so as to furnish an alternating current when using the telephone receiver4 ,for instance1 in making a measurement on a #aaj volt circuit1 assume that the employing of #g1hdj ohms rheostat resistance produces balance1 and that increasing or decreasing this resistance by two ohms1 reverses the galvanometer deflection4 ,this indication that the setting #g1hdj is uncertain1 about #a_/dj of #a per cent4 ,since the rheostat coils are adjusted to an accuracy of only #a_/e of #a per cent41 that will be about the accuracy of the measurement4 ,if the pressure of the standard cell be #a4jah volts1 then #g1hdj #ajj #g1idj4 ,pointing off two places1 gives #gi4dj1 which multiplied by #a4jah gives #hj4hb for the voltage on the circuit4 ,to ,measure ,internal ,resistance of ,cell with ,queen ,acme ,set4--,first compare its voltage on open circuit with the pressure of the testing set battery4 ,then1 shunt the cell with a known resistance1 about #ajj ohms1 and again measure its terminal voltage4 ,the difference between the two values thus obtained1 divided by the value of the shunt resistance1 will give the value of the current4 ,to find the internal resistance1 multiply the value of the shunt resistance by the ratio between the first and second measured values4 ,illustration3 ,fig4 #ehf4--;,evershed portable ohmmeter set4 ,this testing set consists of a direct reading ohmmeter which indicates by direct reading the value of the resistance being tested1 also a portable hand dynamo which provides at any required pressure the current necessary to make the test4 ,it is adapted to the needs of supply stations1 wiring contractors and dynamo builders4 ,it is also useful in testing the insulation of underground and aerial cables1 and is designed so that it can be used by ordinary workmen who are not experienced in handling delicate instruments and who1 by its use1 are able to obtain accurate results4 ,the dynamo is wound for #ajj1 #bjj1 #ejj1 or #a1jjj volts1 and is fitted with spring drum inside the case on which is coiled a twin flexible cord provided with a connector adapted for clamping under the ohmmeter terminals4 ,for instance1 assume that the open circuit voltage of the cell being tested as compared with the voltage of the testing set battery is 4#bab of the latter1 and that when it is shunted with a resistance of #a1jjj ohms1 its terminal voltage is 4#agi4 ,then1 the total resistance is to the #a1jjj ohms shunt resistance as 4#bab is to 4#agi or 74#bab_/4agi7 #a1jjj #a1ahd1 from which deducting the #a1jjj ohms shunt resistance1 gives #ahd ohms as the internal resistance of the cell4 ,illustration3 ,fig4 #ehg4--,leeds and ,northrup fault finder4 ,a lineman's instrument for the location of faults1 crosses1 grounds1 and opens in telephone and telegraph circuits1 and for the measurement of conductor and insulation resistance4 ,illustration3 ,fig4 #ehh4--;,diagram showing arrangement and connections of ,leeds and ,northrup fault finder4 ,it is used to measure conductor resistance1 fault resistance1 to locate faults by four different tests1 and when used with a buzzer and telephone1 to locate opens4 ,the essential feature of the instrument is the uniform resistance ,,ab1 which lies in a circle and which has a value of about #ajj ohms4 ,by a special construction1 it is so arranged that the contact can be made at any point along it1 and it is therefore equivalent to a very high resistance slide wire4 ,it has a moving contact ,c and a uniform scale of #a1jjj divisions4 ,in series with this1 there are the two resistances ,e and ,r which may be short circuited by the switches ,u and ,v4 ,e has exactly the same resistance as the wire ,,ab4 ,r has a resistance of #ajj ohms1 and is the fixed resistance of the bridge arrangement for resistance measurements4 ,the resistances of #a1jjj ohms and #i1jjj ohms connected to the battery post are to protect the battery and the apparatus from excessive current4 ,the #i1jjj ohms may be short circuited by the switch ,w4 ,ammeter ,test with ,queen ,acme ,set4--,connect a low resistance in series with the ammeter and run leads from it to the testing set1 the positive lead to the battery post and the negative lead to the line post ,c 7fig4 #egg74 ,insert a standard cell between the battery posts1 with positive terminal to battery post1 and negative terminal to - battery post4 ,plug commutator as shown in fig4 #ehb4 ,remove #aj1jjj ohms from rheostat1 and #ajj ohms from bridge arm ,b4 ,determine a balance in the usual way by changing the value of the resistance in the rheostat4 ,this operation will balance the difference of pressure at the terminals of the shunt resistance against the standard cell1 and its value is equal to 7#a4dj #ajj7 _/ 7,r #ajj7 #adj _/ 7,r #ajj7 ,to determine the current flowing1 divide the value of the difference of pressure thus obtained by the value of the shunt resistance4 ,illustration3 ,fig4 #ehi4--,resistance measurement with ,leeds and ,northrup fault finder4 ,the diagram shows the proper connections and switch settings for measuring conductor resistance4 ,as in the ordinary slide wire bridge1 the resistance ,x between the two posts #a and #b is obtained from the formula ,x ,a 7#a1jjj - ,a7 ,r4 ,to avoid the necessity of solving in each case the fraction ,a 7#a1jjj - ,a71 a table is furnished with the instrument1 giving the value of this fraction for each value of ,a4 ,the resistance is accordingly determined in each case by simply setting the contact ,c for a balance and reading from the table the resistance opposite the number corresponding to the scale reading and multiplying by #ajj1 the value of ,r4 ,to use an outside battery1 remove the inside battery and connect the outside battery between the posts ,gr and ,ba4 ,the pressure of this battery should not exceed #aaj volts4 ,if it exceed #be volts1 open switch ,w4 ,,example--,with an unknown resistance connected between the posts #a and #b1 the galvanometer showed a balance for a dial reading of #chg4 ,the number opposite #chg in the table is 4#fcac2 hence1 ,x 4#fcac #ajj #fc4ac ohms4 ,illustration3 ,fig4 #eij4--;,diagram of the ,queen standard potentiometer4 ,the circuit arrangement is a method of sub-dividing the main potentiometer wire1 ,,mnopq1 so as to provide for very accurate reading4 ,the secondary voltage1 or that used to supply current to the main potentiometer circuit1 is adjusted by regulating rheostats1 ,fast1 ,medium1 and ,slow so that the current flow is exactly 4#jjja ampere4 ,it is noted that this instrument requires a very small current for its operation4 ,the instrument is direct reading for voltage measurements1 not exceeding #a4d4 ,in order to determine if the current flow through the potentiometer be exactly 4#jjja ampere1 the terminals of the standard cell binding posts are connected in circuit so that the drop over points between which they are connected are exactly equal to the voltage of the standard cell used4 ,binding posts are provided for connection with various standard cells4 ,the unknown voltage to be measured is placed in opposition to the current flow in the potentiometer circuit by connecting to the binding post ,,xemf4 ,observe that polarity is connected as required4 ,the galvanometer with its shunt is placed in the standard cell circuit1 or ,x circuit1 by means of a double pole1 double throw switch4 ,the switch at ,t provides for standard cells of different values and the setting at ,u allows for temperature correction4 ,the range of the instrument in volts can be increased by means of multipliers or volt boxes4 ,illustration3 ,fig4 #eia4 ,illustration3 ,fig4 #eib4 ,illustration3 ,fig4 #eic4 ,illustration3 ,figs4 #eia to #eid4--;,diagrams illustrating loop testing4 ,to properly understand the ,murray or ,varley loop tests1 consider a ,wheatstone bridge 7fig4 #eia7 the arms of which are equal4 ,in loop testing1 the rheostat is replaced by a length of cable and the unknown resistance also by a length of cable1 as in fig4 #eib1 both being similar in resistance per foot4 ,if both lengths be the same1 their resistances are the same and the bridge balances4 ,now shorten one cable and add resistance in series with it until the bridge again balances as in fig4 #eic4 ,the added resistance equals that of the piece cut off4 ,hence1 if the resistance per foot be known1 the length of the shorter piece can be easily calculated4 ,in the ,murray and ,varley tests1 the battery circuit is by ground connections instead of by wire4 ,in the ,murray loop the arrangement is similar to fig4 #eib1 the battery circuit being completed by ground connection through fault in defective cable4 ,fig4 #eid shows the general arrangement of the ,varley loop4 ,loop ,test4--,this is a method of locating a fault in a telegraph or telephone circuit when there is a good wire running parallel with the defective one4 ,in the process1 the good and bad wires are joined at their distant ends and one terminal of the battery is connected to a ,wheatstone bridge1 while the other terminal is grounded4 ,there are different ways of making loop tests as by3 #a4 ,the ,murray loop2 #b4 ,the ,varley loop2 #c4 ,special loop4 ,illustration3 ,fig4 #eie4--,the ,murray loop test4 ,the apparatus is connected as in the figure4 ,the rheostat of the bridge is used in place of the second arm to permit large adjustment4 ,x and ,y are the resistances of the cable between the fault and the points #a and #b respectively4 ,the ,murray ,loop4--,in this test only one of the two regular bridge arms is used1 the other being replaced by the rheostat giving an arm of large adjustment4 ,the connections are shown in fig4 #eie4 ,in making the test1 close key and note the deflection of the needle due to pressure of chemical action at fault1 if any4 ,this is called the false zero4 ,now apply the positive or negative pole of the battery by depressing the battery key1 and balance to the false zero previously obtained by varying the resistance in arms ,a or ,b4 ,then by ,wheatstone bridge formula3 ,,rx,,ay1 and ,l,x,y2 ,y,l-,x1 whence ,x ,a_/7,r,a7 ,y ,l7,r_/7,b,a77 ,illustration3 ,fig4 #eif4--,murray loop method of fault location with ,leeds and ,northrup fault finder4 ,case ,i where there are two wires having equal resistance1 in one of which there is a fault4 ,connect and set switches as shown2 join the good wire to post #a and the faulty wire to post #b4 ,the resistance of ,e is equal to that of ,,ab4 ,from the symmetry of the arrangement1 it is evident that1 if the fault were exactly at the junction between the good and bad wires1 the contact point ,c would rest for a balance at #a1jjj on the scale1 or at #ejj if the fault were half-way along the bad wire2 hence1 at whatever point it comes to rest1 the reading divided by #a1jjj and multiplied by the length of the bad wire is the distance from the instrument to the fault4 ,,example--,in a pair of equal wires1 #e4h miles long1 one is grounded4 ,with the connections made as above1 and the galvanometer balanced for the dial reading #abd1 the distance to the fault is 7#abd #eh7 #a1jjj 4#gaib miles4 ,illustration3 ,fig4 #eig4--,murray loop method of fault location with ,leeds and ,northrup fault finder3 ,case ,,ii1 where the good and bad wires are unequal4 ,the figure shows the connections4 ,it is the ordinary ,murray loop and it is evident that the resistance a1 to the fault will be obtained from the formula a 7,a #a1jjj7 r1 where r is the resistance of the loop1 and ,a is the reading of the contact ,c on its scale4 ,the distance d1 to the fault is obtained from the formula d ,ar 7#a1jjj ,m71 where ,m is the resistance per mile of the faulty wire4 ,,example--,a wire having a resistance ,m of #af4df ohms per mile is grounded4 ,this wire was looped with a wire of unknown resistance and the total resistance of the loop r was measured and found to be #ed4jg ohms4 ,connections were made as in the figure1 and the reading ,a was found to be #ccb4 ,substituting these values in the above formula3 d 7#ccb #ed4jg7 7#a1jjj #af4df7 #a4ji miles4 ,ques4 ,how may the distance from #b to the fault be determined in knots or miles4 ,ans4 ,divide ,y by resistance per knot or mile4 ,the ,varley ,loop4--,this is a method of locating a cross or ground in a telephone or telegraph line or other cable by using a ,wheatstone bridge in a loop formed of a good wire and the faulty wire joined at their distance ends4 ,one terminal of the battery is grounded and the other connected to a point on the bridge at the junction of the ratio arms4 ,the rheostat arm then includes the resistance of the rheostat plus the resistance of the fault1 while the unknown arm includes the resistance of the good wire plus the resistance of the bad wire beyond the fault4 ,when the bridge is balanced1 the unknown resistances may be readily determined by a simple equation4 ,illustration3 ,fig4 #eih4--,the ,varley loop test4 ,the diagram shows the various connections4 ,x and ,y are the resistances of the cable between the fault and the points #a and #b respectively4 ,l is the resistance of the good and bad cable or ,x ,y4 ,in making the ,varley loop test1 the resistance of looped cable or conductors is measured1 and then connected as in fig4 #eih4 ,close the battery key and adjust ,r for balance4 ,when earth current is present1 the best results are obtained when the fault is cleared by the negative pole1 and just before it begins to polarize4 ,if ,x be the resistance from #b to the fault1 then ,x 7,l - ,r7 _/ #b also1 ,x divided by the resistance of the cable or conductor per knot or mile gives the distance of fault in knot or miles4 ,when the resistance of the good wire used to form a loop with the defective wire1 together with that portion of the defective wire from the joint to the fault is less than the resistance of the defective wire from the testing station to the fault1 the resistance ,r must be inserted between point #a and the good conductor1 the defective wire being connected directly to point4 ,the formula in this case is ,x 7,l ,r7 _/ #b ,illustration3 ,figs4 #eii and #fjj4--,varley loop method of fault location with ,leeds and ,northrup fault finder4 ,this method may be used as a check on the ,murray methods4 ,connect the faulty wire to #a1 and measure the resistance of the loop4 ,then throw switches as shown in the fig4 #fjj4 ,let3 a resistance to fault1 d distance to the fault in miles1 ,m resistance of the faulty wire per mile1 r resistance of the loop1 ,r resistance of the coil ,r1 or #ajj ohms1 ,t ,a 7#a1jjj - ,a7 to be read from the table4 ,from the ,wheatstone bridge relation3 a 7r - #ajj,t7 7,t #a71 and d 7r - #ajj,t7 7,t #a7,m4 ,,example--,a wire having a resistance of #af4df ohms per mile is grounded4 ,this was looped with a wire of unknown resistance and the resistance of the loop was found to be #ed4jg ohms4 ,connections were made as in the figure1 and the reading ,a was found to be #bcd4 ,from the table ,t 4#cjee1 and substituting3 d 7#ed4jg - #cj4ee7 7#a4cjee #af4df7 #a4jid miles4 ,special ,loop4--,this method may be used to advantage where the length of the cable or faulty wire only is known and where there are two other wires which may be used to complete the loop4 ,it is not necessary that the resistance of the faulty wire and the length and resistance of the other wires be known4 ,figs4 #fja to #fjd show the connections and method of testing4 ,,example4--,all the wires in a cable #aj1heb ft4 long were found to be grounded so that none of them could be used as good wires4 ,two wires were selected out of another cable going to the same place by a different route and securely joined to one of the grounded wires at the distant end4 ,this grounded wire and one of the good ones were connected as shown in figs4 #fja and #fjb and the reading ,a was found to be #cjg4 ,connections were then made as shown in figs4 #fjc and #fjd and ,a was found to be #faj4 ,what is the value of d8 ,according to formula d ,,al_/,a 7#cjg #aj1hec7_/faj #e1dfa ft4 ,illustration3 ,figs4 #fja and #fjb4--,special loop test with ,leeds and ,northrup fault finder4 ,for the first measurement connect the faulty wire to #b1 either of the good wires1 as ,z1 to #a1 the post ,gr to ground1 and short circuit the coils ,r and ,e by closing switches ,u and ,v as in the figures4 ,balance in the usual way and call the dial reading ,a4 ,for the second measurement1 connect the post ,gr4 7disconnected from ground71 to the other good wire y as shown in figs4 #fjc and #fjd1 and get another balance2 call this reading ,a'4 ,the distance d1 to the fault is determined from the simple formula d ,,al ,a' where ,l is the length of the cable or faulty wire4 ,illustration3 ,figs4 #fjc and #fjd4--,special loop test as made with the ,leeds and ,northrup fault finder4 ,diagram showing connections for the second measurement4 ,the special loop test may be used to advantage where the length of the cable or faulty wire only is known1 and where there are two other wires which may be used to complete the loop4 ,to use an outside battery1 connect one pole to ,ba1 and ground the other4 ,the pressure of this battery must never exceed #aaj volts2 if it be over #be volts1 see that switch ,w is open4 ,the ,potentiometer4--,for the rapid and accurate measurement of voltage1 current1 and resistance1 the potentiometer can be recommended4 ,those in charge of electric light and power companies1 and also those who purchase large amounts of electrical energy are realizing1 more and more1 the necessity of having satisfactory primary standards with which to check their volt-1 ampere-1 and watt-meters4 ,when it is realized that an error of one per cent4 in a commercial instrument means an error of one dollar one way or the other in every one hundred dollars charged1 the need of such standardization apparatus becomes at once apparent4 ,the potentiometer1 it should be noted1 relies for its accuracy1 only upon the constancy and accuracy of resistances and upon standard cells4 ,with the materials now available1 and the skill which has been acquired in their manufacture1 both the resistances and the standard cells are obtainable which are remarkably constant1 and both can be readily checked for accuracy4 ,location of ,opens4--,these measurements are based on the fact that the capacity of wires in a cable is ordinarily a measurable quantity1 which1 in wire of uniform diameter1 is proportionate to length4 ,in making these tests1 a fault finder is used together with a buzzer1 dry cells to operate it1 small induction coil1 and telephone receiver4 ,these instruments are to be found in any telephone exchange4 ,it is best to locate the buzzer at some distance from the fault finder in order that it cannot be heard by the operator4 ,illustration3 ,fig4 #fje4--,to use galvanometer of ,leeds and ,northrup fault finder in series with the battery3 ,set switches as shown1 and connect between posts ,gr4 and #b 7see figs4 #ehg and #ehh74 ,the galvanometer will have the maximum sensibility with the pointer at #a1jjj and the minimum at zero4 ,illustration3 ,fig4 #fjf4--,to measure high resistances1 such as the resistances of faults with ,leeds and ,northrup fault finder4 ,first ,method4--,arrange the switches as shown in the figure4 ,connect posts ,gr4 and #b1 turn the handle until the galvanometer needle comes to rest at an even deflection of ten divisions4 ,call the reading ,a4 ,connect in the unknown resistance between ,gr4 and #b4 ,now close the switch ,w1 so that the figure #a appears on the top of the block1 and again bring the galvanometer to a deflection of ten divisions and call the reading ,b4 ,then ,x 7#aj1jjj ,b ,a7 - #a1jjj4 ,in case ,x be a high resistance1 it will be found that the galvanometer will not deflect ten divisions for any position of the pointer4 ,in such case1 choose a number of divisions which is a factor of ten1 such as #e1 #b1 or #a1 and multiply 7#aj1jjj ,b ,a7 by ten divided by the number chosen1 as #aj_/e1 #aj_/b1 #aj_/a4 ,for example1 for a deflection of two divisions3 ,x 7#aj_/b77aj1jjj ,b ,a7 - #a1jjj4 ,the satisfactory range of the set for high resistance measurement may be increased by using an outside battery of higher voltage4 ,with the contained battery1 satisfactory measurements can be made up to #a or #b megohms4 ,when outside battery is used1 connect one terminal to the post ,ba1 and the other to #b for the reading ,a4 ,connect the battery and unknown resistance in series between these posts for the reading ,b4 ,when an outside pressure of #be volts or over is used1 the switch ,w should not be closed unless there be a resistance in series with the battery of #aj1jjj ohms or over4 ,second ,method4--,for use as a voltmeter to measure high resistances4 7,more convenient but not quite as accurate as first method47 ,set the switches to ,,rv1 ,m and #aj4 ,turn the knurled nut on the galvanometer so as to set the needle to the extreme right hand side of the scale4 ,connect the posts #b and ,gr4 with a short piece of wire4 ,turn the rotating pointer on the scale until the galvanometer needle moves over about #bj scale divisions when the battery key is closed4 ,remove the connection between #b and ,gr4 as the voltmeter terminals4 ,this makes a simple way of testing for various kinds and amounts of trouble4 ,on a wet cable a deflection of #aj to #ae divisions indicates heavy enough trouble to locate with the fault finder4 ,with a little care1 trouble showing only #e or #f divisions can be located4 ,before attempting locations for opens it is well to make the following measurements3 #a4 ,the insulation of the broken wire and the insulation of the good wire with which it is to be compared4 ,this may be done as shown in fig4 #fjf4 ,it is best that the insulation resistance be fairly good1 but experiments indicate that good results can be obtained by the methods which follow1 even when the insulation is as low as #ajj1jjj ohms1 and fair results when as low as #ej1jjj to #ajj1jjj ohms4 ,illustration3 ,fig4 #fjg4--;,diagram of connections in testing for opens with ,leeds and ,northrup fault finder4 ,the apparatus required consists of fault finder1 buzzer1 dry cell to operate buzzer1 small induction coil1 and telephone receiver4 ,connect the battery to the primary of the induction coil1 one terminal of the secondary to the post ,ba1 and the other to the connected wires as shown4 ,set switches ,u and ,v so as to short circuit the two resistance coils4 #b4 ,the resistance between the two sections of the broken wire should be measured4 ,this may be done by joining the broken wire and a good wire at the distant end of the cable and measuring the resistance of the loop4 ,to ensure close locations1 this resistance should be over #ajj1jjj ohms4 ,fair locations can be made when the resistance is much lower and it is worth while to attempt it even if the resistance be as low as #aj1jjj ohms4 ,the difficulty of determining the balance point increases as the resistance decreases4 ,ques4 ,describe the method of locating an open with a fault finder4 ,ans4 7,case ,i7 ,the broken wire will be one of a pair4 ,select another pair in the cable that will have the same capacity per mile and join together the mate of the broken wire and one wire of the other pair4 ,make the connections as shown in fig4 #fjg1 then depress the battery key and move the contact to the point of minimum sound in the telephone4 ,the distance to the break is equal to ,,la 7#a1jjj - ,a71 where ,l is the length of the good wire4 ,,example3 ,a cable #a4de miles long contained a broken wire4 ,it was found that the insulation resistance of the end of this wire was over #aj megohms1 as was that of the good pair selected to test against it4 ,the resistance between the two pieces of the good wire was also over #aj megohms4 ,connections were made as in fig4 #fjg1 and it was found that the balance point was #dgf4 ,accordingly from the table ,a _/ 7#a1jjj - ,a7 #j4ijhd and d #a4de 4#ijhd #a4cag miles4 ,illustration3 ,fig4 #fjh4--;,diagram of connections in testing with ,leeds and ,northrup fault finder for open wire in telegraph and other cables in which the wires are not grouped in pairs4 ,connect the broken wire to #a4 ,select a good wire and join to #b4 ,connect all other wires and ground them1 by connecting to the cable sheath4 ,connect the distant end of the broken wire to the others4 ,ground the end of the induction coil that is not connected to the post ,ba4 ,location of ,opens4--7,case ,,ii7 ,open wire in telegraph or other cables in which the wires are not grouped in pairs4 ,the connections are made as in fig4 #fjh1 and the measurement and calculation exactly as in the preceding case4 ,the accuracy of the location of both of the above methods depends on the good and broken pair1 or the good and broken wires having equal and uniform capacity per unit length4 ,it is not always possible to select wires that are alike in this respect4 ,in such cases1 as for instance1 where there is no good wire in the cable containing the broken wire1 and a good wire has to be selected from another cable1 the method of ,case ,,iii may be used4 ,illustration3 ,fig4 #fji4--;,diagram of connections for reading in testing for opens with ,leeds and ,northrup fault finder1 when broken wire and good wire are not in the same cable4 ,illustration3 ,fig4 #faj4--,leeds and ,northrup potentiometer4 ,it is direct reading from 4#jjjjja volt to #af volts1 and with accessories the range may be extended to #afjj volts1 and currents may be measured up to #cjjj amperes4 ,the instrument has fifteen coils of #e ohms each1 which are in series with an extended wire about #aij long of equal resistance4 ,the electrical circuits are shown in the diagram fig4 #faa4 ,it is well for the user to open up the potentiometer and make himself familiar with its interior construction1 in order to fully understand the operation of the rheostat and other parts4 ,there are no contact resistances in the potentiometer circuit proper4 ,the potentiometer has low internal resistance which gives it the maximum sensibility4 ,compared with high resistance potentiometer1 this is especially advantageous in measuring the electromotive force of thermocouples1 and the fall of potential across standard low resistances4 ,as constructed1 the last one-tenth volt is covered by the extended wire and the handle which carries the contact point on the wire may be manipulated rapidly so that a fluctuating voltage may be accurately followed4 ,when used with any cadmium cell1 the potentiometer is direct reading4 ,the accuracy of the potentiometer resistances can be verified with the facilities of the ordinary laboratory4 ,location of ,opens4--,case ,,iii1 in which the broken wire and good wire are not in the same cable4 ,connect the good wire and broken wire in the same way as shown in fig4 #fjg1 and set the pointer for a balance4 ,call the reading ,a4 ,then connect the good wire and the broken wire at the distant end and set the pointer for a new balance4 ,call this ,a'4 ,the connections for this reading are shown in fig4 #fji4 ,the distance to the break will be d 7,a ,a' ,l7 _/ 7#a1jjj7;,a - ,a'7 ,a ,a'7 where ,l is the total length of the broken wire4 ,illustration3 ,fig4 #faa4--;,diagram showing connections of ,leeds and ,northrup potentiometer4 ,the coils in the series ,,ad are each #e ohms1 and between each two there is a brass block with a reamed hole4 ,a pair of flexible cords with taper plug terminals to fit these holes is furnished4 ,these coils can be measured with an ordinary ,wheatstone bridge and thus compared with each other to a high degree of accuracy1 even if the bridge be not accurate4 ,for potentiometer work1 the essential point is that they should be like each other1 not that they should be accurately any particular value4 ,in the same way the resistance of the extended wire can be compared with the resistance of the coils in ,,ad4 ,its resistance should be #a4a times the value of any coil between ,a and ,d4 ,outside connection with the extended wire may be made by using the posts marked ,,br and -,,ba4 ,this adjustment for balancing an unknown electromotive force is accomplished by the manipulation of the two contact points ,m and ,m'4 ,the coils ,,ad are arranged in a circle1 a revolving switch moving ,m4 ,a checking device enables the operator to set this switch without taking his eye from the galvanometer4 ,the resistance ,s is of such value that when it shunts the wire ,,ob1 the total resistance between ,o and ,b is #a_/aj of the same unshunted4 ,when the shunt is applied1 provided the total current remain the same1 the drop between any two points on ,,ab will be #a_/aj of its previous value4 ,the total current will remain the same provided the total resistance in the circuit remain the same4 ,this is accomplished by making the coil ,k such that it exactly compensates for the reduction in resistance caused by plugging in the shunt coil ,s4 ,the low scale is applied by moving the plug from the position #a to the position 4#a4 ,with this change the potentiometer reads from 4#af volt down by indicated steps of 4#jjjjje volt4 ,the reading is very simple4 ,for instance1 if ,m stand at #a4b and ,m' at #a4ce revolutions1 the reading is #a4bace volts4 ,the resistances of the instrument are wound upon metal spools1 and are therefore able to dissipate a comparatively large amount of energy4 ,this allows the potentiometer to be used for pressure measurements up to #af volts without the use of a volt box4 ,illustration3 ,fig4 #fab4--;,diagram showing actual connections in the rheostat of ,leeds and ,northrup potentiometer4 ,the figure corresponds to ,r of fig4 #faa4 ,the rheostat is mounted in the end of the potentiometer as shown in fig4 #faj4 ,rough adjustment of the potentiometer current is made by means of the variable resistance ,r4 ,fine adjustment is made by means of the variable resistance ,r'4 ,it will be noted that the #bc ohm resistance of this latter rheostat is shunted by a resistance of #f4a ohms1 making possible a very fine regulation4 ,further1 there is in series with the moving contact a resistance of #djj ohms1 which makes the effect of variable contact resistance negligible4 ,only one cell of storage battery should be used4 ,when this battery is fresh1 the plug shown in the figure at #b,r should be inserted at ,r4 ,this gives the greatest resistance in the rheostat circuit4 ,as the cell runs down1 the plug should be changed to #b,r4 ,when both plugs are in1 the rheostat slide wires are in series with the potentiometer circuit4 ,,example3 ,a pair of wires containing one broken wire was connected with a good pair in a different cable as shown in fig4 #fjg4 ,the reading ,a was found to be #ahj4 ,the good and bad wires were then joined at the distant end as in fig4 #fji1 and the reading ,a was found to be #hh4 ,the total length of the bad wire ,,mn was #a4dd miles4 ,required1 the distance to the break4 ,substituting the values in the formula3 d #ahj #hh #a4dd _/ #a1jjj7ahj - #hh7 #ahj #hh 4#baa mile4 ,to ,pick ,out ,faulty ,wires in a ,cable4--,short circuit the coils ,e and ,r with switches ,u and ,v4 ,set the pointer at #a1jjj4 ,connect the post ,gr4 to ground or the cable sheath and apply the wires one after another to the binding post #b4 ,the galvanometer will deflect for a faulty wire4 ,illustration3 ,fig4 #fac4--;,diagram of the ,crompton potentiometer4 ,in this instrument the resistance consists of fourteen coils1 each of #aj ohms1 in series with a straight wire1 also #aj ohms resistance1 thus forming a system of fifteen equal steps4 ,across the whole a pressure of #a4e volt is applied from a secondary cell1 thus providing 4#aj volt per step4 ,any fraction is then tapped off by means of a radial switch on the resistance coils and a sliding contact on the wire4 ,the standardization is performed by adjusting a resistance in series with the whole until the standard cell employed indicates1 by means of the galvanometer ,g1 a balance at the point which represents its electromotive force on the basis given above4 ,ques4 ,what is a potentiometer8 ,ans4 ,an arrangement of carefully standardized resistances for measuring voltages in comparison with a standard cell4 ,it is used for accurate measurement of voltages1 currents1 and resistances4 ,in place of a series of standardized resistances1 a slide wire may be used as in fig4 #fad4 ,ques4 ,describe one form of potentiometer4 ,ans4 ,as shown in fig4 #fad1 it consists of a fine ,german silver wire about #c feet long stretched between the binding posts ,a1 ,b1 which are attached to a wooden base carrying a scale divided into #a1jjj equal parts4 ,there are three circuits1 the terminal ,a being included in each1 one including the battery1 and the other two the galvanometer4 ,a three point switch connects the galvanometer in series with the standard cell ,,sc1 or the cell to be tested ,c1 the circuits being completed by leads terminating in the sliding contacts ,m and ,s4 ,illustration3 ,fig4 #fad4--;,diagram of potentiometer showing method of measuring the voltage of a cell4 ,the potentiometer is simply a high resistance wire of uniform diameter stretched between two binding posts1 ,a and ,b1 in such a way that contact can be made at its ends and along its length4 ,necessary circuits are plainly shown in the figure2 ,,sc1 is a standard cell and ,c1 the cell to be tested4 ,m1 and ,s are sliding contacts1 connecting with the slide wire4 ,ques4 ,describe the method of measuring the voltage of a cell with a potentiometer4 ,ans4 ,fig4 #fad shows a method of comparing a pressure with that of a standard cell and is applicable whether the pressure of the cell to be tested be greater or less than that of the standard cell4 ,in making the test the switch ,f is first closed1 then the other switch is moved to ,d1 and ,m adjusted till galvanometer shows no deflection2 similarly1 the switch is moved to ,g1 and ,s adjusted till galvanometer shows no deflection4 ,then1 ,c3,,sc ,,as3,,am4 from which ,c ,,sc ,,as ,,am4 ,,example4--,let #a4jaf volts be the known voltage of the standard cell ,,sc1 and the scale reading of ,,as be #feg1 and of ,,am1 #bbe as in the figure1 then ,c 7#a4jaf #feg7 _/ #bbe #b4iff volts ,the arrangement may1 however1 be made direct reading1 that is1 the slide wire may have a scale of volts instead of lengths or resistances1 as follows3 ,suppose the standard cell to have a pressure of #a4dcd volts1 the sliding contact ,m is placed at the reading #a4dcd1 and the adjustable resistance varied till the galvanometer shows no current4 ,this means that the pressure between ,a and ,m is #a4dcd1 and consequently the pressures all along the slide can be read off the scale in volts4 ,hence1 when ,s has been adjusted to balance1 the pressure of ,c is read off the scale in volts4 ,illustration3 ,fig4 #fae4--,to measure a pressure greater than #a4f volts with ,leeds and ,northrup potentiometer by using a volt box or multiplier4 ,to measure high voltages it is necessary to connect the voltage across high resistance and to measure on the potentiometer a definite fraction of the total drop4 ,in the figure1 ,,ab is a high resistance of which ,,cb is 4#ath1 ,,db 4#jath1 and ,,eb 4#jjath of the total resistance4 ,the potentiometer reading is accordingly multiplied by #aj1 #ajj1 or #a1jjj1 depending upon whether the switch ,m is set on ,c1 ,d1 or ,e4 ,resistance boxes for this purpose are called volt boxes1 and are constructed to multiply the potentiometer readings by #aj1 #ajj1 and #a1jjj4 ,in using them1 it is only necessary to connect the unknown ,e4,m4,f4 at the posts so marked1 and the potentiometer to the posts marked ,p4 ,the potentiometer reading is taken as above and multiplied by a factor depending upon the position of the switch ,m1 which factor is indicated upon the box4 ,it is essential in making these connections that the polarity be carefully observed4 ,how to ,use a ,potentiometer4--,all connections must be made as indicated by the stamping on the instrument4 ,particular attention must be given to the polarity of the standard cell1 of the battery1 and of the voltage1 the corresponding and - signs being marked4 ,if used with a wall galvanometer having a telescope and scale1 it will be found convenient to place the potentiometer so that the telescope is directly over the glass index of the extended wire1 thus permitting the observer to read the galvanometer deflections and potentiometer settings without changing his position4 ,potentiometer ,current4--,a medium sized storage cell will be found desirable1 producing a steady current4 ,errors in measurements are frequently made by using an unsteady current4 ,setting for ,standard ,cell4--,set the standard cell to correspond with the certified pressure of the standard cell as given in its certificate4 ,in using the potentiometer shown in fig4 #faj1 place the plug in hole #a1 and see that it is always in this position when checking against the standard cell4 ,place the double throw switch at ,,std4 ,,cell4 ,adjust the regulating rheostat until the galvanometer shows no deflection4 ,in making the first adjustment use the key marked ,r#a2 when a balance is almost attained1 use key ,r#b1 and for the final adjustment use key marked ,r#j4 ,this cuts out the resistance in series with the galvanometer and gives the maximum sensibility4 ,measurement of ,unknown ,pressure4--,the potentiometer 7fig4 #faj71 as ordinarily used1 gives direct readings for voltages up to and including #a4f volts4 ,for pressures higher than #a4f volts1 a volt box or multiplier should be used4 ,after obtaining the standard cell balance1 as previously described1 place the double throw switch in the position marked ,e4,m4,f4 ,the balance for the unknown ,e4,m4,f4 is obtained by manipulating the tenths switch and rotating the contact on the extended potentiometer wire4 ,the final position of the two contacts in conjunction with the position of the plug at the left of the instrument indicates the voltage under test4 ,as directed above1 use key ,r#a for rough adjustment1 ,r#b for intermediate adjustment1 and key ,r#j for final adjustment4 ,plug at #a gives readings for voltage directly from settings of tenths switch and extended wire contact4 ,plug at 4#a shunts the potentiometer circuit so that the voltage measured is 4#a of the reading taken directly from the scale4 ,hence1 the readings taken from the setting of the tenths switch and the slide wire contact must be divided by #aj4 ,to ,balance ,galvanometer for ,unknown ,voltage4--,place plug in hole #a 7fig4 #faj7 for voltages up to #a4f1 and in hole 4#a for voltages up to 4#af4 ,rotate the tenths switch until a condition of balance is obtained exactly or approximately4 ,to secure an exact balance1 rotate the contact on the extended wire4 ,the unknown voltage can now be read directly from the position of the tenths switch and the extended wire contact if plug be at #a1 or by dividing by #aj if plug be at 4#a4 ,,example4--,a balance was obtained with the tenths switch at #a4c1 the extended wire contact at #agf and the plug at #a4 ,the voltage under test1 therefore1 is #a4cagf4 ,if the plug at 4#a had been used1 the same reading would have indicated 4#acagf4 ,to ascertain if the current in the potentiometer circuit has altered during a measurement1 it is only necessary to plug in at #a1 place the double throw switch on ,,std4 ,,cell and close the galvanometer key4 ,no deflection indicates that the current has not changed4 ,if the galvanometer deflect1 the regulating rheostat must again be adjusted until the galvanometer shows no deflection4 ,to ,measure ,voltages from #a4f to #af4--,pressures up to #af volts may be measured by using a greater voltage across the ,,ba posts 7fig4 #fae74 ,for this purpose a battery of about #bj volts should be used4 ,insert the large plug at 4#a and throw the switch to ,,std4 ,,cell1 then balance the galvanometer by means of the regulating rheostat4 ,when the rheostat has been set to secure a balance1 insert the large plug at #a1 set the switch on ,e4,m4,f4 and read the voltage in the usual manner4 ,multiply the reading by #aj4 ,illustration3 ,fig4 #faf4--,measurement of current with potentiometer4 ,this is done by measuring the drop in volts across a known low resistance4 ,in the figure ,s is the standard resistance1 and on it are the pressure terminals pp1 and the current terminals ,,cc4 ,the potentiometer is connected to the shunt through the posts marked ,p4 ,the resistance between the points pp is adjusted to an even fraction of an ohm4 ,these resistances are so chosen that in order to determine the current passing through the shunt1 after having obtained a potentiometer balance it is only necessary to multiply the potentiometer reading by a simple factor4 ,for instance1 in using a 4#ja ohm standard4 ,it is only necessary to multiply the potentiometer reading by #ajj1 which gives the current reading in amperes2 similarly1 a 4#a ohm requires multiplication by #aj1 and a 4#jja ohm by #a1jjj4 ,care of ,potentiometer4--,the slide wire1 although protected to a great extent by the hood1 in time accumulates dust and dirt with a thin film of oxide4 ,this will tend to increase the resistance in this part of the circuit owing to poor contact4 ,this wire should1 therefore1 be cleaned occasionally4 ,to do this1 unscrew the stop against which the hood strikes when turned to read zero2 then remove the hood and rub the entire slide wire vigorously with a soft cloth dipped in vaseline4 ,do not use emery or sand paper as this will destroy the uniformity of the slide wire4 ,clean also the steel contact which rubs on the wire1 as this becomes glazed after much use4 ,when the potentiometer is not in use1 the hood should be screwed all the way down1 and the lid put in place to exclude dust4 ,if it be used in a chemical factory1 laboratory1 or any place where acid fumes are prevalent1 this latter precaution is important1 because the fumes may attack the slide wire4 ,it is also well to keep the contact surfaces of the switch studs clean and bright by wiping them occasionally with a soft cloth dipped in vaseline4 ,illustration3 ,res4 of leads on each end is equal to #aj divisions of slide wire4 ,the slide wire is divided into #ajjj parts--#bj for the leads1 or #ihj divisions4 ,calibrated scale on ,galvanometer3 ,fig4 #fag4--;,diagram of ,leeds and ,northrup bridge for locating faults in power circuits1 showing arrangement of the connections including the lead cables and galvanometer contacts4 ,make connections as shown4 ,the clamps must be so fastened at ,a and ,c that the contact resistances will be very small4 ,this contact resistance will figure as an error in the measurement4 ,if1 for instance1 the contact resistance were equal to 4#jja of an ohm1 and the wire were of such a size that 4#jja of an ohm were equal to the resistance of #bj feet of the cable1 there would be an error of #bj feet in the location of the fault4 ,for this reason all contact resistances throughout the loop from ,a to ,c must be extremely small4 ,the battery is to be connected to the posts marked ,ba41 and the post marked ,gr4 is to be grounded4 ,it will very frequently happen that the ground is to the cable sheath or some other conductor4 ,in this case1 the binding post ,gr4 should be grounded to this conductor4 ,sufficient battery should be used to give a readable deflection on the galvanometer for a small movement of the contact on the bridge wire4 ,the fault is located by the usual ,murray formula4 ,if1 for instance1 the galvanometer show no deflection when the contact is at #cjj on the scale1 it would indicate that the fault is at a distance from ,a equal to 4#jjc of the total length of the loop from ,a to ,c4 ,a testing current of five amperes may be used with this bridge4 ,in cases of necessity1 this current may be increased to eight amperes1 but when this current is used it should not be allowed to pass through the bridge for a longer time than is necessary4 ,it frequently happens that small faults which have a very high resistance develop in high pressure cables4 ,such faults are likely to break down and result in damage and should be located4 ,it is usually impossible to locate these faults until they have been partially carbonized4 ,this must be done by applying a sufficiently high voltage between the cable and the sheath 7or whatever it is grounded on7 to break down the fault4 ,in order to prevent the breaking down process from resulting in a serious burn out a high resistance must be placed in the circuit which will prevent an excessive current1 or the circuit must be carefully fused4 ,the former procedure is the better4 ,location of ,faults where the ,loop is ,composed of ,cables of ,different ,cross ,sections4--,faults in loops of this character may be located with the same degree of accuracy as those in loops of a uniform cross section1 provided the length and cross section of each length of cable are known4 ,an example will illustrate the method3 ,in the diagram1 fig4 #fag1 assume the length of the cable ,,ae to be #eej yards of #be1jjj cir4 mil41 ,,ef1 #ejj yards of #dj1jjj cir4 mil41 and ,,fc1 #a1jej yards of #cj1jjj cir4 mil4 ,these lengths must be reduced by calculation to equivalent lengths of one size1 and for this purpose it is best to select the largest size4 ,the results of this calculation are as follows3 #eej yds4 of #be1jjj cir4 mil4 #hhj yds4 of #dj1jjj cir4 mil4 #ejj #dj1jjj #ejj #dj1jjj #a1jej #cj1jjj #a1djj #dj1jjj ,this makes the total resistance of the loop equivalent to #b1ghj yards of #dj1jjj cir4 mil4 ,if the contact show a balance for a reading of #cgb4e1 this indicates that the fault is at a distance of #cgb4e_/a1jjj of #b1ghj #a1jce4e equivalent yards4 ,of this1 #hhj are in the stretch ,a ,e4 ,consequently the fault is3 #a1jce4e - #hhj #aee4e yards from ,e4 ,illustration3 ,fig4 #fah4--,the ,fischer portable cable testing set1 designed for locating crosses1 grounds and breaks in cables1 also for conductor and liquid resistance measure4 ,the distinguishing feature of the set is the master switch4 ,by means of this switch1 connections can be made for the various tests by a single movement1 thus avoiding the labor and time which have to be expended in interchanging the connections and memorizing the rather complicated scheme of connections4 ,,chapter ,,xxviii ,,ammeters1 ,,voltmeters ,,and ,,wattmeters4 ,an ammeter or ampere meter is simply a commercial form of galvanometer so constructed that the deflection of the needle indicates directly the strength of current in amperes4 ,a good ammeter should have a very low resistance so that very little of the energy of the current will be absorbed2 the needle should be dead beat1 and sufficiently sensitive to respond to minute variations of current4 ,according to the principle of operation1 ammeters and voltmeters are classified as3 #a4 ,moving iron2 #b4 ,moving coil2 #c4 ,solenoid or plunger2 #d4 ,magnetic vane2 #e4 ,hot wire2 #f4 ,electrostatic2 #g4 ,astatic2 #h4 ,inclined coil2 #i4 ,fixed and movable coil4 ,again1 they are divided according to their use into two classes3 #a4 ,portable type2 #b4 ,switchboard type4 ,milli-ammeters or milli-voltmeters are instruments in which the scale is graduated to read directly in thousandths of an ampere or thousandths of a volt respectively4 ,ques4 ,describe the moving iron type instrument4 ,ans4 ,the arrangement of the working parts are shown in fig4 #fbj4 ,a soft iron needle ,n1 is pivoted inside of a coil ,c1 and is held out of line with the axis of the coil by means of a permanent magnet ,m1 when the instrument is idle4 ,in this position1 a pointer ,p1 which is attached to the needle1 stands at the zero mark of the scale ,s4 ,if a current be passed through the coil1 magnetic lines of force are set up in its center1 which tend to pull the needle into line with them1 and therefore with the axis of the coil4 ,this pull is resisted by the permanent magnet ,m1 and the amount of deflection of the needle from the zero position depends upon the strength of the current or the voltage according as the coil is wound to indicate amperes or volts4 ,illustration3 ,fig4 #fbj4 ,moving iron type instrument4 ,the essential parts are3 ,n1 soft iron needle2 ,c1 coil2 ,m1 permanent magnet2 ,p1 pointer2 ,s1 scale4 ,current passing through the coil acts on the needle1 causing it to turn against the restraining force due to the influence of the permanent magnet4 ,illustration3 ,fig4 #fba4--,moving coil type instrument4 ,the essential parts are3 ,a1 spiral spring2 ,c1 coil2 ,k1 soft iron core2 ,m1 permanent magnet2 ,p1 pointer2 ,s1 scale4 ,current passing through the coil causes the moving system to turn against the restraining force due to the influence of the permanent magnet4 ,ques4 ,describe a moving coil instrument4 ,ans4 ,this type of instrument is shown in fig4 #fba4 ,it consists of a moving coil ,c1 to which is attached the pointer1 and which is pivoted between the poles of a permanent magnet ,m4 ,the coil moves between these poles and a fixed soft iron core ,k1 and is held in the normal position by two spiral springs ,a1 above and below the core4 ,the springs also serve to make electrical connection with the coil ,c4 ,when a current passes through the coil1 magnetic lines are set up in it which are at an angle to those passing from one pole of the permanent magnet to the other4 ,the lines of force1 which formerly passed from one pole of the magnet to the other by straight lines or by short curved ones1 are stretched on account of the field produced by the current in the coil1 and1 in trying to shorten themselves1 tend to twist the coil through an angle4 ,this tendency to move is resisted by the two spiral springs1 hence the coil moves until equilibrium is established between the two opposing forces4 ,the amount of deflection of the pointer depends1 either upon the current strength1 or the voltage according to the winding of the coil4 ,illustration3 ,fig4 #fbb4--,keystone voltmeter2 view showing the moving element being withdrawn by loosening one screw4 ,these instruments are constructed on the d',arsonval system1 the moving element being shown in detail in figs4 #fbc and #fbd4 ,the entire system is mounted upon a solid metal base plate4 ,the permanent field magnet is made of a single piece of magnet steel1 and the pole pieces are of soft steel1 permanently secured to the magnet in order that the distribution of the magnetic flux will not be changed by removal and replacement of the pole piece4 ,accordingly the moving mechanism is mounted separately from the field1 so that it can be readily lifted from the field without removing the pole pieces4 ,the function of the core is to secure a uniform field4 ,the moving coil is wound upon a form of aluminum1 which serves the purpose of damping by the generation of eddy currents4 ,the winding of the coil is of fine copper wire1 to which current is conveyed by means of the controlling springs and which1 in the case of a voltmeter1 is connected in series with a resistance1 and in the case of an ammeter1 across the terminals of a shunt4 ,ques4 ,how does the winding differ in ammeters and voltmeters8 ,ans4 ,an ammeter coil consists of a few turns of heavy wire 7when designed to carry the full current71 while a voltmeter coil is wound with many turns of fine wire4 ,thus1 the ammeter is of low resistance1 and the voltmeter of high resistance4 ,illustration3 ,fig4 #fbc4--,new moving element of ,keystone instruments1 weight #a4b grams4 ,fig4 #fbd4--,moving element of ,keystone instruments assembled in bearing4 ,the moving element consists of coil1 counterpoise and pointer4 ,the mechanical connections are made by means of screws and steady pins4 ,in order to adjust for slight set or subset of spring under long use a zero adjuster is provided by means of which this set can be connected and the pointer brought back to zero4 ,ques4 ,why is a high resistance coil used with a voltmeter8 ,ans4 ,as actually constructed1 most voltmeters are simply special forms of ammeter4 ,from ,ohm's law1 the current through a given circuit equals the pressure at its terminals divided by its resistance4 ,hence1 if a high resistance be connected in series with a sensitive ammeter that will measure very small currents1 then the current passing through the circuit is directly proportional to the voltage at its terminals1 and the instrument may be calibrated to read volts4 ,illustration3 ,figs4 #fbe and #fbf4--;,connections for series and shunt ammeters4 ,when the construction is such that all the current passes through the instrument1 it is connected as in fig4 #fbe1 but where the instrument is designed to take only a fraction of the current1 it is connected across a shunt1 as in fig4 #fbf1 a definite proportion of the current passing through the instrument and the remainder through the shunt4 ,ques4 ,into what two classes may ammeters be divided8 ,ans4 ,they are classed as series or shunt according to the way they are designed to be connected with the circuit4 ,ques4 ,what determines the mode of connecting ammeters8 ,ans4 ,when the wire of the ammeter coil is large enough to carry the whole current1 it is connected in the circuit in series as shown in fig4 #fbe4 ,if1 however1 the wire be small1 the instrument is connected in parallel with a shunt of low resistance1 so that it only carries a small part of the current1 as in fig4 #fbf4 ,for circuits which carry large currents1 the shunt connection is always used1 because otherwise the coil of the ammeter would have to be very heavy and the instrument correspondingly bulky4 ,ques4 ,how are shunt ammeters arranged to correctly measure the current8 ,ans4 ,the coil is arranged so that a definite proportion of the whole current passes through it4 ,a large conductor of low resistance is connected directly between the two terminals or binding posts of the instrument2 the coil is connected as a shunt around a definite part of this main conductor2 then1 since the two are connected in parallel and each branch has a definite resistance1 the current divides between the two branches directly in proportion to their relative conductivities1 or inversely according to their resistances4 ,the coil1 therefore1 takes a definite part of the whole current1 and the force moving it and its pointer away from the zero position is directly proportional to the whole current4 ,hence1 by providing a proper scale1 the value of the entire current will be indicated4 ,illustration3 ,figs4 #fbg and #fbh4--,westinghouse ammeter shunts4 ,these shunts are used where heavy currents are to be measured4 ,the shunt is connected in series with the bus bar or circuit to be measured1 and its terminals are connected by means of small leads to the ammeter or other instrument4 ,these shunts are designed to have approximately #ej millivolts drop at full rated current4 ,they are intended primarily for ,westinghouse meters1 but can be used satisfactorily with any meter requiring #ej millivolts for full scale deflection4 ,ques4 ,how is a voltmeter connected8 ,ans4 ,a voltmeter is always connected to the two points1 whose difference of potential is to be measured4 ,for instance1 to measure the voltage between the two sides ,a and ,b of the circuit shown in fig4 #fbi1 one terminal of the voltmeter is connected to wire ,a1 and the other to wire ,b4 ,if the drop or difference in voltage through a certain length of wire ,l1 of a circuit1 as from ,a to ,b in fig4 #fcj is to be determined1 one terminal of the voltmeter is connected to ,a and the other to ,b4 ,in a similar manner is found the drop through a lamp4 ,illustration3 ,fig4 #fbi4--,voltmeter connection for measuring the pressure in an electric circuit4 ,the voltmeter is connected in parallel in the circuit at the point where the voltage is to be measured4 ,fig4 #fcj4--,voltmeter connection for measuring the drop or fall in voltage in a certain length of wire1 as for instance1 the length between the points ,a and ,b4 ,the voltmeter is shunted between the two points whose pressure difference is to be measured4 ,ques4 ,what is the difference between a voltmeter and an ammeter8 ,ans4 ,a voltmeter measures pressure1 while an ammeter measures current4 ,as actually constructed1 most voltmeters are simply special forms of ammeter4 ,if a high resistance be connected in series with a sensitive ammeter that will measure very small currents1 then the current passing through the circuit is directly proportional to the pressure or voltage at its terminals and the instrument may be calibrated to read volts4 ,ques4 ,explain the term calibrate4 ,ans4 ,to calibrate a measuring instrument is to determine the variations in its readings by making special measurements1 or by comparison with a standard4 ,illustration3 ,fig4 #fca4--,weston ammeter2 view showing shunt enclosed within the instrument4 ,weston instruments are direct reading and dead beat4 ,although the scales have practically uniform divisions1 it is not assumed in the calibration that they are uniform1 and the scales are not printed or engraved4 ,the method of calibration consists in laying out each large division of the scale by comparing the instrument with a standard1 and then inking in the division lines so found4 ,the smaller divisions between the large ones are then equally spaced and marked by a mechanical method4 ,illustration3 ,fig4 #fcb4--,weston portable voltmeter1 inspector's style4 ,this instrument is provided with a reversing key4 ,instead of the regular binding posts1 pins are used with which connections are made by means of contact cups attached to flexible cords4 ,these contact cups are convenient in making connections1 or in changing quickly from one range to the other1 if the instrument have a double scale4 ,connections for the different ranges are made in precisely the same way as with the regular double scale voltmeters4 ,for the upper scale values1 the contact pin to the right and the front contact pin to the left being taken1 and for the lower scale values1 the left contact cup is changed to the rear contact pin4 ,ques4 ,describe a solenoid or plunger ammeter4 ,ans4 ,this type consists of a plunger or soft iron core arranged to enter a solenoid4 ,current being passed through the wire of the solenoid causes the core to be more or less attracted against a restraining force of gravity or springs4 ,a pivoted pointer attached to the core indicates the current value on a graduated dial as shown in fig4 #fcc4 ,ques4 ,what are the objections to plunger instruments8 ,ans4 ,they are not reliable for small readings1 and are readily affected by magnetic fields4 ,illustration3 ,fig4 #fcc4--,plunger type instrument4 ,the current to be measured passes through the solenoid1 producing a magnetic effect on the soft iron plunger which tends to draw it into the coil1 and thus cause the pointer to move over the graduated scale4 ,the distance the rod moves depends on the value of the restraining force 7which may be springs or gravity71 the coil winding1 and strength of current4 ,the winding consists of a few turns of heavy wire for an ammeter1 and a large number of turns of fine wire when constructed as a voltmeter4 ,since the iron has a certain amount of residual magnetism1 the deflection with smaller following large currents is more than would be produced by the same current following a smaller one4 ,the instrument therefore is less reliable than the usual types4 ,ques4 ,describe a magnetic vane instrument4 ,ans4 ,it consists of a small piece of soft iron or vane mounted on a shaft that is pivoted a little off the center of a coil as shown in fig4 #fcd4 ,the principle upon which the instrument works is that a piece of soft iron placed in a magnetic field and free to move will move into such position as to conduct the maximum number of lines of force4 ,the current to be measured is passed around the coil producing a magnetic field through the center of the coil4 ,the magnetic field inside the coil is strongest near the inner edge1 hence1 the vane will move against the restraining force of a spring so that the distance between it and the inner edge of the coil will be as small as possible4 ,a pointer1 attached to the vane shaft moves over a graduated dial4 ,illustration3 ,fig4 #fcd4--,magnetic vane instrument4 ,a soft iron vane1 eccentrically pivoted within a coil carrying the current to be measured1 is attracted toward the position where it will conduct the greatest number of magnetic lines of force against the restraining force of a spring or equivalent4 ,ques4 ,describe an inclined coil instrument4 ,ans4 ,as shown in fig4 #fce1 a coil carrying the current1 is mounted at an angle to a shaft to which is attached a pointer4 ,a bundle of iron strips is mounted on the shaft4 ,a spring restrains the shaft and holds the pointer at the zero position when no current is flowing4 ,when a current is passed through the coil1 the iron tends to take up a position with its longest sides parallel to the lines of force1 which results in the shaft being rotated and the pointer moved on the dial1 the amount of movement depending upon the strength of the current in the coil4 ,the coils for large sizes are generally wound with a few turns of flat insulated copper ribbon4 ,the instruments are adapted to either direct or alternating currents but are recommended for alternating currents4 ,illustration3 ,fig4 #fce4--,thompson inclined coil ammeter4 ,it is constructed on the magnetic vane principle in which an iron vane is attracted by the magnetic field due to the coil1 so as to turn itself parallel with the axis of the coil1 the latter being inclined with respect to the axis of the vane4 ,the voltmeter of this type has a similarly placed stationary coil1 but in place of the iron vane1 is provided with a moving coil in series with the other coil4 ,the restraining force in each case being that due to springs4 ,figs4 #fcf and #fcg show the actual construction of inclined coil instruments4 ,illustration3 ,figs4 #fcf and #fcg4--,thompson inclined coil portable indicating instruments4 ,fig4 #fcf1 ammeter type ,p interior2 fig4 #fcg1 wattmeter1 type ,p1 interior4 ,these instruments1 though primarily designed for use on alternating current circuits1 may also be used on direct current circuits1 by making reversed readings and taking the mean as the true indication4 ,the voltmeters and wattmeters are constructed on the dynamometer principle and the ammeters1 on the magnetic vane principle4 ,the voltmeters and wattmeters are provided with a contact key which may be locked in position1 enabling the instruments to be left constantly in circuit4 ,the movements of the pointer are damped by means of an air vane2 there is also a friction damping device operated by a small button to check excessive oscillations of the pointer4 ,the inclined coil instruments are so designed that the torque is sufficiently high to insure the pointer assuming a definite position with each change in current value4 ,ques4 ,what is the principle of the hot wire instrument8 ,ans4 ,its action depends upon the heating of a conductor by the current flowing through it1 causing it to expand and move an index needle or pointer1 the movements of which1 by calibration1 are made to correspond to the pressure differences producing the actuating currents4 ,ques4 ,what are the characteristics of hot wire instruments8 ,ans4 ,voltmeters of this type are not affected by magnetic fields1 and as their self-induction is small1 they can be used on either direct or alternating currents2 but they possess certain serious defects3 they consume more current than the other types2 cannot be constructed for small readings2 are liable to burn out on accidental overloads2 and are somewhat vague in the readings near the zero point and are sometimes inaccurate in the upper part of the scale4 ,ques4 ,describe the construction and operation of the ,whitney hot wire instruments4 ,ans4 ,as shown in fig4 #fch1 a wire ,,ax1 of non-oxidizable metal1 of high resistance and low temperature coefficient1 passes over a pulley ,b mounted on the shaft ,c4 ,the ends of the wire are attached to the plate ,e at its ends ,f and ,g1 the wire being insulated from the plate at ,g4 ,a spring ,h holds the wire in tension and takes up the slack due to the expansion caused by the heating of the wire when a current passes through it4 ,the current flows only in the portion of the wire marked ,a1 between the plate ,e and the pulley ,b up to the point ,k where the connection is shown4 ,when a current flows through the wire ,a1 the spring takes up the slack1 pulls ,a around ,b1 and causes ,b to rotate upon its shaft ,c4 ,it is clear1 that a pointer attached to ,c1 would indicate on a scale the movement of ,b and ,c1 but as this movement is very slight1 a magnifying device will be required4 ,this device consists of a forked rod ,l1 rigidly attached to the shaft ,c1 and carrying at its lower end a silk fibre fastened to the fork and passing around a pulley ,m1 to which a pointer ,n is attached4 ,for direct current measurements only an electromagnetic system is used4 ,illustration3 ,fig4 #fch4--;,diagram showing principle and construction of the ,whitney hot wire instruments4 ,the action of instruments of this type depends on the heating of a wire by the passage of a current causing the wire to lengthen4 ,this elongation is magnified by suitable mechanism and transmitted to the pointer of the instrument4 ,ques4 ,what is the principle of electrostatic instruments8 ,ans4 ,the action of these instruments depends upon the fact that two conductors attract one another when any difference of electric pressure exists between them4 ,if one be delicately suspended so as to be free to move1 it will approach the other4 ,illustration3 ,fig4 #fci4--,kelvin electrostatic voltmeter2 a form of instrument designed for measuring high pressures up to #bjj1jjj volts4 ,the instrument1 as illustrated1 consists of fixed and movable vanes with terminals connecting with each4 ,these vanes which act as condensers take charges proportional to the potential difference between them1 resulting in a certain attraction which tends to rotate the movable disc against the restraining force of gravity4 ,in the figure aa and b are two fixed vanes and c a movable vane1 carrying a pointer and having a proper weight at its lower end4 ,ques4 ,describe the ,kelvin electrostatic voltmeter4 ,ans4 ,a simple form consists1 as shown in fig4 #fci1 of a metal case containing a pair of highly insulated plates1 between which a delicately mounted paddle shaped needle is free to move4 ,when the needle is connected to one side of a circuit and the stationary plates to the other side1 the needle is attracted and moves between them as indicated by the pointer4 ,adjusting screws at the lower end of the needle allow it to be balanced so that its center of gravity is somewhat below the center of suspension4 ,gravity then is the restraining force4 ,the range of the instrument may be changed by hanging different weights upon the needle4 ,by increasing the number of blades the instrument can be made to measure as low as #cj volts4 ,the form having two stationary blades and one movable blade is suitable for measuring from #bjj to #bj1jjj volts4 ,the quadrant electrometer or laboratory form will measure a fraction of a volt4 ,illustration3 ,fig4 #fdj4--,thompson astatic instrument without cover4 ,when current passes through the coils of the moving element1 the lines of force parallel to the shaft produce a torque which tends to turn the shaft and cause the needle to travel across the scale4 ,this action is1 of course1 opposed by the magnetic field at right angles to the shaft acting on the two pieces of magnetic metal4 ,these astatic instruments have no controlling springs4 ,the two small silver spirals which conduct the current to and from the armature are made of untempered silver and exert no force as springs4 ,the actuating and restraining forces are dependent upon the same electromagnets4 ,the damping effect in these instruments is produced by an aluminum disc moving in a magnetic field1 and is proportional to the square of the magnet strength4 ,ques4 ,explain the construction and principle of the ,thompson astatic instruments4 ,ans4 ,the fields of these instruments are electromagnets wound for any specified voltage and provided with binding posts separate from the current posts of the instrument4 ,the moving coils are mounted upon an aluminum disc and are located in a magnetic field which is parallel to the shaft and astatically arranged4 ,two small pieces of magnetic metal are rigidly mounted on the shaft and the astatic components of the magnetic field1 which are perpendicular to the shaft1 tend to keep the pieces of magnetic metal in their initial positions4 ,when current passes through the coils of the moving element1 the lines of force parallel to the shaft produce a torque which tends to turn the shaft and cause the needle to travel across the scale4 ,this action is1 of course1 opposed by the magnetic field at right angles to the shaft acting on the two pieces of magnetic metal4 ,there are thus no restraining springs1 current being conveyed to the moving coil by torsionless spirals of silver wire4 ,thompson astatic instruments can be provided with polarity indicators1 a red disc showing on the scale card where the poles are reversed4 ,the effect of external fields is eliminated by the astatic arrangement of the fields and the moving parts4 ,a field which tends to increase the torque on one side of the armature diminishes it to a corresponding degree on the other side4 ,the damping effect in these instruments is produced by an aluminum disc moving in a magnetic field4 ,illustration3 ,fig4 #fda to #fdb4--,multipliers for ,western standard portable voltmeters4 ,multipliers are resistance boxes1 the coils in which are highly insulated1 and are adjusted so that the readings of the instrument may be multiplied by any desired constant4 ,multipliers are usually constructed so that the indications of the pointer1 multiplied by #b1 #e1 #aj1 #bj or #ej1 will give the voltage of the circuit4 ,by the use of multipliers the range of voltmeters may be increased to any practical limit4 ,illustration3 ,fig4 #fdc4--,portable multiplier for portable voltmeter4 ,a multiplier is used for increasing the readings of voltmeters1 and consists of resistance coils placed in a portable case4 ,a multiplier is connected in series with the voltmeter and must be adjusted for the instrument with which it is to be used1 because the resistance coil must be a multiple of the voltmeter resistance4 ,for instance1 a multiplier with a value of #aj1 used with a #f volt voltmeter or #eba ohms would measure about #e1bae ohms2 one with a value of #dj1 would equal about #bj1hfj ohms4 ,the multiplier #aj would give a total scale value of #fj1 and the multiplier #dj1 a total scale value of #bdj volts to the #f volt instrument4 ,a multiplier is of considerable value in that it does away with the necessity of having a number of voltmeters of different ranges4 ,the instrument here illustrated has a range of #aej volts4 ,ques4 ,what are multipliers8 ,ans4 ,these are extra resistance coils which are connected in series with a voltmeter for increasing its capacity or readings4 ,they are put up in portable boxes1 and must be adjusted for each particular voltmeter as the resistance of a multiplier coil must be a multiple of the resistance of the voltmeter itself4 ,ques4 ,what is an electro-dynamometer8 ,ans4 ,an instrument for measuring amperes1 volts1 or watts by the reaction between two coils when the current to be measured is passed through them4 ,one of the coils is fixed and the other movable4 ,illustration3 ,figs4 #fdd to #fde4--,western standard portable shunts4 ,the milli-voltmeters used in connection with these shunts read directly in amperes4 ,shunts of different capacities can be adjusted to the same instrument1 and it can1 therefore1 be used to measure a current of #b1jjj amperes with the same degree of accuracy as a current of #a ampere4 ,in selecting shunts of different capacities for use in connection with one instrument it should be considered that the higher ranges must be even multiples of the lower one in order to suit the same scale on the instrument4 ,ques4 ,describe the ,siemens' electro-dynamometer4 ,ans4 ,the essential parts are shown in fig4 #fdf4 ,the fixed coil ,a1 composed of a number of turns of wire is fastened to a vertical support1 and surrounded by the movable coil ,b of a few turns1 or often of only one turn4 ,the movable coil is suspended by a thread and a spiral spring ,c1 below the dials which are fastened at one end to the movable coil and at the other end to a milled headed screw ,d1 which can be turned so as to place the planes of the coil at right angles to each other1 and to apply torsion to the spring to oppose the deflection of the movable coil for this position when a current is passed through the coils4 ,the ends of the movable coil dip into two cups of mercury ,e1 ,e'1 located one above the other and along the axis of the coils so as to bring the two in series when connected to an external circuit4 ,the arrows show the direction of current through the two coils4 ,an index pointer ,f is attached to the movable coil4 ,the upper end of this pointer is bent at a right angle1 so that it swings over the dial between two stop pins ,g1 ,g'1 and rests directly over the zero line when the planes of the coils are at right angles to each other4 ,a pointer ,h is attached to the torsion screw ,d1 and sweeps over the scale of the dial4 ,the spring is the controlling factor in making the measurement4 ,illustration3 ,fig4 #fdf4--;,diagram of ,siemens' electro-dynamometer4 ,it consists of two coils on a common axis1 but set in planes at right angles to each other in such a way that a torque is produced between the two coils which measures the product of their currents4 ,this torque is balanced by twisting a spiral spring through a measured angle of such degree that the coils shall resume their original relative positions4 ,if the instrument be used for measuring current1 the coils are connected in series1 and the reading is then proportional to the square of the current4 ,if used as a wattmeter1 one coil carries the main current and the other a small current1 which is proportional to the pressure4 ,the reading is then proportional to the power in the circuit4 ,fig4 #fdg4--;,diagram showing connections of ,siemens' electro-dynamometer as arranged to read watts4 ,illustration3 ,figs4 #fdh to #fej4--,wright demand indicator4 ,this is a device for registering the maximum ampere demand of appreciable duration in any electrical circuit4 ,it may be used on either direct or alternating current circuits4 ,the essential features and principle are as follows3 ,a liquid is hermetically sealed in a glass vessel consisting of two bulbs connected by a ,u tube1 and a central tube called the index tube1 connected to the upper end of the right hand side of the ,u4 ,around the left hand or heating bulb1 is placed a band of resistance metal1 through which the current to be measured is passed1 or a definite shunted portion of it4 ,the heating effect of the current increases the temperature of the left hand bulb1 causing the air to expand which forces the liquid up the right hand side of the ,u tube and into the index tube1 where it remains until the indicator is reset4 ,the height of the liquid in the index tube as shown by the scale1 indicates the maximum current which has passed through the indicator4 ,it is the difference in temperature of the air in the two bulbs which causes the flow of the liquid4 ,any change in external temperature causes equal effect in both bulbs and therefore does not affect the reading4 ,illustration3 ,figs4 #fea and #feb4--,weston illuminated dial station voltmeter and ammeter4 ,the voltmeter has two indices1 a pointed index for close readings and an index called the normal index1 which enables a slight deviation from the normal voltage to be seen from a long distance4 ,the normal index is inside the case and terminates in a circular disc of blackened aluminum4 ,the disc is adjusted from the outside of the case by hand1 by means of the knurled knob seen on the front of the case1 so that it is directly below the point of normal voltage4 ,when the indicating index reaches the point of normal voltage1 the disc of the normal index appears in the center of the circular opening of the indicating index1 a narrow ring of white being visible1 encircling the disc of the normal index4 ,the ammeter depends for its operation upon the fall of potential between two points of the circuit carrying the main current1 and requires a difference of only about 4#je volt to give a full scale deflection4 ,when a maximum deflection is secured1 the current passing through the instrument is never more than 4#jg ampere irrespective of the total capacity of the instrument4 ,a separate shunt is used which is placed at the back of the switchboard4 ,in many cases1 a special shunt can be dispensed with and a short section of the mains on the switchboard1 or the mains leading from the dynamo1 can be used instead4 ,on the basis of one square inch cross section per #a1jjj amperes1 a length of about #e feet of copper conductor would be required as a shunt1 in which case however1 this section of the conductor must be adjusted with precision4 ,ques4 ,explain the operation of the ,siemen's electrodynamometer4 ,ans4 ,in fig4 #fdf1 when a current is passed through both coils1 the movable coil is deflected against a stop pin1 then the screw ,d is turned in a direction to oppose the action of the current until the deflection has been overcome and the coil brought back to its original position4 ,the angle through which the pointer of the torsion screw was turned is directly proportional to the square root of the angle of torsion4 ,to determine the current strength in amperes1 the square root of the angle of torsion is multiplied by a calculated constant furnished by the makers of this instrument4 ,illustration3 ,fig4 #fec4--,thompson watt hour meter 7type ,c-#f74 ,this form is furnished with side connections1 the line wires entering at the left and the load wires at the right4 ,both sides of the system are carried through the meter in all sizes up to and including the #ej ampere size4 ,in meters of larger ampere capacities1 a voltage tap is used4 ,ques4 ,how is the electrodynamometer adapted to measure volts or watts8 ,ans4 ,when constructed as a voltmeter1 both coils are wound with a large number of turns of fine wire making the instrument sensitive to small currents4 ,then by connecting a high resistance in series with the instrument it can be connected across the terminals of a circuit whose voltage is to be measured4 ,when constructed as a wattmeter1 one coil is wound so as to carry the main current1 and the other made with many turns of fine wire of high resistance suitable for connecting across the circuit4 ,with this arrangement1 the force between the two coils will be proportional to the product of amperes by volts1 hence1 the instrument will measure watts4 ,illustration3 ,fig4 #fed4--;,interior view of ,thompson watt hour meter 7type ,c-#f74 ,capacity3 #e to #fjj amperes1 two wire1 and #e to #cjj amperes1 three wire2 #ajj to #bej volts4 ,the meter is supported by three lugs1 the upper one of which is keyholed1 and the lower right hand one slotted4 ,this permits rapid and accurate levelling as the top screw can be inserted and the meter hung thereon approximately level4 ,the right hand screw may then be placed in position and the meter adjusted as may be required before forcing the screw home4 ,ques4 ,describe briefly the construction of the ,thompson recording wattmeter4 ,ans4 ,it consists of four elements3 #a1 a motor causing rotation2 #b1 a dynamo providing the necessary load or drag2 #c1 a registering device1 the function of which is to integrate the instantaneous values of the electrical energy to be measured2 and #d1 means of regulation for light and full load4 ,illustration3 ,,how ,,to ,,read ,a ,,meter ,fig4 #fee4--,recording dials of watt hour meter1 illustrating method of reading electric meters4 ,the unit of measurement of electrical energy is the watt hour4 #a1jjj watt hours make or equal #a kilowatt hour4 ,some electric meters have #d dials1 the extreme right hand dial of which registers in kilowatt hours1 while others have #e dials1 the extreme right hand dial of which registers in tenths of kilowatt hours4 ,in making out bills to customers the extreme right hand dial of a #e dial meter is ignored in order that the state of meter shown on bills uniformly requires the addition of #c ciphers to correctly express the registration in watt hours4 ,each division on the right hand dial 7ignoring the #eth dial mentioned7 denotes #a1jjj watt hours or #a kilowatt hour2 on the next dial #aj kilowatt hours1 on the next dial #ajj kilowatt hours and on the left hand dial #a1jjj kilowatt hours4 ,one complete revolution of any dial causes the hand on the dial immediately to its left to move forward one division4 ,to take a statement from the meter begin at the left and set down for each dial the lower figure next to each hand1 not necessarily the figure nearer the hand4 ,in the above example the statement is #a1gbf kilowatt hours or #a1gbf1jjj watt hours4 ,subtract the previous statement to arrive at registration for a given period4 ,some meters are subject to a multiplying constant so stated on their face and the registration of such meters must be multiplied by the constant as shown1 to determine the actual consumption of electrical energy4 ,the constant is the measure of the mechanical adjustment in the register of the meter and is the ratio between the registration of the dial hands and the true consumption4 ,this adjustment is made always by the manufacturer of the meter and is never changed in service4 ,ques4 ,what is the action of the motor in the ,thompson watt hour meter8 ,ans4 ,it rotates at very slow speed1 and since there is no iron in its fields and armature1 it has very little reverse voltage4 ,its armature current1 therefore1 is independent of the speed of rotation1 and is constant for any definite voltage applied at its terminals4 ,illustration3 ,fig4 #fef4--;,interior of ,thompson watt hour meter 7type ,c-#f7 showing armature1 small commutator and gravity brushes4 ,a spherical armature moving within circular field coils is the construction adopted in this meter4 ,the armature is wound on a very thin paper shell1 stiff enough to withstand the strain due to winding and subsequent handling4 ,the wire composing the armature is of the smallest gauge consistent with mechanical strength4 ,the field coils1 as before stated1 are circular1 and are placed as near each other as possible1 one on either side of the armature1 with the internal diameter just sufficient to give the necessary clearance for the rotating element4 ,this construction prevents magnetic leakage4 ,ribbon wire is employed for the field coils1 thus economizing space and further carrying out the idea of concentration4 ,the torque of this motor being proportioned to the product of its armature and field currents1 must vary directly as the energy passing through its coils4 ,in order then1 that the motor shall record correctly it is necessary only to provide some means for making the speed proportional to the torque4 ,this is accomplished by applying a load or drag1 the strength of which varies directly as the speed4 ,ques4 ,explain the operation of the ,thompson recording wattmeter4 ,ans4 ,there being no iron in either field or armature of the motor element1 no considerations of saturation are involved4 ,the torque or pull of the armature is dependent upon the product of the field and armature strength4 ,the strength of the field--there being no iron--varies directly with the current in the field4 ,thus the strength of the field with #aj amperes flowing to the load is exactly twice the strength of the field with #e amperes flowing to the load4 ,the strength of the armature is dependent on the voltage of the system to which it is connected1 the armature element of the meter being practically a voltmeter4 ,there is1 therefore1 a torque or pull varying directly with the strength of the armature multiplied by the strength of the field1 or1 in other words1 varying directly with the watt load1 and except in so far as influenced by friction1 the speed of rotation varies directly with the torque or pull4 ,the currents generated in the disc armature consist of eddy currents1 which circulate within the mass of the disc4 ,installation of ,wattmeters4--,the various types of wattmeter differ so widely either in mechanical details1 or operating principles1 that it is customary for manufacturers to furnish detailed instructions for the installation of their meters4 ,such instructions should be carefully followed in all cases1 but the following will be found generally applicable to all types of motor meter3 #a4 ,after unpacking the meter1 and before opening the case or cover1 clean the latter carefully to remove all adhering particles of dust and excelsior4 #b4 ,the proper location for the meter should be one where there is no vibration4 ,when this location has been selected1 nail or screw upon the walls1 a board somewhat larger than the dimensions of the back of the meter1 and upon this board hang the meter by the top hanger4 #c4 ,after hanging the meter1 open or remove the case or cover1 and if necessary1 put the mechanism in order according to instructions furnished by the manufacturer4 #d4 ,in order to operate satisfactorily1 the meter should hang plumb1 so that the spindle of the revolving element will be vertical1 and the horizontal planes through the armature and retarding disc will be level4 ,many complaints relative to meters being slow on light loads1 are invariably due to the fact that the meters have been installed out of plumb,b4 #e4 ,in making the circuit connections1 be very careful that the positive lead or wire is placed in the positive binding post of the meter4 ,this precaution is essential for insuring an accurate and sensitive measurement on small loads4 #f4 ,when a meter of the commutated motor type sparks at the brushes at starting1 it is an indication that the commutator is dusty4 ,clean it with a piece of closely woven cotton tape #a_/d-;inch in width4 #g4 ,meters should never be allowed to remain with their covers off1 in the testing room1 station1 or any other place4 ,in order to get the best service1 and to give them long life they must be kept clean4 ,b ,,note4--,the most practical and accurate method of plumbing a meter is to level it by means of a small brass weight placed upon the retarding disc4 ,place the weight upon the front or back upper surface of the disc1 close to the edge4 ,if the disc and weight rotate toward the right1 move the bottom of the meter in the same direction so as to raise the disc on the right4 ,when the disc is level1 the weight and disc will remain stationary when the weight is placed on either the front or the back of the disc4 ,next1 place the weight on the disc close to the edge on either side4 ,if the disc rotate towards the front1 swing the bottom of the meter away from the wall or board until the disc remains stationary when the weight is placed upon it on either side4 ,if the disc rotate toward the back1 raise it up on that side by bringing the top of the meter away from the wall or board4 ,it is possible that the second levelling operation will alter the position of the disc obtained by the first operation1 therefore1 the first should be repeated1 and after that the second also1 until the disc remains stationary when the weight is placed at any point upon its surface4 ,this method of levelling is more reliable than any method in which a spirit level is employed4 ,illustration3 ,fig4 #feg4--;,interior view of ,thompson watt hour meter 7type ,,cq74 ,the capacities of this type range from #ej to #djj amperes inclusive1 two wire1 and #ej to #bjj amperes inclusive1 three wire1 and for voltages of from #ajj to #fjj volts4 ,these meters are made with either front or back connections4 ,in front connected meters the positions of the leading-in wires and cables are the same as in the type ,c-#f1 fig4 #fed1 so that either type of meter may be installed in the same location4 ,illustration3 ,fig4 #feh4--,specimen record from ,general ,electric recording ammeter4 ,the record is made on a band of specially prepared paper four inches wide and sixty feet in length4 ,on this paper are ruled lines corresponding to time1 and the instrument calibration4 ,the lines ruled across the paper represent time2 those ruled lengthwise represent volts1 amperes1 or watts1 depending upon the instrument construction4 ,this form of paper has the advantage of permitting the use of time divisions of equal length throughout the entire range of the recording pen4 ,the recording pen is attached to the moving element in such a manner that its motion is transmitted in a straight line parallel to the time division on the chart4 ,as the paper is unwound and passed under the recording pen1 it is paid into a space at the bottom of the instrument case4 ,to assist in removing paper1 the instrument is provided with a stripper1 which enables the paper to be torn off evenly and without damage4 ,the paper feeding mechanism is simple4 ,by means of suitable gearing1 the clock drives a drum having peg teeth which engage the holes located near the edge of the paper4 ,these teeth not only feed the paper under the recording pen1 but also give it a definite and accurate position along the axis of the drum4 ,the feeding drum is driven by a friction clutch4 ,illustration3 ,fig4 #fei4--,westinghouse type ,,cw-#f watt hour meter with cover off4 ,this meter is of the commutator type without iron in the magnetic circuit4 ,the spherical armature is closely surrounded by circular field coils which provide the shortest magnetic path and smallest magnetic leakage1 thus securing high torque with small consumption of energy4 ,the armature winding is wound on a hollow sphere of prepared paper which is moulded in corrugated form to secure strength4 ,uniform brush tension is maintained by gravity4 ,each brush consists of two small round wires placed side by side and held against the commutator by a small counterweight whose distance from the fulcrum is adjustable4 ,the current winding consists of two flat coils of strap copper1 one clamped rigidly on either side of the central mounting frame which supports the armature bearings4 ,these coils are connected either in series or parallel1 depending on the capacity4 ,in three wire meters one of the coils is connected in series with each side of the line4 ,the retarding element consists of a light aluminum disc rotating between two pairs of permanent magnets4 ,the magnets are prepared by a special aging process to insure permanence4 ,full load adjustment is made by shifting the position of the permanent magnets4 ,ample light load adjustment or friction compensation is provided by means of the movable coil1 which can be shifted horizontally or radially on loosening one screw4 ,the meter registers directly in kilowatt hours4 ,illustration3 ,fig4 #ffj4--;,interior of ,thompson prepayment watt hour meter4 ,the actuating force is a large flat coil spring enclosed in a barrel or drum to which its outside end is attached4 ,the operating knob winds this main spring by turning the drum4 ,the spring has many turns and as the operation of the device never equals one whole turn1 the spring always exerts a practically constant force4 ,the rate device consists of a small train of gears secured to the front of the frame directly back of the register4 ,each device is marked with the price per kw-hr4 for which it should be used4 ,the switch is of the double pole double break type with leaf contacts4 ,the coin receptacles are placed at the back of the meter4 ,to make an advance payment1 the winding knob is turned so that the arrow points upward4 ,a quarter dollar is then inserted in the slot and the knob turned to the right1 the coin serving as a key which operates the mechanism within the device1 turning the registering wheel and placing the coin to the credit of the customer4 ,if the circuit be open when the coin is deposited the same motion of the knob which moves the registering mechanism closes the circuit switch contained within the case4 ,the dial contains a scale marked in plain figures over which a pointer passes indicating the number of coins remaining to the credit of the depositor4 ,when the first coin is deposited and the knob turned closing the main switch1 the pointer rests opposite the first division on the scale4 ,if a second coin be deposited before the current purchased with the first coin has been consumed1 a second motion of the knob will bring the pointer opposite the second division on the scale4 ,twelve coins can thus be deposited consecutively1 after which the slot is automatically closed and further prepayment cannot be made until the value of one or more coins has been consumed4 ,whenever energy to the value of one coin has been delivered through the meter1 the escapement is mechanically released turning the pointer back one division4 ,this process continues until all the energy has been delivered for which payment has been made4 ,thus the depositor can ascertain at any time how much energy can be obtained without further payment4 ,when all energy has been delivered1 the circuit switch is opened so that no more current can be obtained until one or more coins have been deposited4 ,the indicating mechanism shows only the number of coins which stand to the credit of the customer1 but1 by consulting the meter dial1 one can determine what fractional part of the prepayment next to be cancelled remains to the credit of the customer4 ,a coin or washer larger than the coin for which the device is designed cannot be introduced into the receiving slot and a smaller one will not operate the device4 ,how to test a meter4--,a simple test for ascertaining whether a customer's meter is fast or slow,c1 may be applied as follows3 #a4 ,turn off the lamps and other power consuming devices in the house and then note the reading of the meter dial and the exact time of day2 #b4 ,turn on as quickly as possible about one-tenth of all the lamps in the house and allow them to burn for about two hours2 #c4 ,at the end of two hours1 turn off the lamps as quickly as possible and note the reading of the meter dial4 ,the difference between the first and second readings of the dial will be the indicated consumption of two hours1 and if this be greater than the amount of power that ought to be consumed by the number of lamps turned on1 the meter is fast1 but if it be less1 the meter is slow4 ,the best results obtained by this method are only approximations1 however1 on account of the variations in the watts consumed by the different makes of lamp1 the uncertainty as to the actual voltage on the line at the time of the test1 and the lack of knowledge as to the age of the lamps4 ,therefore1 if the meter test within five per cent41 or do not record more nor less than one-twentieth of the assumed lamp consumption it is safe to assume that the meter is correct as the result of the test is not likely to be any closer to the truth4 ,c ,,note4--,a meter operates under more varied and exacting conditions than almost any other piece of apparatus4 ,it is frequently subjected to vibration1 moisture and extremes of temperature2 it must register accurately on varying voltages and various wave forms2 it must operate for many months without any supervision or attention whatever2 and1 in spite of all these conditions1 it is expected to register with accuracy from a few per cent4 of its rated capacity to a #ej per cent4 overload4 ,as a meter is a type of machine1 its natural tendency is to run slow2 but occasionally1 through accident1 a meter may run fast4 ,when a meter runs fast the consumer is paying a higher rate per kilowatt hour than his contract calls for4 ,he is being discriminated against4 ,the periodic testing of meters is therefore a necessity and is an indication of the honesty of intention of the manager toward the customers of his company4 ,meters controlling a very large amount of revenue may be tested as often as once a month1 while the ordinary run of meters should be tested at least once a year1 once in eighteen months1 or once in two years1 the period varying with different companies1 different types and different civic requirements4 ,commutator type meters1 having comparatively heavy moving elements with consequent rapid increase in friction due to wear on the jewel and bearings1 and a commutator also increasing in friction with age1 must have frequent and expert attention to insure their accuracy under all conditions4 ,ques4 ,how should a roughened commutator be cleaned and smoothed8 ,ans4 ,by means of tape4 ,illustration3 ,fig4 #ffa4--;,internal connections of ,sangamo watt hour meter 7type ,d74 ,a1 copper disc armature1 submerged in mercury2 ,b1 bridge wire between binding posts1 for main load current1 when both sides of the line are carried through the meter2 ,,ct1 compounding series turns around pressure circuit magnet1 building up field as load increases1 to compensate for falling off in speed otherwise found2 ,d1 aluminum damping1 or brake disc1 controlling speed of meter2 ,e1 copper contact ears1 imbedded in insulating wall of mercury chamber1 leading current into and out from armature2 ,f1 hardwood float on armature proportioned to give slight lift to entire moving system1 when armature and float are immersed in mercury2 ,h1 soft steel disc above permanent magnets1 riveted to fine pitch screw working in bracket above1 so that adjustment of the disc up or down gives variation in damping effect of permanent magnets1 and therefore of main speed4 ,k1 clamp slider with thumb screw1 for obtaining light load adjustment by moving ,k to right or left1 as may be necessary4 ,k spans and connects parallel wires of light load adjustment1 ,,br and ,,rr'4 ,,mm1 powerful permanent magnets1 acting on disc ,d1 giving main speed control for meter4 ,n1 high resistance heavy wire1 forming part of series adjustment between armature and any shunt with which meter may be used1 to set drop through meter correct for drop of the shunt4 ,p1 spirally laminated soft steel ring1 moulded in mercury chamber above the armature space1 to act as a return for magnetic lines of force from and to energizing magnet below4 ,r1 resistance card unit1 in series with pressure circuit coils2 in #aaj volt meters one card is used1 in #bbj volt meters two cards1 or one card and a thermocouple4 ,,br1 small brass wire1 connected to ingoing end of pressure circuit coils and forming ,,rr' and the slides ,k the light load adjustment4 ,,rr'1 high resistance wire having opposite ends connected to ears ,,ee by low resistance wires4 ,current energizing the pressure circuit coils ,,sc passes from ,,rr' through ,k to ,,br and thence to the coils1 and if ,k be near the end of ,,rr' and ,,br1 the least compensation is obtained2 if near right end1 maximum light load compensation is obtained4 ,s1 shaft or spindle4 ,in actual meter ,s is divided1 the lower shaft carrying armature ,a1 and the upper shaft damping disc ,d4 ,,sa1 series resistance adjustment1 for setting meter to correct drop for shunt4 ,,sc and ,,sc'1 pressure coils connected in series4 ,,tt1 binding posts at bottom of meter4 ,y1 laminated soft steel yoke1 carrying coils ,,sc and ,,sc'1 and giving a powerful and concentrated magnetic field on the armature4 ,w1 worm1 driving recording dial train4 ,,ww1 worm wheel4 ,illustration3 ,fig4 #ffb4--;,interior view of ,columbia watt hour meter 7type ,d71 showing construction and principal parts and connections4 ,the armature winding consists of three coils approximately circular in shape4 ,the coils are form wound1 interlocked with one another and with the light impregnated fibre disc which serves as a spacer for them4 ,the aluminum damper disc has the conventional anti-creep provision in the shape of the three small soft iron plugs1 mounted close to the central staff1 which the illustration shows4 ,these in their revolution come successively within the influence of an adjustable iron screw which is magnetized by an extension from one of the damper magnets4 ,the angular relationship of the armature windings and of the three iron plugs is such that at the time that the armature is exerting a maximum torque the magnetized screw is exerting the maximum pull to hold back a given plug and conversely when the armature pull is a minimum the magnetic screw is attracting a plug with the maximum effort to cause ahead rotation4 ,the irregularities of torque are in this way smoothed out4 ,the commutator has three segments and is made of chemically pure silver4 ,each brush is formed of a length of phosphor bronze wire bent like a hair pin and secured at its ,u end to a brass sleeve1 which in turn is secured to an insulated stud by a set screw4 ,an extension on the sleeve carries a micrometer screw brush adjustment4 ,the main speed adjustment is secured by providing a soft iron bridge plate1 bridging over the extremities of each magnet end and adjustable1 by means of a set screw and lock nut1 to any desired distance therefrom4 ,this gives a regular micrometer means of varying the effective magnet strength4 ,interposed between the series coil and the permanent magnets is a heavy soft iron shield to guard the magnets against disturbance by short circuiting4 ,light load adjustment is obtained by providing in the coil circuit a series of small resistance spools1 equipped with pin terminals1 to which connection can be selectively made by means of a split bushing terminal on a flexible cord4 ,this series of spools is strung on a metal arbor located within the case4 ,illustration3 ,fig4 #ffc4--;,diagram showing internal connections of the ,duncan watt hour meter4 ,its operation depends upon the principle of the well known electro-dynamometer1 in which the electromagnetic action between the currents in the field coils and an armature produces motion in the latter4 ,it also embodies the other two necessary watt hour meter elements required for the speed control and registration of the revolutions of the armature1 these being embodied in the drag magnet and disc1 and the meter register respectively4 ,the motion of the armature is converted into continuous rotation by the aid of a commutator and brushes1 the commutator being connected to the armature coils and carried on the same spindle therewith4 ,waste of ,electricity in ,lighting4--,in large residences where a good many servants are employed or in any place where the power consumed is not directly under the supervision of the person who must pay the bills1 a great deal of waste usually occurs4 ,if the meter be read before retiring1 the reading in the morning will show how much energy was consumed during the night1 which will show in turn how many lamps were burning all night4 ,a great deal of light can be saved by placing the lamps so that they will throw the light where it is needed and by placing small lamps such as #h candle power and #d candle power in places where not much light is needed1 such as bathrooms1 halls1 cellars1 etc4 ,when the lamps get old and dim they should be replaced with new ones1 as it costs about the same to burn an old lamp as a new one4 ,an old #af candle power lamp which is very dim will give only about #h candle power and use about as much current as is required for a new #af candle power4 ,if the dim light be light enough1 it should be replaced by an #h candle power lamp1 which will not consume as much power as the old #af candle power4 ,,chapter ,,xxix ,,operation ,,of ,,dynamos ,before ,starting a ,dynamo or ,motor4--,when the machine has been securely fixed1 it should be carefully examined to see that all parts are in good order4 ,the examination should be made as follows3 #a4 ,the field magnet circuit should first be inspected to see that none of the wires or connections have broken or have become loose1 and that the coils are correctly connected2 #b4 ,the caps of the bearings should be taken off1 and these and the journals carefully cleaned of all grit and dirt4 ,they should then be oiled1 and the caps replaced and screwed up by hand only2 #c4 ,the gaps between the outer surface of the armature and the polar faces should be examined in order to ascertain whether any foreign body1 such as a small screw or nail has lodged therein4 ,if such be the case1 it should be carefully removed with a bit of wire2 #d4 ,the guard plates protecting the armature windings should be removed1 and the windings carefully inspected by slowly rotating the armature1 to see that they are not damaged1 and that the insulation is perfect4 ,the armature should then be finally rotated by hand to see that it revolves freely1 and that the bearings are securely fixed2 #e4 ,the commutator should be examined to see that it is not damaged in any way through one or more of the segments being knocked in1 or the lugs being forced into contact with one another2 #f4 ,the brush holders and brushes should be inspected to see that the former work freely on the spindle1 and that the hold off catches work properly1 are clean and make good contact with the brush holders or flexible leads2 #g4 ,having ascertained that the machine is not injured in any way1 and that the armature revolves freely1 the brushes should be adjusted4 ,in the subsequent working of the dynamo it will of course be unnecessary to follow the whole of these proceedings every time the machine is started1 as it is extremely unlikely that the machine will be damaged from external causes while working without the attendant being aware of the fact4 ,adjusting the ,brushes4--,the adjustment of the brushes upon the commutator requires careful attention if sparking is to be avoided4 ,there are two adjustments to be made3 #a4 ,for pressure2 ,the brushes must bear against the commutator segments with sufficient pressure for proper contact4 #b4 ,for lead4 ,the brushes must have the proper angular advance 7positive or negative1 according as the machine is a dynamo or motor7 to prevent sparking4 ,ques4 ,at what point on the commutator should the brushes bear8 ,ans4 ,the points upon the commutator at which the tips of the brushes 7carried by opposite arms of the rocker7 bear1 should be1 in bipolar dynamos1 at opposite extremities of a diameter4 ,in multipolar dynamos the positions vary with the number of poles and the nature of the armature winding4 ,ques4 ,what provision is made to facilitate the correct setting of the brushes8 ,ans4 ,setting marks are usually cut in the collar of the commutator next to the bearing4 ,illustration3 ,figs4 #ffd and #ffe4--;,diagrams illustrating how to set brushes4 ,some brush holders require brushes set with the direction of rotation of the commutator1 and others1 set against the direction of rotation4 ,in fig4 #ffd is shown a brush holder of the first class1 which must always be set as indicated by the arrow4 ,if set in the opposite direction1 trouble will ensue1 as an inspection of the figure will show1 because the surface of the commutator and the brush would form a toggle joint1 and the brush would tend to dig into the commutator and either break itself or bend the brush rigging4 ,in fig4 #ffe is shown a brush holder of the second type4 ,this brush is set against the direction of rotation1 but an inspection of the cut will show that there is1 in this case1 no tendency for the brush to dig into the commutator surface4 ,each type of brush holder1 of which there are several1 should be adjusted as recommended by the manufacturer to secure proper working4 ,ques4 ,how are the brushes set by these marks8 ,ans4 ,the tips of all the brushes carried by one arm of the rocker are set in correct line with the commutator segments marked out by one setting mark1 and the tips of the brushes carried by the other arm or arms are set in correct line with the segments marked out by the other mark or marks4 ,if one or more of the brushes in a set be out of line with their setting mark1 it will be necessary to adjust the brushes up to this mark by pushing them out or drawing them back1 as may be required1 afterwards clamping them in position4 ,when adjusting the brushes1 the armature should always be rotated1 so that the setting marks are horizontal4 ,the rocker can then be rotated into position1 and the tips of both sets of brushes conveniently adjusted to their marks4 ,in those brush holders provided with an index or pointer for adjusting the brushes1 the setting marks upon the commutator are absent1 length of the pointer being so proportioned that when the tips of the brushes are in line with the extreme tips of the pointers1 the brushes bear upon the correct positions on the commutator4 ,illustration3 ,fig4 #fff4--,method of soldering cable to carbon brush4 ,drill a hole in the end1 also in the side of the brush1 as shown in the sketch1 and after thoroughly tinning the pig-tail1 place it in the end hole and fill the holes up with solder through the side hole4 ,another method is to drill a hole through the carbon so that the cable will just slip through1 countersink the edge of the hole a little1 clean the cable thoroughly and pass it through the hole4 ,then with any good flux and solder1 fill the countersunk part on both sides4 ,ques4 ,what should be done after adjusting the brushes to their correct positions upon the commutator8 ,ans4 ,their tips or rubbing ends should be examined while in position to see that they bed accurately on the surface of the commutator4 ,in many instances it will be found that this is not the case1 the brushes sometimes bearing upon the point or toe1 and sometimes upon the heel1 so that they do not make contact with the commutator throughout their entire thickness and width4 ,the angle of the rubbing ends will therefore need to be altered by filing to make them lie flat4 ,ques4 ,how is the proper brush contact secured8 ,ans4 ,when the brushes do not bed properly they should be refitted to secure proper contact4 ,ques4 ,how is the pressure adjustment made8 ,ans4 ,this is effected by regulating the tension of the springs provided for the purpose upon the brush holders4 ,ques4 ,with what pressure should the brushes bear against the commutator8 ,ans4 ,the tension of the springs should be just sufficient to cause the brushes to make a light yet reliable contact with the commutator4 ,the contact must not be too light1 otherwise the brushes will vibrate1 and thus cause sparking2 nor must it be too heavy1 or they will press too hard upon the commutator1 grinding1 scoring and wearing away the latter and themselves to an undesirable extent1 and moreover1 giving rise to heating and sparking4 ,the correct pressure is attained when the brushes collect the full current without sparking1 while their pressure upon the commutator is just sufficient to overcome ordinary vibration due to the rotation of the commutator4 ,illustration3 ,figs4 #ffg to #ffi4--,method of winding cables with marlin4 ,when connecting the feeders and dynamo and service leads to a switchboard1 the wires are often served with marlin4 ,by serving is meant to tightly wrap the wires of each set together with marlin4 ,a tool for serving may be made as in fig4 #ffg1 using a piece of oak #b ins4 wide1 #g_/h in4 thick and #ad ins4 long1 having four holes drilled through it1 as shown4 ,the marlin is passed through the holes1 commencing at the hole nearest the handle1 the object being to cause a strain on the marlin at the point where it passes around the wire1 so that the marlin may be wrapped tightly4 ,it is necessary to serve the first four or five inches by hand1 pushing the winding into the conduit as far as possible4 ,this acts as an additional protection to the wires where they leave the conduit4 ,the serving is continued1 as in fig4 #ffh1 to within four or five inches of the first lug by means of the serving tool1 passing the ball of marlin around the wires with the serving tool4 ,the wires are then bent in shape1 as in fig4 #ffi4 ,to serve the wires properly it is necessary to tie the ends of the wires taut4 ,the wires should be straightened and run together so as to be parallel1 being bound with tape at different points to keep them so4 ,when the serving is complete the marlin should be thoroughly painted with a moisture resisting compound4 ,the marlin serving will stiffen the wires and they can be bent very neatly to avoid touching the bus bars of the board4 ,when painted the marlin hardens so that it is difficult to bend the wires after the paint has dried4 ,it then requires a strong pressure to bend them4 ,the marlin acts as an additional insulation and mechanical protection to the wires1 and while no harm would result from the wires coming in contact with the bars while thus protected1 it looks better to bend them so as to avoid touching the bars4 ,direction of ,rotation4--,this is sometimes a matter of doubt and often results in considerable trouble4 ,as a general rule1 a dynamo is intended to run in a certain direction2 either right handed or left handed according to whether the armature1 when looked at from the pulley end1 revolves with or against the direction of the hands of a clock4 ,dynamos are usually designed to run right handed1 but the manufacturers will make them left handed if so desired4 ,it may be necessary to reverse the direction of rotation of a dynamo1 if the driving pulley to which it has to be connected happen to revolve left handed1 or if it be necessary to bring the loose side of the belt on top of the pulley1 or to place the machine in a certain position on account of limited space4 ,the direction of rotation of ordinary series1 shunt1 or compound bipolar dynamos may be reversed by simply reversing the brushes without changing any of the connections1 then changing the point of contact of the brush tips #ahj4 ,in multipolar dynamos1 a similar change1 amounting to #ij for a four pole machine1 and #de for an eight pole machine1 will reverse their direction of rotation4 ,it will be understood that under these conditions1 the original direction of the current and the polarity of the field magnets will remain unchanged4 ,this rule does not apply to arc dynamos and other machines1 which have to be run in a certain direction only1 in order to suit their regulating devices4 ,if the direction of current generated by a dynamo be opposite to that desired1 the two leads should be reversed in the terminals1 or the residual magnetism should be reversed by a current from an outside source4 ,illustration3 ,fig4 #fgj4--,method of assembling core discs4 ,for this operation two wooden horses should be provided to support the core at a convenient height1 as shown in the illustration4 ,starting a ,dynamo4--,having followed the foregoing instructions1 all keys1 spanners1 bolts1 etc41 should be removed from the immediate neighborhood of the machine1 and the dynamo started4 ,illustration3 ,figs4 #fga and #fgb4--,tinning block for electric soldering tool4 ,it is made with two soft bricks4 ,one brick is used to support the soldering tool1 and the other to contain the tinning material and to furnish a material which will keep the copper bit bright enough to receive its coating of tin4 ,fig4 #fga represents a section of the tinning brick1 which is scooped out on top as shown by the lower line4 ,into one end of the hollow in the brick1 some sal-ammoniac is placed to help tin the copper bit4 ,sal-ammoniac is a natural flux for copper and aids greatly in keeping the tool well tinned4 ,next1 some melted solder is run into the hollow of the brick1 and lastly enough resin to fill the cavity nearly to the top4 ,when the tool is not in use1 the electricity is switched off and the tool permitted to lie in the resin4 ,if it be desired to repair the tin coating a little when the tool is in use1 the latter is rubbed on the brick below the layer of solder1 and the layer of resin4 ,if the tool be in very bad condition1 it may be pushed into the sal-ammoniac once or twice and then rubbed in the solder again4 ,it requires but little heat to keep the brick and its contents ready for use4 ,in fact1 the brick is a fair non-conductor of heat and prevents the escape of heat from one side of the tool4 ,when momentarily not in use1 the tool remains in the solder which becomes melted underneath the layer of resin4 ,when the copper bit becomes too hot1 it will begin to volatilize the resin1 thus calling attention to this fact1 whereupon1 the electricity should be turned off from the tool4 ,ques4 ,how should a dynamo be started8 ,ans4 ,a dynamo is usually brought up to speed either by starting the driving engine1 or by connecting the dynamo to a source of power already in motion4 ,in the first case1 it should be done by a competent engineer1 and in the second case by a person experienced in putting on friction clutches to revolving shafts1 or in slipping on belting to moving pulleys4 ,illustration3 ,fig4 #fgc4--;,connections for two shunt wound dynamos to run in parallel4 ,the positive lead of each machine is connected to the same bus bar4 ,in starting1 if but one machine is to be used1 the dynamo is first brought up to speed and the voltage regulated by means of the rheostat ,r and the voltmeter ,v4 ,the main switch is then thrown in4 ,the connections for the field are taken off the dynamo leads so that the opening of the main switch will not open the field circuit and for this reason the field will begin to build up as soon as the machine is started4 ,when but one of the machines is running1 the idle machine is brought up to speed with the main switch open1 and the voltage regulated by means of the rheostat and voltmeter until the voltages of the machines are the same4 ,then the main switch is thrown in and the load on the machines 7which is ascertained by the ammeters7 is equalized by means of the rheostats4 ,should there be any great difference in voltages1 the higher one will run the other as a motor without changing the direction of rotation4 ,the field current will remain unchanged1 and the armature current of the low dynamo will be reversed1 which will cause it to run as a motor in the same direction as it ran as a dynamo4 ,when dynamos feeding current to motors are to be shut down1 the switches on the motors should first be opened4 ,otherwise some of the motor fuses will blow4 ,as the voltage goes down the motors will draw more current to do the work4 ,if a plant be shut down with the motor switches in it will generally be found impossible to start a shunt dynamo1 the low resistance in the mains not allowing enough current to flow around the shunt fields to energize them4 ,ques4 ,should the brushes be raised out of contact in starting8 ,ans4 ,the brushes should not be in contact in starting if there be any danger of reverse rotation1 as might happen when the dynamo is driven by a gas engine4 ,aside from this1 it is desirable that the brushes be in contact1 because they are more easily and better adjusted1 and the voltage will come up slowly1 so that any fault or difficulty will develop gradually and can be corrected1 or the machine stopped before any injury is done4 ,illustration3 ,fig4 #fgd4--;,connections for two shunt dynamos to run on the three wire system4 ,the two machines are connected in series1 three wires being carried from them1 one from the outside pole of each machine and one from the junction of the two machines4 ,the voltage between the outside wires is equal to the combined voltage of the two machines and the voltage between the outside and the central or neutral wire is equal to the voltage of the corresponding machine4 ,if the load on each side of the system be equal1 there will be no current in the neutral wire1 while if the loads be unequal1 the neutral wire will have to carry the difference in current between the two outside wires4 ,ques4 ,how should a series machine be started8 ,ans4 ,the external circuit should be closed1 otherwise a closed circuit will not be formed through the field magnet winding and the machine will not build up4 ,ques4 ,what is understood by the term build up8 ,ans4 ,in starting1 the gradual voltage increase to maximum4 ,illustration3 ,fig4 #fge4--;,connections for two compound wound dynamos to run in parallel4 ,the series fields of the machines are connected together in parallel by means of wire leads or bus bars1 which connect together the brushes from which the series fields are taken4 ,this is known as the equalizer and is shown by the line running to the middle pole of the dynamo switch4 ,by tracing out the series circuits it will be seen that current from the upper brush of either dynamo has two connections to its bus bar4 ,one of these leads through its own field1 and the other1 by means of the equalizer bar1 through the fields of the other dynamo4 ,as long as both machines are generating equally there is no difference of pressure between the brushes of either1 but should the voltage of one be lowered1 current from the other would flow through its fields and thereby raise the voltage1 and at the same time reduce its own until both are equal4 ,the equalizer may then be called upon to carry much current1 but to have the machines regulate closely it should be of low resistance4 ,it may also be run as shown by the dotted lines1 but this will leave all the machines alive when any one is generating4 ,the ammeters should be connected as shown4 ,if they were on the other side they would come under the influence of the equalizing current and would indicate wrong1 either too high or too low4 ,the equalizer switch should be closed a little before the main switches are closed4 ,ques4 ,how should a shunt or compound machine be started8 ,ans4 ,all switches controlling the external circuits should be opened1 as the machine excites best when this is the case4 ,if the machine be provided with a rheostat or hand regulator and resistance coils1 these latter should all be cut out of circuit1 or short circuited1 until the machine excites1 when they can be gradually cut in as the voltage rises4 ,when the machine is giving the correct voltage1 as indicated by the voltmeter or pilot lamp1 the machine may be switched into connection with the external or working circuits4 ,ques4 ,in starting a shunt dynamo1 should the main line switch be closed before the machine is up to voltage or after8 ,ans4 ,if the machine be working on the same circuit with other machines1 or with a storage battery1 it is1 or course1 necessary to make the voltage of the machine equal to that on the line before connecting it in the circuit4 ,if the machine work alone1 the switch may be closed either before or after the voltage comes up4 ,the load will be thrown on suddenly if the switch be closed after the machine has built up its voltage1 thus causing a strain on the belt1 and possibly drawing water over the engine cylinder4 ,on the other hand1 if the switch be closed before the voltage of the machine has come up1 the load is picked up gradually1 but the machine may be slow or may even refuse to pick up at all4 ,ques4 ,why does a shunt machine pick up more slowly if the main switch be closed first8 ,ans4 ,because the resistance of the main line is so much less than that of the field that the small initial voltage due to the residual magnetism causes a much larger current in the armature than in the shunt field4 ,if this be too large1 the cross and back magnetizing force of the armature weakens the field more than the initial field current strengthens it1 and so the machine cannot build up4 ,ques4 ,if a shunt dynamo will not pick up1 what is likely to be the trouble8 ,ans4 ,the speed may be too slow2 the resistance of the external circuit may be too small2 the brushes may not be in proper position2 some of the electrical connections in the dynamo may be loose1 broken or improperly made2 the field may have lost its residual magnetism4 ,illustration3 ,figs4 #fgf and #fgg4--;,diagrams of ground detectors4 ,fig4 #fgf1 a ground detector switch suitable for mounting on a switch board4 ,the two arms pivoted at their upper ends are connected with an insulating bar ,a and make contact at their lower ends with two brass strips and a contact button1 which are connected to the bus bars and ground1 respectively4 ,when the arms are moved to the left1 the positive bus bar is connected to the ground through the voltmeter ,v4 ,in fig4 #fgg is another form of ground detector4 ,this is known as a lamp ground detector4 ,on a #aaj volt system two ordinary lamps are connected in series1 while the line connecting the lamps is connected to the ground through a snap switch ,s4 ,when current is on1 the two lamps will burn with equal brilliancy1 but at a lower candle power4 ,when the switch ,s is closed1 if the two lines be clear1 the brilliancy of the lamps will not be affected1 but if there be a ground on the positive side1 one lamp will burn brighter1 the brightness depending on the resistance of the ground4 ,if there be a dead ground1 the lamp will burn to its full candle power4 ,ques4 ,what is the indication that the connections between the field coils and armature are reversed8 ,ans4 ,if the machine build up when brought to full speed1 the connections are correct1 but if it fail to build up1 the field coils may be improperly connected4 ,illustration3 ,fig4 #fgh4--,method of correcting reversed polarity in large shunt dynamo by transposing the shunt field leads1 and then starting up the machine4 ,as soon as the voltmeter registers any voltage1 the dynamo may be stopped and the field leads restored to their original position1 when it will be found that the residual magnetism in the pole pieces will usually bring the dynamo up to its polarity and proper voltage4 ,this method has the disadvantages1 of the uncertainty as to the machine building up1 and that a temporary wire must probably be run from the switchboard to one terminal of the field circuit1 which is usually connected to a terminal back of the dynamo frame1 so that the flow of current through the field coils may be reversed4 ,with dynamos having laminated field magnet cores of comparatively low residual magnetism1 this method may suffice1 but in the case of solid field magnetic cores it is not practical4 ,a better method is to disconnect the shunt field leads and temporarily extend them to some other source of direct current4 ,if the current be of higher voltage than the coils are designed for1 as for instance #aaj volt dynamo and available current #ejj volt1 caution must be exercised and a suitable resistance be provided to protect the coils4 ,a #ejj volt coil1 however1 may be supplied from #aaj volt circuit1 providing the field winding to be energized is equipped with a cut off switch having a discharge resistance1 so that it may be used to close and break the circuit when the temporary leads have been connected4 ,if the field windings be not so provided1 a bank of lamps or some other non-inductive resistance must be connected across the leads between the field magnet coils and the point at which the circuit is to be opened and closed4 ,this is to provide a path for the discharge of the induced electromotive force4 ,the circuit should not remain closed more than a few seconds if the full voltage can be applied4 ,it is well1 however1 to leave the current on long enough to run the machine up to about half speed and make sure1 by means of a voltmeter1 that the polarity has been corrected4 ,when this has been ascertained the dynamo should be stopped and the field winding leads returned to their proper terminals4 ,then the voltage will be brought up in the right direction1 provided the work has been done correctly4 ,this can be tested by connecting a voltmeter across the terminals of the armature1 or by means of a magnetic needle placed at a short distance from one of the pole pieces in such a position that it does not point to the north pole4 ,if the field coils be improperly connected1 the current due to the initial voltage will weaken the field magnetism and thus prevent the machine building up1 and when the field circuit is closed the voltmeter reading will be reduced1 or the magnetic needle will be less strongly attracted4 ,ques4 ,what will be the result if the connections of some of the field coils of a dynamo be reversed8 ,ans4 ,if one-half the number of coils oppose the other half1 the field magnetism will be neutralized and the machine will not build up at all2 but if one of the coils be opposed to the others1 the machine might build up1 but the generated voltage will be low1 and there will be considerable sparking at some of the brushes4 ,ques4 ,how may it be ascertained which coil is reversed8 ,ans4 ,in all dynamos there should be an equal number of positive and negative poles1 and in almost all of them the poles should be alternately positive and negative4 ,therefore1 if a pocket compass be brought near the pole pieces1 and it show that there are more poles of one kind than the other1 the indication is that one or more of the coils are reversed1 and the improper sequence of alternation will determine which one is wrongly connected4 ,ques4 ,when a dynamo loses its residual magnetism1 how can it be made to build up8 ,ans4 ,by temporarily magnetizing the field4 ,to do this a current is passed through it from another dynamo1 or from the cells of a small primary battery4 ,usually1 this will set up sufficient initial magnetism to allow the machine to build up4 ,the battery circuit should be broken before the machine has built up to full voltage4 ,ques4 ,what should be done if a dynamo become reversed by a reversal of its field magnetism due to lightning1 short circuit1 or otherwise8 ,ans4 ,the residual magnetism should be reversed by a current from another dynamo1 or from a battery2 but if this be not convenient1 the connections between the machine and the line should be crossed so that the original positive terminal of the dynamo will be connected to the negative terminal of the line1 and vice versa4 ,illustration3 ,fig4 #fgi4--,method of correcting reversed polarity in compound wound dynamo4 ,the polarity may be reversed without disconnecting or changing the wire4 ,the figure shows two compound dynamos1 and essential connections4 ,the current from any machine connected to the equalizer bar by its equalizer switch will divide1 a portion going through the series field winding of the other machines connected to the bus bar1 the division being determined by the resistance of the different sets of coils4 ,for instance1 assume that ,no4 #a dynamo has had its polarity reversed and that ,no4 #b is running connected to the bus bar4 ,the method of reversing the polarity of ,no4 #a machine is as follows3 ,no4 #a machine should be at rest and then make sure that the circuit breaker and negative switch are open and that any other special connections to other machine or station lighting circuits are open4 ,then close the positive and equalizer switches1 thus allowing a part of the current from the other dynamo to pass through the equalizer connection and through the series field winding of ,no4 #a machine in the usual direction1 which will magnetize the magnetic core4 ,if ,no4 #a machine be a large unit and ,no4 #b a small unit1 it will be necessary to cut out the resistance of the shunt field circuits by means of the rheostat1 if it be desired to maintain its bus bar voltage at its normal point4 ,this will rob the series winding of any other machines which may be connected to the bus bars and will lower the voltage slightly4 ,no4 #a machine is then brought up to full speed when it will be found to have recovered its correct polarity4 ,the positive switch may be readily opened1 watching the bus bar voltage closely as it will rise when the current is restricted again to the series field winding of the other machines4 ,the dynamo will then be ready to cut in with the other machines as soon as the voltage has been brought up to the proper point1 or it may be shut down until required4 ,ques4 ,can a dynamo be reversed by reversing the connections between the field coils and the armature8 ,ans4 ,no1 for if these connections be reversed1 the machine will not build up4 ,ques4 ,will a dynamo build up if it become reversed8 ,ans4 ,yes4 ,ques4 ,then what is the objection to a reversed dynamo8 ,ans4 ,since the direction of current of a reversed dynamo is also reversed1 serious trouble may occur if it be attempted to connect it in parallel1 with other machines not reversed4 ,attention while ,running4--,when a dynamo is started and at work1 it will need a certain amount of attention to keep it running in a satisfactory and efficient manner4 ,the first point to be considered is the adjustment of the brushes4 ,if this be neglected1 the machine will probably spark badly1 and the commutator and brushes will frequently require refitting to secure good contact4 ,ques4 ,what may be said with respect to the lead of the brushes8 ,ans4 ,the lead in all good dynamos is very small1 and varies with the load and class of machine4 ,the best lead to give to the brushes can in all cases be found by rotating the rocker and brushes in either direction to the right or left of the neutral plane until sparking commences1 increasing with the movement4 ,the position midway between these two points is the correct position for the brushes1 for at this position the least sparking occurs1 and it is at this position that the brushes should be fixed by clamping the rocker4 ,illustration3 ,fig4 #fhj4--,method of taking temperature4 ,in taking the temperature of a hot part1 it is convenient to use a thermometer in which the scale of degrees has been etched on the stem4 ,bind this to the heated part1 having first taken the precaution to cover the bulb with waste to prevent the radiation of heat and take the reading when the column of mercury has ceased to rise4 ,the question which most often presents itself to the attendant is how hot can the various parts of a dynamo or motor become and yet be within the safe limit4 ,the degree of heat can be determined by applying the hand to the various parts4 ,if the heat be bearable it is entirely harmless1 but if the heat become unbearable to the hand for more than a few seconds1 the safety limit has been reached and the machine should be stopped and the fault located4 ,of course when the solder begins to melt at the commutator connections and shellac begins to fry out of the armature and an odor of burnt cotton begins to pervade the air1 the safe limit has been far exceeded1 and in most cases1 as a matter of fact serious damage is the result4 ,to be more definite1 no part of the dynamo or motor should be allowed to rise in temperature more than #hj degrees ,f4 above the temperature of the surrounding air1 excepting in the case of commutators where no solder has been used to connect the leads4 ,these can be allowed to rise to a still higher temperature4 ,ques4 ,how does the lead vary in the different types of dynamo8 ,ans4 ,in series dynamos giving a constant current1 the brushes require practically no lead4 ,in shunt and compound dynamos the lead varies with the load1 and therefore the brushes must be rotated in the direction of rotation of the armature with an increase of load1 and in the opposite direction with a decrease of load4 ,in cases where the dynamos are subjected to a rapidly varying or fluctuating load1 it is of course not possible to constantly shift the brushes as the load varies1 therefore the brushes should be fixed in the positions where the least sparking occurs at the moment of adjustment4 ,if at any time violent sparking occur1 which cannot be reduced or suppressed by varying the position of the brushes by rotating the rocker1 the machine should be shut down at once1 otherwise the commutator and brushes are liable to be destroyed1 or the armature burnt up4 ,this especially refers to high tension machines4 ,ques4 ,what should be done if the brushes begin to spark excessively8 ,ans4 ,first1 look at the ammeter to see if an excessive amount of current is being delivered2 second1 see if the brushes make good contact with the commutator1 and if the latter have a bar too high1 or too low1 and an open circuit4 ,illustration3 ,figs4 #fha and #fhb4--,remedies for leakage of oil from self-oiling bearings4 ,if there be sufficient space1 a metal ring may be attached to the shaft as in fig4 #fha4 ,with this arrangement the high speed of the shaft will carry the oil outside of the ring and throw it off in the oil reservoir4 ,another way is to insert a tin apron1 as shown in fig4 #fhb at ,t1 which will serve to drain the oil which may creep along the shaft1 and also cut off the draft from the pulley which may suck the oil out of the bearing4 ,sometimes a tin fan is attached to the pulley1 which tends to drive the oil back into the bearing1 and which also assists in keeping the box cool4 ,ques4 ,what should be done if the current be excessive8 ,ans4 ,if the current exceed the rated capacity by more than #ej per cent41 and continue for more than a few minutes1 the main switch should be opened1 otherwise the machine may be seriously injured4 ,ques4 ,how does an excessive current injure a dynamo8 ,ans4 ,by causing it to overheat which destroys the insulation of the armature1 commutator1 etc4 ,lubrication4--,the shaft bearings of dynamos may be lubricated by sight feed oilers or oil rings4 ,the latter method is almost universally used4 ,an oil well is provided in the hollow casting of the pedestals as shown in fig4 #gbh4 ,oil rings revolve with the shaft and feed the latter with oil1 which is continuously brought up from the reservoir below4 ,the dirt settles to the bottom and the upper portion of the oil remains clear for a long period1 after which it is drawn off through the spigot and a fresh supply poured in through openings provided in the top4 ,the latter are often located directly over the slots in which the rings are placed1 so that the bearings can be lubricated directly by means of an oil cup1 if the rings fail to act or the reservoir become exhausted4 ,illustration3 ,fig4 #fhc4--;,imaginative view of a shaft showing its rough granular structure4 ,in operation these minute irregularities interlock and act as a retarding force1 or frictional resistance4 ,hence1 the necessity for lubrication--a lubricant presents a thin intervening film against which the surfaces rub4 ,ques4 ,what kind of oil can should be used in filling the reservoir1 or oil cups8 ,ans4 ,one made of some non-magnetic material such as copper1 brass1 or zinc4 ,if iron cans be used1 they are liable to be attracted by the field magnets1 and thus possibly catch in the armature4 ,ques4 ,what is the indication of insufficient lubrication8 ,ans4 ,the bearings become unduly heated4 ,ques4 ,what precaution should be taken with new dynamos8 ,ans4 ,they are liable to heat abnormally and for the first few days they should be carefully watched and liberally supplied with oil4 ,after a dynamo has been running for a short time under full load1 its armature imparts a certain amount of heat to the bearings1 a little more also to the bearing on the commutator end of shaft2 beyond this there is no excuse for excessive heating4 ,the latter may result from various causes1 some of which are given with their remedies1 as follows3 #a4 ,a poor quality of oil1 dirty or gritty matter in the oil2 #b4 ,journal boxes too tight2 #c4 ,rough journals1 badly scraped boxes2 #d4 ,belt too tight2 #e4 ,bearings out of line2 #f4 ,overloaded dynamo2 #g4 ,bent armature shaft4 ,ques4 ,what is the allowable degree of heating8 ,ans4 ,it may be taken as a safe rule that no part of a working dynamo should have a temperature of more than #hj ,fahr4 above that of the surrounding air4 ,accordingly1 if the temperature of the engine room be noted before applying the thermometer to the machine1 it can at once be seen if the latter be working at a safe temperature4 ,in taking the temperature1 the bulb of the thermometer should be wrapped in a woolen rag4 ,the screws and nuts securing the different connections and cables should be examined occasionally1 as they frequently work loose through vibration4 ,illustration3 ,fig4 #fhd4--;,diagram illustrating forces acting on a dynamo armature4 ,in the figure the normal field magneto-motive force is in the direction of the line #a1 #b1 produced by the field circuit ,g1 if there were no current in the armature4 ,but as soon as the armature current flows1 it produces the opposing force #c1 #d1 which must be combined with #a1 #b to give the resulting force to produce magnetism and hence voltage4 ,the resultant #a1 #e1 if #c1 #d be large enough1 does not differ much from the original force #a1 #b4 ,or1 expressed in a more physical way1 the brushes ,e1 ,f1 rest on the commutator and all the turns embraced by twice the angle #f1 #c1 ,f1 oppose the flow of flux through the armature core as well as all the turns embraced by twice the angle1 #g1 #c1 ,e4 ,the remaining turns distort the flux1 making the pole corners at ,a and ,b denser1 and at ,c and ,d rarer4 ,so that all the effect is to kill an increase of flux1 or voltage4 ,this cross magnetism tends also to decrease the flow of flux1 for the extra ampere turns required to force the flux through the dense pole tips are greater than the decreased ampere turns relieved by the reduction of flux at the other pole tips2 this follows1 since iron as it increases in magnetic density requires ampere turns greater in proportion than the increase of flux4 ,instructions for ,stopping ,dynamos4--,when shutting down a machine1 the load should first be gradually reduced1 if possible1 by easing down the engine2 then when the machine is supplying little or no current1 the main switch should be opened4 ,this reduces the sparking at the switch contacts1 and prevents the engine racing4 ,when the voltmeter almost indicates zero1 the brushes should be raised from contact with the commutator4 ,this prevents the brushes being damaged in the event of the engine making a backward motion1 which it often does1 particularly in the case of a gas engine4 ,on no account1 however1 should the brushes be raised from the commutator while the machine is generating any considerable voltage2 for not only is the insulation of the machine liable to be damaged1 but in the case of large shunt dynamos1 the person lifting the brushes is liable to receive a violent shock4 ,ques4 ,what attention should the machine receive after it has been shut down8 ,ans4 ,it should be thoroughly cleaned4 ,any adhering copper dust1 dirt1 etc41 should be removed from the armature by dusting with a stiff brush1 and the other portions of the machine should be thoroughly cleaned with linen rags4 ,waste should not be used1 as it is liable to leave threads or fluff on the projecting parts of the machine1 and on the windings of the armature1 which is difficult to remove4 ,ques4 ,what attention should be given to the brushes and brush gear8 ,ans4 ,they should be examined and thoroughly cleaned4 ,if necessary the brushes should be refitted and readjusted4 ,all terminals1 screws1 bolts1 etc41 should be carefully cleaned and screwed up ready for the next run4 ,the brush holders should receive special attention1 as when dirty1 they are liable to stick and cause sparking4 ,all dirt and oil should be removed from the springs1 contacts1 pivots1 and other working parts4 ,it is advisable at stated intervals to entirely remove the brush holders from the rocker arms1 and give them a thorough cleaning by taking them to pieces1 and cleaning each part separately with emery cloth and benzoline or soda solution4 ,another point to which particular attention should be given is the cleaning of the brush rocker4 ,this being composed wholly of metal1 and the two sets of positive and negative brushes being only separated from it by a few thin insulating washers1 it follows that if any copper dust given off by the brushes be deposited in the neighborhood of these washers1 there is considerable liability for a short circuit of the machine to occur by the dust bridging across the insulation4 ,ques4 ,what further attention should be given8 ,ans4 ,it is a good plan1 when the machine has been thoroughly cleaned and all connections made secure1 to occasionally test the insulation of the different parts4 ,if a record be kept of these tests1 any deterioration of the insulation can at once be detected1 localized and remedied before it has become sufficiently bad to cause a breakdown4 ,as a means of protecting the machine from any moisture1 dirt1 etc41 while standing idle1 it is advisable to cover it with a suitable waterproof cover4 ,,chapter ,,xxx ,,coupling ,,of ,,dynamos ,series and ,parallel ,connections4--,when it is necessary to generate a large and variable amount of electrical energy1 as must be done in central generating stations1 apart from the question of liability to breakdown1 it is neither economical nor desirable that the whole of the energy should be furnished from a single dynamo4 ,since the efficiency of a dynamo is dependent upon its output at any moment1 or the load at which it is worked 7the efficiency varying from about #ie per cent4 at full load to #hj per cent4 at half load71 it is advisable in order to secure the greatest economy in working1 to operate any dynamo as near full load as possible4 ,under the above circumstances1 when the whole of the output is generated by a single dynamo this can evidently not be effected1 for the load will naturally fluctuate up and down during the working hours1 as the lamps1 motors1 etc41 are switched into and out of circuit2 hence1 although the dynamo may be working at full load during a certain portion of the day1 at other times it may probably be working below half load1 and therefore the efficiency and economy in working in such an arrangement is very low4 ,ques4 ,how is maximum efficiency secured with variable load8 ,ans4 ,it is usual to divide up the generating plant into a number of units1 varying in size1 so that as the load increases1 it can either be shifted to machines of larger size1 or when it exceeds the capacity of the largest dynamo1 the output of one can be added to that of another1 and thus the dynamos actually at work at any moment can be operated as nearly as possible at full load4 ,ques4 ,what should be noted with respect to connecting one dynamo to another8 ,ans4 ,it is necessary to take certain precautions 7as later explained7 in order that the other dynamos may not be affected by the change1 and that they may work satisfactorily together4 ,ques4 ,what are the two methods of coupling dynamos8 ,ans4 ,they are connected in series1 or in parallel4 ,in coupling dynamos in series1 the current capacity of the plant is kept at a constant value1 while the output is increased in proportion to the pressures of the machines in circuit4 ,when connected in parallel1 the pressures of all the machines are kept at a constant value1 while the output of the plant is increased in proportion to the current capacities of the machines in circuit4 ,coupling ,series ,dynamos in ,series4--,series wound dynamos will run satisfactorily together without special precautions when coupled in series1 if the connections be arranged as in fig4 #fhe4 ,the positive terminal of one dynamo is connected to the negative terminal of the other1 and the two outer terminals are connected directly to the two main conductors or bus bars through the ammeter ,a1 fuse ,f1 and switch ,s4 ,if it be desired to regulate the pressure and output of the machines1 variable resistances1 or hand regulators ,r1 ,r#a1 may be arranged as shunts to the series coils as shown1 so as to divert a portion or the whole of the current therefrom4 ,series ,dynamos in ,parallel4--,simple series wound dynamos not being well adapted for the purpose of maintaining a constant pressure1 are in practice seldom coupled in parallel2 the conditions or working1 however1 derive importance from the fact that compound dynamos1 being provided with series coils1 are subject to similar conditions when working in parallel1 which is frequently the case4 ,ques4 ,what may be said with respect to coupling two or more plain series dynamos in parallel8 ,ans4 ,the same procedure cannot be followed as in the case of plain shunt dynamos1 for the reason that if the voltage of the dynamo to be coupled be exactly equal to that of the bus bars when connected in parallel1 the combination will be unstable4 ,illustration3 ,fig4 #fhe4--;,diagram showing method of coupling series dynamos in series4 ,r and ,r' are two hand regulators which are placed in shunt across the coil terminals to regulate the pressure and output of the machine4 ,ques4 ,why is this8 ,ans4 ,if1 from any cause1 the pressure at the terminals of one of the dynamos fall below that of the others1 it immediately takes a smaller proportion of the load2 as a consequence1 the current in its field coils is reduced1 and a further fall of pressure immediately takes place4 ,this again causes the dynamo to relinquish a portion of its load1 and again occurs a further fall of pressure4 ,thus the process goes on1 until finally the dynamo ceases to supply current1 and the current from the other dynamos flowing in its field coils in the reverse direction reverses its magnetism1 and causes it to run as a motor against the driving power in the opposite direction to that in which it previously ran as a dynamo4 ,under such circumstances the armature is liable to be destroyed if the fuse be not immediately blown1 and in any case it is subjected to a very detrimental shock4 ,this tendency to reverse in series dynamos can be effectually prevented by connecting the field coils of all the dynamos in parallel4 ,illustration3 ,fig4 #fhf4--;,diagram showing method of coupling series dynamos in parallel4 ,in the diagram ,a1 ,a'1 are ammeters2 ,f1 ,f'1 fuses2 ,s1 ,s'1 switches4 ,ques4 ,how are the field coils of all the dynamos connected in parallel8 ,ans4 ,this is effected in practice by connecting the ends of all the series coils where they join on to the armature circuit by a third connection1 called the equalizing connection1 or equalizer1 as shown in fig4 #fhf4 ,ques4 ,what is the effect of the equalizer8 ,ans4 ,the immediate effect is to cause the whole of the current generated by the plant to be divided among the series coils of the several dynamos in the inverse ratio of their resistance1 without any regard as to whether this current comes from one armature1 or is divided among the whole4 ,the fields of the several dynamos being thus maintained constant1 or at any rate being caused to vary equally1 the tendency for the pressure of one dynamo to fall below that of the others is diminished4 ,shunt ,dynamos in ,series4--,the simplest operation in connection with the coupling of dynamos1 and the one used probably more frequently in practice than any other1 is the coupling of two or more shunt dynamos to run either in series or in parallel4 ,when connected in series1 the positive terminal of one machine is joined to the negative of the other1 and the two outer terminals are connected through the ammeter ,a1 fuses ,f1 ,f'1 and switch ,s1 to the two main conductors or omnibus bars as represented in fig4 #fhg4 ,the machine will operate when the connections are arranged in this manner1 if the ends of the shunt coils be connected to the terminals of their respective machines4 ,shunt ,dynamos in ,parallel4--,the coupling of two or more shunt dynamos to run in parallel is effected without any difficulty4 ,this method of coupling dynamos is one that is very frequently used4 ,fig4 #fhh illustrates diagrammatically the method of arranging the connections4 ,the positive and negative terminals of each machine are connected respectively to two massive insulated copper bars1 shown at the top of the diagram1 called omnibus bars1 through the double pole switches1 ,s1 ,s'1 and the double pole fuses ,f1 ,f'4 ,ammeters1 ,a1 ,a' are inserted in the main circuit of each machine1 and serve to indicate the amount of current generated by each4 ,an automatic switch or cutout1 ,ac1 ,ac'1 is also shown as being included in the main circuit of each of the machines1 although this appliance is sometimes dispensed with4 ,the pressure of each of the machines is regulated independently by means of the hand regulators ,r1 ,r'1 inserted in series with the shunt circuit4 ,the shunt circuits are represented as being connected to the positive and negative terminals of the respective machines1 but in many cases where the load is subjected to sudden variations1 and when a large number of machines is connected to the bus bars1 the shunt coils are frequently connected direct to these4 ,in such circumstances this method is preferable1 as by means of it the fields of the idle dynamos can be excited almost at once direct from the bus bars by the current from the working dynamos2 hence1 if a heavy load come on suddenly1 no time need be lost in building up a new machine previous to switching it into parallel4 ,the pressure of the lamp circuit is given by a voltmeter whose terminals are placed across the bus bars2 and the pressure at the terminals of each of the machines is indicated by separate voltmeters or pilot lamps1 the terminals of which are connected to those of the respective machines4 ,illustration3 ,fig4 #fhg4--;,diagram showing method of coupling shunt dynamos in series4 ,the ends of the shunt coils may be connected to the terminals of their respective machine1 or they may be connected in series as shown4 ,ques4 ,describe a better method of parallel connection4 ,ans4 ,better results are obtained by connecting both the shunt coils in series with one another1 so that they form one long shunt between the two main conductors1 the same as in fig4 #fhg4 ,when arranged in this way1 the regulation of both machines may be effected simultaneously by inserting a hand regulator 7,r7 in series with the shunt circuit as represented4 ,illustration3 ,fig4 #fhh4--;,diagram showing method of coupling shunt dynamos in parallel4 ,switching ,dynamo ,into and ,out of ,parallel4--,in order to put an additional dynamo in parallel with those already working1 it is necessary to run the new dynamo up to full speed1 and1 when it excites1 regulate the pressure by means of a hand regulator until the voltmeter connected to the terminals of the machines registers one or two volts more than the voltmeter connected to the lamp circuit1 and then close the switch4 ,the load upon the machine can then be adjusted to correspond with that upon the other machines by means of the hand regulator4 ,ques4 ,in connecting a shunt dynamo to the bus bars1 must the voltage be carefully adjusted8 ,ans4 ,there is little danger in overloading the armature in making the connection hence the pressure need not be accurately adjusted4 ,it is1 in fact1 common practice in central stations to judge the voltage of the new dynamo merely by the appearance of its pilot lamp4 ,ques4 ,how is a machine cut out of the circuit8 ,ans4 ,when shutting down a machine1 the load or current must first be reduced1 by gradually closing the stop valve of the engine1 or inserting resistance into the shunt circuit by means of the hand regulator2 then when the ammeter indicates nine or ten amperes1 the main switch is opened1 and the engine stopped4 ,by following this plan1 the heavy sparking at the switch contacts is avoided1 and the tendency for the engine to race1 reduced4 ,ques4 ,what precaution must be taken in reducing the current8 ,ans4 ,care must be taken not to reduce the current too much4 ,ques4 ,why is this necessary8 ,ans4 ,there is danger that the machine may receive a reverse current from the other dynamos1 resulting in heavy sparking at the commutator1 and in the machine being driven as a motor4 ,ques4 ,what provision is made to obviate this danger8 ,ans4 ,dynamos that are to be run in parallel are frequently provided with automatic cutouts1 set so as to automatically switch out the machine when the current falls below a certain minimum value4 ,dividing the ,load4--,if a plant1 composed of shunt dynamos running in parallel1 be subjected to variations of load1 gradual or instantaneous1 the dynamos will1 if they all have similar characteristics1 each take up an equal share of the load4 ,if1 however1 as is sometimes the case1 the characteristics of the dynamos be dissimilar1 the load will not be shared equally1 the dynamos with the most drooping characteristics taking less than their share with an increase of load1 and more than their share with a decrease of load4 ,if the difference be slight1 it may be readily compensated by means of the hand regulator increasing or decreasing the pressures of the machines1 as the load varies4 ,if1 however1 the difference be considerable1 and the fluctuations of load rapid1 it becomes practically impossible to evenly divide the load by this means4 ,under such circumstances1 the pressure at the bus bars is liable to great variations1 and there is also liability of blowing the fuses of the overloaded dynamos1 thus precipitating a general breakdown4 ,to cause an equal division of the load among all the dynamos1 under such circumstances1 it is needful to insert a small resistance in the armature circuits of such dynamos as possess the straightest characteristics1 or of such dynamos as take more than their share of an increase of load4 ,by suitably adjusting or proportioning the resistances1 the pressures at the terminals of all the machines may be made to vary equally under all variations of load1 and each of the machines will then take up its proper share of the load4 ,coupling ,compound ,dynamos in ,series4--,since compound dynamos may be regarded as a combination of the shunt and series wound machines1 and as no special difficulties are encountered in running these latter in series1 analogy at once leads to the conclusion that compound dynamos under similar circumstances may be coupled together with equal facility4 ,ques4 ,how are compound dynamos connected to operate in series8 ,ans4 ,the series coils of each are connected as in fig4 #fhe1 and the shunt coils are connected as a single shunt as in fig4 #fhg1 which may either extend simply across the outer brushes of the machines1 so as to form a double short shunt1 or may be a shunt to the bus bars of external circuit1 so as to form a double long shunt4 ,illustration3 ,fig4 #fhi4--;,coupling compound dynamos in series2 short shunt connection4 ,the dotted lines indicate the changes that would be made for long shunt connection4 ,compound ,dynamos in ,parallel4--,machines of this type will not run satisfactorily together in parallel unless all the series coils are connected together by an equalizing connection1 as in series dynamos4 ,the method of arranging the connections as adopted in practice1 being illustrated in fig4 #fij4 ,by means of it idle machines are completely disconnected from those at work4 ,ques4 ,how is the equalizer connected8 ,ans4 ,the equalizer is connected direct to the positive brushes of all the dynamos1 a three pole switch being fitted for disconnecting it from the circuit when the machine to which it is connected is not working4 ,the two contacts of the switch are respectively connected to the positive and negative conductors1 while the central contact is connected to the equalizer4 ,illustration3 ,fig4 #fij4--;,diagram showing method of coupling compound dynamos in parallel4 ,switching a ,compound ,dynamo ,into and ,out of ,parallel4--,if the characteristics of all the dynamos be similar1 and the connections arranged as in figs4 #fij1 or #fia1 the only precaution to be observed in switching a new machine into parallel is to have its voltage equal1 or nearly equal to that of the bus bars previous to closing the switch4 ,if this be the case1 the new machine will take up its due share of the load without any shock4 ,illustration3 ,fig4 #fia4--;,diagram showing another and better method of coupling compound dynamos in parallel4 ,with this arrangement the idle machines are completely disconnected from those at work4 ,the same reference letters are common in both diagrams4 ,s1 ,s' are switches2 ,f1 ,f' fuses2 ,a1 ,a' ammeters1 which indicate the total amount of current generated by each of the machines2 ,,ac1 ,,ac'1 automatic switches1 arranged for automatically switching out a machine in the event of the pressure at its terminals being reduced through any cause2 ,r1 ,r1' are hand regulators1 inserted in the shunt circuits of each of the machines1 by means of which the pressures of the individual machines may be varied and the load upon each adjusted4 ,the pressure at the bus bars is given by the voltmeter ,v1 one terminal of which is connected to each of the bars2 a second voltmeter may be used1 to give the pressure of any individual machine1 by connecting voltmeter keys to the terminals of each of the machines1 or a separate voltmeter may be used for each individual machine4 ,the only essential difference between figs4 #fij and #fia is1 that in fig4 #fij the equalizer is connected direct to the positive brushes of all the dynamos1 while in fig4 #fia the equalizer is brought up to the switchboard and arranged between the two bus bars1 a switch being fitted for disconnecting it from the circuit when the machine to which it is connected is not working4 ,ques4 ,how is a compound dynamo1 running in parallel1 cut out of circuit8 ,ans4 ,the load is first reduced to a few amperes1 as in the case of shunt dynamos1 either by easing down the engine1 or by cutting resistance into the shunt circuit by means of the hand regulator1 and then opening the switch4 ,previous to this1 however1 it is advisable to increase the voltage at the bus bars to a slight extent1 as while slowing down the engine the load upon the outgoing dynamo is transferred to the other dynamo armatures1 and the current in their series coils not being increased in proportion1 the voltage at the bus bars is consequently reduced somewhat4 ,equalizing the ,load4--,when a number of compound dynamos of various output1 size1 or make1 are running together in parallel1 it frequently happens that all their characteristics are not exactly similar1 and therefore the load is unequally distributed1 some being overloaded1 while others do not take up their proper share of the work4 ,,note4--,the action of an equalizing bar in equalizing the load on compound dynamos run in parallel may be explained as follows3 ,the compound winding of a dynamo raises the pressure in proportion to the current flowing through it1 and if1 in a system of parallel operated compound dynamos without the equalizing connection1 the current given by one machine were slightly greater than the currents from the others1 the pressure of that machine would increase4 ,with this increase in pressure above the other machines1 a still greater current would flow1 and so raise the pressure further4 ,the effect is therefore cumulative1 and in time the one dynamo would be carrying too great a proportion of the whole current of the system4 ,with the equalizing connection1 whatever the current flowing from each machine1 the currents in the various compound windings are all equal1 and so the added pressure due to the compound winding is practically the same in each machine4 ,any inequality in output from the machines is readily eliminated by adjusting the shunt currents by means of the shunt rheostats4 ,when compound wound dynamos are operated in parallel1 the equalizer bar insures uniform distribution among the series coils of the machines4 ,,note4--,to secure the best results in parallel operation1 dynamos should be of the same design and construction and should possess as nearly as possible the same characteristics2 that is1 each should respond with the same readiness1 and to the same extent1 to any change in its field excitation4 ,any number of such machines may be operated in parallel4 ,the usual practice is to connect the equalizer and the series field to the positive terminal1 though if desired1 they may be connected to the negative terminal2 both however1 must be connected to the same terminal4 ,the resistance of the equalizer should be as low as possible1 and it must never be greater than the resistance of any of the leads from the dynamos to the bus bar4 ,sometimes a third wire is run to the switchboard from each dynamo and there connected to an equalizer bar1 but the usual practice is to run the equalizer directly between the dynamos and to place the equalizer switches on pedestals near the machines4 ,this shortens the connections and leads to better regulation4 ,the positive and equalizer switches of each machine differ in pressure only by the slight drop in the series coil1 and in some large stations these two switches are placed side by side on a pedestal near the machine4 ,in such cases1 the equalizer and positive bus bars are often placed under the floor near the machines1 so that all leads may be as short as possible4 ,if all the dynamos be of equal capacity1 all the leads to bus bars should be of the same length1 and it is sometimes necessary to loop some of them4 ,if the difference be small1 it may be compensated by means of the hand regulator2 if large1 however1 other means must be taken to cause the machines to take up their due proportion of the load4 ,if the series coils of the several dynamos be provided with small adjustable resistances1 in the form of ,german silver or copper ribbon inserted in series with the coils1 the distribution of the current in the latter may be altered by varying the resistance attached to the individual coils4 ,the effect of the series coils upon the individual armatures in raising the pressure may be adjusted1 and the load thus evenly divided among the machines4 ,shunt and ,compound ,dynamos in ,parallel4--,it is not practicable to run a compound dynamo and a shunt dynamo in parallel1 for1 unless the field rheostat of the shunt machine be adjusted continually1 the compound dynamo will take more than its share of the load4 ,,chapter ,,xxxi ,,dynamo ,,fails ,,to ,,excite ,this trouble is of frequent occurrence in both old and new machines4 ,if a dynamo fail to excite1 the operator should first see that the brushes are in the proper position and making good contact1 and that the external circuit is open if the machine be shunt wound1 and closed if series wound4 ,in starting a dynamo it should be remembered that shunt and compound machines require an appreciable time to build up1 hence1 it is best not to be too hasty in hunting for faults4 ,the principal causes which prevent a dynamo building up are3 #a4 ,brushes not properly adjusted2 #b4 ,defective contacts2 #c4 ,incorrect adjustment of regulators2 #d4 ,speed too low2 #e4 ,insufficient residual magnetism2 #f4 ,open circuits2 #g4 ,short circuits2 a4 ,in external circuits2 b4 ,in dynamo4 #h4 ,wrong connections2 #i4 ,reversed field magnetism4 ,brushes not ,properly ,adjusted4--,if the brushes be not in or near their correct positions1 the whole of the voltage of the armature will not be utilized1 and will probably be insufficient to excite the machine4 ,if in doubt as to the correct positions1 the brushes should be rotated by means of the rocker into various points on the commutator1 sufficient time being given the machine to excite before moving them into a new position4 ,defective ,contacts4--,if the different points of contact of the connections of the machine be not kept thoroughly clean and free from oil1 etc41 it is probable that enough resistance will be interposed in the path of the exciting current to prevent the machine building or exciting4 ,each of the contacts should therefore be examined1 cleaned1 and screwed up tight4 ,ques4 ,which of the contacts should receive special attention8 ,ans4 ,the contact faces of the brushes and surface of the commutator4 ,these are very frequently covered with a slimy coating of oil and dirt1 which is quite sufficient to prevent the machine exciting4 ,incorrect ,adjustment of ,regulators4--,when shunt and compound machines are provided with field regulators1 it is possible that the resistance in circuit may be too great to permit the necessary strength of exciting current passing through the field windings4 ,accordingly1 the fault is corrected by cutting out more or less of the resistance4 ,the field coils of series machines are sometimes provided with short circuiting switches or resistances arranged to shunt the current across the field coils4 ,if too much of the current be shunted across1 the switch should be opened1 or if there be a regulator1 it should be so adjusted that it will pass enough current through the field windings to excite the machine4 ,speed too ,low4--,in shunt and compound dynamos there is a certain critical speed below which they will not excite4 ,if the normal speed of the machine be known1 it can be seen whether the failure to excite arises from this cause1 by measuring the speed of the armature with a speed indicator4 ,in all cases it is advisable1 if the machine do not excite in the course of a few minutes1 to slightly increase the speed4 ,as soon as the voltage rises1 the speed may be reduced to its regular rate4 ,illustration3 ,fig4 #fib4--,method of testing for break by short circuiting the terminals of the machine4 ,if the external circuit test out apparently all right1 and there be no defective contacts in any part of the machine1 and all short circuiting switches1 etc41 be cut out of circuit1 the machine still refusing to excite1 short circuiting the terminals of the machine should be tried4 ,this should be done very cautiously1 especially in case of a high tension machine4 ,it is advisable to have1 if possible1 only a portion of the load in circuit1 and the short circuit should be effected as shown in the figure4 ,the short circuit may be made by momentarily bridging across the two terminals of the machine with a single piece of wire4 ,as this1 however1 is liable to burn the terminals1 a better plan is to fix a short piece of scrap wire in one terminal1 and then with another piece of insulated wire make momentary contacts with the other terminal and the short piece of wire4 ,if the machine excite1 it will be at once evident by the arc which occurs between the two pieces of wire4 ,as the voltage of a series machine when induced to build in this manner generally rises very rapidly1 great care should be taken that the contact is at first only momentary1 merely a rubbing or scraping touch of the wires4 ,the contact may be prolonged if the machine do not excite at the first contact4 ,compound wound machines can often be made to excite quickly by short circuiting their terminals in this manner4 ,insufficient ,residual ,magnetism4--,this fault is not of frequent occurrence2 it takes place chiefly when the dynamo is new1 and may be remedied by passing the current from a few storage cells1 or from another dynamo1 for some time in the proper direction through the field coils4 ,if a heavy current1 such as is obtainable from a storage battery1 be not available1 and the machine be shunt or compound wound1 a few primary cells arranged as in fig4 #fic will generally suffice4 ,illustration3 ,fig4 #fic4--,method of overcoming insufficient residual magnetism4 ,the flexible lead ,l of the dynamo ,d is disconnected from the positive terminal of the machine1 and is connected to the negative or zinc pole of the battery ,b1 the other or positive carbon pole being connected to the terminal1 from which the lead was removed1 and shunt circuit ,s4 ,as thus arranged1 it will be seen that the battery ,b is in series with the armature and shunt circuit1 and therefore its voltage will be added to any small voltage generated in the armature4 ,when the machine is started1 the combined voltages will probably be able to send sufficient current through the shunt to excite the machine4 ,as the voltage rises and the strength of the current in the shunt windings increases1 the flexible lead may be again inserted into the terminal from which it was removed4 ,the battery will thus be short circuited1 and may be cut out of circuit without any danger of breaking the shunt circuit1 and thus causing the machine to demagnetize4 ,open ,circuits4--,dynamos are affected by open circuits in different ways1 depending upon the type4 ,series machines require closed circuit to build up1 while an open circuit is necessary with the shunt machine4 ,an open circuit may be due to3 #a1 broken wire or faulty connection in the machine2 #b1 brushes not in contact with commutator2 #c1 safety fuse blown or removed2 #d1 circuit breaker open2 #e1 switch open2 #f1 external circuit open4 ,if the trouble be due merely to the switch or external circuit being open1 the magnetism of a shunt machine may be at full strength1 and the machine itself may be working perfectly1 but if the trouble be in the machine1 the field magnetism will probably be very weak4 ,open circuits are most likely to occur in3 #a4 ,the armature circuit2 #b4 ,the field circuit2 #c4 ,the external circuit4 ,when the open circuit is due to the brushes not making good contact1 simple examination generally reveals the fact4 ,ques4 ,what causes breaks in the field circuit8 ,ans4 ,bad contacts at the terminals1 broken connections1 or fracture of the coil windings4 ,ques4 ,how is the field circuit tested for breaks8 ,ans4 ,the flexible leads attached to the brushes are removed from their connections with the field circuit1 and the latter is then tested for conductivity with a galvanometer4 ,ques4 ,where is a break likely to occur in a shunt machine8 ,ans4 ,in the hand regulator through a broken resistance coil or bad contact4 ,very frequently the fault occurs in the connecting wires leading from the machine to the hand regulator fixed upon the switchboard1 or in the short wires connecting the field coils to the terminals or brushes4 ,the insulation of a broken wire will sometimes hold the two ends together so as to defy any but the most careful inspection or examination2 therefore1 in order to avoid loss of time1 it is advisable to disconnect the wires if possible1 and test each separately for conductivity with a battery and galvanometer connected1 as in fig4 #fid4 ,if the fault be not located in the various connections1 the magnet coils should be tested with the battery and galvanometer coupled up as in fig4 #gjf1 care being first taken to disconnect the ends of each of the coils4 ,a faulty coil will not show any deflection of the galvanometer4 ,illustration3 ,fig4 #fid4--,method of testing dynamo for short circuits4 ,in the figure1 one pole of the battery ,b is placed in contact with the frame of the machine at a point which has previously been well scraped and cleaned2 the other pole is connected to one of the galvanometer terminals as shown4 ,the other terminal of the galvanometer is connected to each of the dynamo terminals ,t ,t under test in turn4 ,if a deflection of the needle be produced when the galvanometer terminal is in contact with either1 the terminals are in contact with the frame1 and they should then be removed1 and the fault repaired by additional insulation or by reinsulating4 ,ques4 ,at what point of a shunt coil does a break usually occur8 ,ans4 ,at the point where the wire passes through the flanges of the spool or bobbin4 ,ques4 ,how should the coil be repaired8 ,ans4 ,in most cases a little of the wood or metal of which the flange is made can be gouged or chipped out1 and a new connecting wire soldered on to the broken end of the coil without much difficulty4 ,if it be necessary to take the magnets apart at any time1 care should be taken in putting them together again to wipe all faces perfectly clean1 and screw up firmly into contact1 and to see that the connections of the coils are made as they were before being taken apart4 ,if the faulty coil cannot be repaired quickly1 and the machine is urgently required1 the coil may be cut out of circuit entirely1 or short circuited1 and the remaining coils coupled up so as to produce the correct polarity in the pole pieces4 ,illustration3 ,fig4 #fie4--,watson armature discs4 ,each lamination is made from low carbon electrical steel of high magnetic permeability4 ,each disc is annealed and afterwards varnished4 ,ques4 ,what trouble is liable to be encountered in operating after cutting out a coil8 ,ans4 ,the remaining coils are liable to heat up to a greater extent than formerly1 owing to the increased current1 hence it is advisable to proceed cautiously in starting the dynamo1 since the temperature may exceed a safe limit4 ,if this occur1 a resistance may be put in circuit with the field coils1 or the speed of the dynamo reduced4 ,illustration3 ,fig4 #fif4--;,fort ,wayne pedestal type commutator truing device4 ,when this device is used1 the armature is revolved in its own bearings by means of a handle clamped to the pulley4 ,the tool has a horizontal travel of #ba ins41 7being #c ins4 wide inside the fastening bolt in the base71 and a vertical adjustment of #ab ins41 adapting it to machines with commutators up to #cf ins4 in diameter4 ,illustration3 ,fig4 #fig4--;,fort ,wayne yoke type commutator truing device for machines having brush mechanism mounted on a yoke carried by the field frame4 ,it consists of a carriage for the tool holder having a screw feed and a bracket for attaching to the brush yoke4 ,the bracket replaces two brush holder brackets on the brush yoke1 and is made to fit the yoke of the particular machine on which it is to be used4 ,ques4 ,what kind of dynamo is affected by breaks in the external circuit8 ,ans4 ,a series dynamo4 ,ques4 ,name the kind of break that is difficult to locate4 ,ans4 ,a partial break4 ,short ,circuits4--,in a series or compound dynamo a short circuit or heavy load will overload the machine and cause the fuses to blow4 ,a shunt machine will not excite under these circumstances1 for the reason that practically the whole of the current generated in the armature passes direct to the external circuit1 and the difference of potential between the shunt terminals is practically nil4 ,ques4 ,what should be done if it be suspected that the failure to excite arises from this cause8 ,ans4 ,the main leads should be taken out of the dynamo terminals1 then1 if due to this cause1 the machine will excite4 ,ques4 ,what parts of a dynamo are specially liable to be short circuited8 ,ans4 ,the terminals1 brush holders1 commutator1 armature coils and field coils4 ,ques4 ,how are the terminals liable to be short circuited8 ,ans4 ,the terminals of the various circuits of the machine are liable to be short circuited1 either through metallic dust bridging across the insulation1 or through the terminals making direct contact with the frame of the machine4 ,the various terminals should be examined1 and if the fault cannot be located by inspection1 they should each be disconnected from their circuits and tested with a battery and galvanometer arranged as in fig4 #fid4 ,ques4 ,what precaution should be taken with the brush holders8 ,ans4 ,since1 they are liable to be short circuited through the rocker by metallic dust lodging in the insulating washers1 they should be kept clean4 ,ques4 ,how are the brush holders tested8 ,ans4 ,a galvanometer and battery are connected in series with one terminal of the galvanometer connected to one set of brushes2 the unconnected terminal of the battery is then connected with the other set of brushes4 ,a deflection of the needle will indicate a short circuit4 ,illustration3 ,fig4 #fih4--;,field coil testing with telephone receiver4 ,in the method here shown1 a telephone receiver is connected in series with two symmetrically placed coils ,a and ,b4 ,very little sound will be heard when the flux through the two coils ,,ab is the same2 but if a short-circuited coil is being tested1 the fluxes through the coils ,a1 ,b will not be equal and a noise can be heard in the receiver4 ,ques4 ,what is the effect of a short circuit in the field coils or field circuit8 ,ans4 ,the machine generally refuses to excite4 ,ques4 ,how are the field coils tested for short circuit8 ,ans4 ,by measuring the resistance of each coil with an ohmmeter or ,wheatstone bridge4 ,the faulty coils will show a much less resistance than the perfect coils4 ,the fault may also be discovered and located by passing a strong current from a battery or another dynamo through each of the coils in turn1 and observing the relative magnetic effects produced by each upon a bar of iron held in their vicinity4 ,the short circuit may be in the terminals or connections1 and these should first be examined and tested4 ,some series dynamos are provided with a resistance1 arranged in parallel or shunt with the field coils1 to divert a portion of the current therefrom1 and thus regulate the output4 ,when making a series dynamo excite1 all resistances and controlling devices should be temporarily cut out of circuit by opening the shunt circuit4 ,series machines have frequently a switch which short circuits the field coils4 ,care should be taken that this is open1 or otherwise the machine will not excite4 ,illustration3 ,fig4 #fii4--,watson armature complete4 ,the armature coils are form wound1 heavily insulated and so mounted on the core as to insure rapid dissipation of heat by ventilation4 ,each coil is protected by an insulating sheath and tape covering before mounting4 ,the armature is baked after the coils are mounted to drive out all moisture1 then1 while hot1 is treated with insulating compound and again baked twelve hours4 ,wrong ,connections4--,when a machine is first erected1 the failure to build up may be due to incorrect connections4 ,the whole of these latter should therefore be traced or followed out1 and compared with the diagrams of dynamo connections given in figs4 #aij to #aih4 ,sometimes errors are made in connecting the field coils1 causing them to act in opposition4 ,this may occur when the dynamo is a new one or the coils have been removed for repairs4 ,it may be caused either through the coils having been put on the field cores the wrong way1 or through incorrect coupling up4 ,under these circumstances1 the dynamo1 if bipolar1 will fail to excite2 and if multipolar1 poles will be produced in the yokes1 etc4 ,it may be remedied by removing one of the coils from the core and putting it on the reverse way1 or by reversing its connections4 ,the correctness of connections of all the coils should be verified4 ,in compound dynamos it sometimes happens that the machine will excite properly1 but that the series coils tend to reverse the polarity of the dynamo1 thus reducing the voltage as the load upon the machine increases4 ,this may be detected when the machine is loaded by short circuiting the series coils1 not the terminals4 ,if the voltage rise in doing this1 the series coils are acting in opposition to the shunt coils1 and the connections of the series coils must be reversed4 ,reversed ,field ,magnetism4--,this is sometimes caused by the nearness of other dynamos1 but is generally due to reversed connections of the field coils4 ,under such conditions the field coils tend to produce a polarity opposed to the magnetization to which they owe their current1 and therefore the machine will refuse to excite until the field connections are reversed1 or a current is sent from another dynamo or a battery through the field coils in a direction to produce the correct polarity in the pole pieces4 ,,chapter ,,xxxii ,,armature ,,troubles ,a large proportion of the mishaps and breakdowns which occur with dynamos and motors arise from causes more strictly within the province of the man in charge than in that of the designer4 ,the armature1 being a complex and delicately built structure1 is subject in operation to various detrimental influences giving rise to faults4 ,many of the faults which occur are avoided by operators better informed as to the electric and magnetic conditions which obtain in the running of the machine1 especially the mechanical stresses on the copper inductors due to the magnetic field and the necessity of preserving proper insulation4 ,the chief mishaps to which armatures are subject are as follows3 #a4 ,short circuits2 a4 ,in individual coils2 b4 ,between adjacent coils2 c4 ,through frame or core2 d4 ,between sections of armature2 e4 ,partial short circuits4 #b4 ,grounds2 #c4 ,breaks in armature circuit4 ,short ,circuit in ,individual ,coils4--,this is a common fault1 which makes its presence known by a violent heating of the armature1 flashing at the commutator1 flickering of the light on lighting circuits1 and by a smell of burning varnish or overheated insulation4 ,when these indications are present1 the machine should be stopped at once1 otherwise the armature is liable to be burnt out4 ,the fault is due either to metallic dust lodging in the insulation between adjacent bars of the commutator1 or to one or more convolutions of the coils coming into contact with each other1 either through a metallic filing becoming embedded in the insulation or damage to the insulation4 ,illustration3 ,fig4 #gjj4--,method of locating short circuited armature coil4 ,disconnect the external and field circuits from the armature1 and pass a large current--say from #bj to #ajj amperes--from a battery 7,b7 or another dynamo through the whole armature by means of the brushes4 ,then1 having previously well cleaned the commutator1 measure the difference of potential between adjacent segments all round the commutator 7,c71 by means of a voltmeter or galvanometer 7,g71 the terminals of which are connected to adjacent segments1 as shown4 ,the short circuited coil or coils will be located by the difference of potential between the corresponding segments being little or nothing4 ,it may be remarked1 however1 that this is not always a decisive test4 ,in some cases the short circuit may be intermittent1 or may disappear as soon as the armature ceases to rotate4 ,in such cases1 the short circuit is caused by the wire coming into contact through the action of the centrifugal forces developed by the rotation of the armature4 ,ques4 ,how is the faulty coil located8 ,ans4 ,when the machine is stopped1 the faulty coil1 if not burnt out1 can generally be located by the baked appearance of the varnish or insulation1 and by its excessive temperature over the rest of the coils1 being detected also by the baked appearance of the varnish or insulation4 ,ques4 ,what should be done if the machine do not build1 and it be suspected that the fault is due to short circuited armature coils8 ,ans4 ,the field magnets should be excited by the current from a storage battery or another dynamo1 and1 having raised the brushes from contact with the commutator1 the armature should be run for a short time4 ,in stopping1 the faulty coil or coils may be located by the heat generated by the short circuit4 ,when the dynamo is started for the purpose of localizing a short circuit1 precautions should be taken1 and the machine only run for a few minutes at a time until the faulty coil is detected4 ,when the faulty coil has been located1 the insulation between the segments of the commutator to which its ends are connected should be carefully examined for anything that may bridge across from segment to segment1 and scraped clean4 ,if the commutator be apparently all right1 the fault probably lies in the winding4 ,the insulation of the winding should be carefully examined1 and any metallic filings or other particles discovered therein carefully removed1 and a little shellac varnish applied to the faulty part4 ,illustration3 ,fig4 #gja4--,test for break in armature lead4 ,clean the brushes and commutator1 and apply current from a few cells of battery having a telephone receiver in circuit as shown in the figure4 ,if the machine have more than two brushes1 connect the leads to two adjoining brushes and raise the others4 ,now rotate the armature slowly by hand and there will be a distinct click in the receiver as each segment passes under the brushes until one brush bears on the segment at fault1 when the clicking will cease4 ,in making this test1 the brushes must not cover more than a single segment4 ,ques4 ,if the insulation on adjacent conductors has been abraded1 how should it be repaired8 ,ans4 ,a small boxwood or other hardwood wedge1 coated with shellac varnish should be driven in tightly between the wire2 this will generally be sufficient4 ,illustration3 ,fig4 #gjb4--;,bar to bar test for open circuit in coil or short circuit in one coil or between segments4 ,if1 in testing as in fig4 #gja1 on rotating the armature completely around1 the receiver indicate no break in the leads1 connect the battery leads directly to the brushes1 as shown in the above figure1 and touch the connections from the receiver to two adjacent bars1 working from bar to bar4 ,the clicking should be substantially the same between any two commutator bars2 if the clicking suddenly rise in tone between two bars1 it indicates a high resistance in the coil or a break 7open circuit74 ,ques4 ,if a faulty coil cannot be quickly repaired and the dynamo be needed1 what should be done8 ,ans4 ,the coil may be cut out of circuit1 and the corresponding commutator segments connected together with a piece of wire 7of a size proportionate to the amount of current to be carried71 soldered to each4 ,it will not be necessary to cut out and remove the entire coil4 ,if the active portions only be separated so that they do not form a closed circuit1 it will answer the purpose4 ,if the wires be cut with a chisel at the point where they pass over the ends of the core1 and the ends separated1 it will be quite as effective as removing the entire coil4 ,it is wise1 of course1 to rewind the coil at the first opportunity4 ,illustration3 ,fig4 #gjc4--;,alternate bar test for short circuit between sections4 ,where two adjacent commutator bars are in contact1 or a coil between two segments becomes short circuited1 the bar to bar test described in fig4 #gjb will detect the fault by the telephone receiver remaining silent4 ,if a short circuit be found1 the leads from the receiver should then include or straddle three commutator bars1 as here shown4 ,the normal click will then be twice that between two segments until the faulty coils are reached1 when the clicking will be less4 ,when this happens1 test each coil for trouble and1 if individually they be all right1 the trouble is between the two4 ,to test for a ground place one terminal of the receiver on the shaft or frame of the machine1 and the other on the commutator4 ,if there be a click it indicates a ground4 ,move the terminal about the commutator until the least clicking is heard and at or near that point will be found the contact4 ,grounds in field coils can be located in the same manner4 ,short ,circuits between ,adjacent ,coils4--,in ring armatures the presence of this fault does not necessarily imply that the machine will not build2 in drum armatures1 wound into a single layer of conductors1 it entirely prevents this occurring4 ,reference to a winding diagram will show that adjacent coils are during a certain period of the revolution at the full difference of pressure generated by the machine4 ,hence1 if any two adjacent coils be connected together or short circuited1 the whole of the armature will be practically closed on itself1 any current generated flowing within the armature only4 ,large drum armatures wound with compressed and stranded bars and connectors are particularly susceptible to this fault1 a slight blow generally forcing one or more of the strands into contact with the adjacent bars1 thus short circuiting the armature1 and rendering it practically useless so far as the generation of current is concerned4 ,in this class of short circuit in drum armatures1 the method of locating the faulty coils by exciting the field1 and running the armatures on open circuit1 does not apply1 for the reason that the whole armature will be heated equally4 ,a method of locating such fault is illustrated in fig4 #gjd4 ,this applies to drum wound armatures4 ,faults of this description can frequently be discovered by a careful inspection of the windings of the armature without recourse to testing4 ,when located1 the fault can usually be repaired with a hardwood wedge1 as explained above1 or a piece of mica or vulcanized fibre cemented in place with shellac varnish4 ,illustration3 ,fig4 #gjd4--,method of locating short circuits between adjacent armature coils4 ,fasten a monkey wrench to the rim of the pulley1 or a crank to the shaft4 ,now1 excite the fields1 and1 to make the effects more marked1 connect the coils in parallel4 ,when this has been done it will require considerable force to rotate the armature1 and then it will move quite slowly1 except at one position4 ,when this position has been found1 mark the armature at points in the center of the pole pieces at points ,a and ,b and at both ends of the armature4 ,the explanation is that both halves of the armature oppose one another at this position2 but when not at these points a continuous circuit is formed1 and the resultant magnetic effect is considerable4 ,the cross or short circuit is nearly always found on the commutator end in the last half of the winding1 where the wires pass down through the first half terminals4 ,this applies to an unequal winding4 ,in armatures where the windings are equal1 it is as liable to occur at one point as at another4 ,with this method a defect can be found and remedied in a few moments1 for it has always been a simple matter to repair it when discovered4 ,these results can be observed in a perfect armature by connecting the opposite sections of the commutator4 ,short ,circuits between ,sections through ,frame or ,core of ,armature4--,detection of this fault can be effected by the methods described above1 and by disconnecting the whole of the armature coils from the commutator and from each other1 and testing each separately with a battery and galvanometer coupled up as in fig4 #gje1 one wire being connected to the shaft and the other to the end of the coil under test4 ,as a rule1 there is no way of remedying this fault other than unwinding the defective coils1 reinsulating the core1 and rewinding new coils4 ,illustration3 ,fig4 #gje4--,method of locating short circuits between coils through armature core4 ,the galvanometer1 battery and coil to be tested are connected in series as shown1 and then the unconnected terminal of the galvanometer is brought into contact with the shaft4 ,if then some portion of the insulation of the wire has been abraded or destroyed1 thus bringing the bare wire into contact with the metal core1 as at ,a in the figure1 the needle of the galvanometer will be deflected since a closed circuit is formed through the core and wire4 ,if the insulation be perfect1 the needle will not be deflected4 ,it will thus be seen that in the conductivity test 7fig4 #gjj7 it is necessary that the needle should be deflected1 or turned1 to prove that all is right1 while in the insulation test the converse holds good2 if the needle be deflected1 it proves that the insulation is broken down4 ,short ,circuits between ,sections through ,binding ,wires4--,this fault is the result of a loose winding1 and is caused by the insulation upon which the binding wires are wound giving way1 thus bringing coils at different pressures together4 ,as a consequence of the heavy current which flows1 the binding wires are as a rule unsoldered or burned4 ,the location of the fault can therefore be effected by simple inspection4 ,to remedy1 it will be necessary to unwind and rewind on new binding wires1 on bands of mica or vulcanized fibre1 soldering at intervals to obviate flying asunder4 ,partial ,short ,circuits in ,armatures4--,this is usually due to the presence of moisture in the windings4 ,to remedy the fault1 the armature should be taken out and exposed to a moderate heat1 or subjected to a current equal to that ordinarily given by the dynamo4 ,under the action of heat or of this current the moisture will be gradually dispersed4 ,when thoroughly dry1 and while still warm1 a coat of shellac should be applied to the whole of the windings4 ,illustration3 ,fig4 #gjf4--,method of testing for breaks4 ,the instruments are connected as shown4 ,b is the battery1 ,g the galvanometer1 and ,s the coil of wire being tested4 ,one terminal of the battery is connected to a terminal of the galvanometer1 and the other to one of the ends of the coil under test4 ,the other terminal of the galvanometer is connected to the other end of the coil4 ,if the connecting wires be making good electrical contact with the respective terminals1 and the wire of coil being tested be unbroken1 the needle of the galvanometer will be deflected as soon as a closed circuit is made by the end of the coil coming into contact with the galvanometer terminal4 ,if the wire of the coil be broken in some part or the ends of the connecting wires do not make good electrical contact with the terminals1 the needle will not be deflected4 ,in order to prevent mistakes1 it is advisable to test the battery and galvanometer connections and contacts by short circuiting or bringing the ends of the wire connecting the terminal of the galvanometer and negative pole or the battery together before starting to test the circuit or coil4 ,if the needle be deflected1 the connections are all right2 if not deflected1 there is a bad contact somewhere1 which must be made good before the test can proceed4 ,burning of ,armature ,coils4--,the reason for the burning of an armature coil may be explained as follows3 ,the coil1 segments1 and the short circuit between the segments form a closed circuit of low resistance so that it is only necessary to have a low pressure set up in the active portion of the coil to force a very large current through the coil and the short circuited commutator bars4 ,the heating effect of this current is sufficient to burn out the coil4 ,illustration3 ,fig4 #gjg4--,watson field coils4 ,automatic machinery is employed to wind these coils2 after winding1 they are bound with tape1 then baked to expel all moisture1 and while hot1 are saturated with an insulating compound and again baked for twelve hours to make them practically oil and water proof4 ,heavy flexible leads are brought out to avoid danger of breaking or other damage4 ,cutting ,out ,damaged ,armature ,coils4--,to cut out a damaged coil from an armature1 first1 disconnect the coil from the commutator1 and after cutting off the leads1 insulate the exposed parts with tape4 ,then connect the commutator bars 7which were connected with the leads7 with a wire of the same size as the wire winding4 ,to remove the coil entirely1 cut the band wire or remove the wedges1 and lift up a sufficient number of leads and coils to permit of the removal of the damaged coil4 ,grounds in ,armatures4--,these faults occur when the armature coils become connected to the frame or core of the armature4 ,when this grounding is confined to a single coil1 it is not in itself liable to do damage4 ,a simple method of locating a grounded coil is illustrated in fig4 #gjh4 ,illustration3 ,fig4 #gjh4--,method of locating grounded armature coil4 ,b is a battery or dynamo circuit giving a current of a few amperes through the armature by its own brushes 7#a and #b74 ,at ,g is placed a roughly made galvanometer1 to carry some #be amperes or so1 one terminal being in connection with the shaft of the armature1 and the other attached to a movable brush #c4 ,since the function of the particular galvanometer is simply to show a deflection when a current is passing1 and to mark zero when there is none1 a coil of thick wire with a pocket compass in the center will do all that is required1 but care must be taken to remove it sufficiently far away from the disturbing effects of the armature magnetism4 ,the manner of testing is as follows3 ,assume a steady current to be flowing from battery ,b through the armature2 touch the commutator with brush #c1 and a current will flow through ,g4 ,slowly rotate the armature or the brush #c until the galvanometer ,g shows no deflection4 ,the coil in contact with #c will be found to be grounded4 ,a hand regulator or rheostat ,r may be inserted in series with the battery or dynamo circuit to regulate the strength of the current passing4 ,ques4 ,what is the advantage of this test8 ,ans4 ,the damaged coil can be located without unsoldering the coils from the commutator1 which is sometimes a difficult operation without proper tools2 further1 the fault can frequently be repaired without disconnecting any of the wires if its exact position be determined4 ,magneto ,test for ,grounded ,armatures4--,a magneto test for grounded armatures is not to be recommended1 as armatures often possess sufficient static capacity to cause a magneto to ring even though there be no leak4 ,this is due to the alternating current given by the magneto for when the circuit has capacity it acts as a condenser and at each revolution of the armature of the magneto a rush of current goes out and returns1 charging the surfaces of the conductor alternately in opposite directions1 and ringing the bell during the process4 ,illustration3 ,fig4 #gji4--,method of binding armature winding4 ,complete appliances for handling armatures in making repairs are usually not available with most street railway companies1 since they are so seldom required4 ,when needed1 therefore1 some temporary contrivance must be resorted to for help in the dilemma4 ,should an armature burn out1 some local concern that makes coils and rewinds armatures may be available to do the work2 again1 it will be necessary to send to the manufacturers for a man1 as soon as coils can be made ready for the work4 ,in no case should any but an experienced man be given charge of this work4 ,but if there be any doubt as to whether the armature is really burnt out1 let a competent man be the judge4 ,when a large armature needs repairing1 a pair of chain tongs can be used on some part of the shaft when putting in the coils1 and a block and tackle1 as shown1 can be used1 when putting on the band wires4 ,do not finish one band and then cut off the wire1 but run it over for the next1 etc4 ,then solder and trim off the wires4 ,breaks in ,armature ,circuit4--,a partial or complete break in the armature circuit is always accompanied by heavy sparking at the commutator1 but not1 as a rule1 by an excessive heating of the armature or slipping of the belt1 and this enables the fault to be distinguished from a short circuit4 ,the faulty part can always be readily located by the flat which it produces upon the surface of the commutator4 ,the armature circuit being open at the faulty part1 heavy sparking results at every half revolution as the brushes pass over it1 and as a consequence the corresponding segments become pitted or flattened with respect to the others2 they may easily be discovered on examination4 ,breaks in the armature circuit may occur in either the commutator or in the coils of the armature4 ,to ascertain whether it be in the latter1 carefully examine the winding of the faulty coil4 ,the defect may be sought for more particularly at the commutator end of the armature1 as breaks in the wire are most frequent where the connections are made with the commutator segments4 ,if no break can be discovered1 try passing a heavy current through the faulty coil by means of the brushes4 ,if a partial break exist with sufficient contact to pass a current1 the coil will be heated at that point and may be discovered by running the fingers over the coil4 ,when located1 the fault may be repaired by rewinding the coil1 or carefully cleaning the broken ends and jointing4 ,the fault may also be temporarily repaired by soldering the adjacent commutator segments together without disconnecting the coil4 ,,chapter ,,xxxiii ,,care ,,of ,,the ,,commutator ,,and ,,brushes ,for satisfactory operation1 the brushes and commutator must be kept in good condition4 ,to this end the main thing to be guarded against is the production of sparks at the brushes4 ,if care be taken in the first instance to adjust the brushes to their setting marks1 and to regulate their pressure upon the commutator1 and afterwards to attend to the lead as the load varies1 so that little or no sparking occurs1 and also to keep the brushes and commutator free from dirt1 grit1 excessive oil1 etc41 the surface of the commutator will assume a dark burnished appearance and wear will practically cease4 ,under these circumstances the commutator will run cool1 and will give very little trouble4 ,in order to maintain these conditions it will only be necessary to see that the brushes are kept in proper condition and fed forward to their setting marks1 as they wear away1 and that the commutator is occasionally polished4 ,if the pressure of the brushes upon the commutator be too great1 or their adjustment faulty1 or the commutator be allowed to get into a dirty condition1 sparking will result1 and1 if not at once attended to and remedied1 the brushes will quickly wear away1 and the surface of the commutator will be destroyed4 ,as this action takes place1 in the earlier stages1 the surface of the commutator will become roughened or scored1 resulting in jumping of the brushes1 and increased sparking2 in the later stages1 the commutator will become untrue and worn into ruts1 moreover1 owing to the violent sparking which takes place through this circumstance1 the machine will quickly be rendered useless,d4 ,d ,,note4--,in operating dynamos having metal brushes1 it is of importance to keep the commutator smooth and glossy4 ,to accomplish this1 it is necessary to keep the commutator and brushes clean and free from grit1 and to occasionally lubricate the commutator with some light oil1 such as ordinary machine oil4 ,this should be done daily if the machine be in constant use4 ,keep the brushes resting upon the commutator with just enough pressure to insure a good firm contact4 ,this will be found to be much less than the springs are capable of exerting4 ,a good method to follow in cleaning the machine is as follows3 ,loosen the brush holder thumb screws and tilt the brushes off the commutator 7or1 if box brush holders be used1 take them out of their holders74 ,then run the machine and hold a clean cloth against the commutator4 ,after the commutator is clean1 hold against it a cloth or piece of waste moistened with machine oil and reset the brushes4 ,if for any reason the brushes begin to cut or score the commutator1 it may be readily detected by holding the finger against the commutator2 the ridge may be easily felt by the finger4 ,this should be attended to at once in the following manner3 ,tilt back the brushes 7or if box brushes are used take them out of their holders71 and hold lightly against the commutator a piece of ,no4 #jj sandpaper well moistened with oil1 passing it back and forth until the surface is perfectly smooth4 ,then wipe off the commutator with a clean piece of cloth or waste and lubricate with another clean piece moistened with oil and reset the brushes4 ,ques4 ,how is the commutator easily tested as to the condition of its surface8 ,ans4 ,it is readily tested by resting the back of the finger nail upon it while in motion2 the nail being very sensitive to any irregularities1 indicates at once any defect4 ,ques4 ,what causes grooves or ridges to be cut in the commutator8 ,ans4 ,they result from using brushes with hard burnt ends which are not pliable2 also by too great a pressure of the brush upon the commutator surface4 ,sparking at the brushes is expensive and detrimental1 chiefly because it results in burning the brushes and also the commutator1 necessitating their frequent renewal4 ,every spark consumes a particle of copper1 torn from the commutator or brush4 ,the longer the sparking continues1 the greater the evil becomes1 and the remedy must be applied without delay4 ,ques4 ,what kind of oil should be used on the commutator8 ,ans4 ,mineral oil4 ,ques4 ,what attention should be given to the brushes8 ,ans4 ,at certain intervals1 according to the care taken to reduce sparking and the length of time the machine runs1 the brushes will fray out or wear unevenly1 and will therefore need trimming4 ,they should then be removed from the brush holders and their contact ends or faces examined4 ,if not truly square1 they should be filed or clipped with a pair of shears1 the course of treatment differing with the type of brush4 ,if the machine be fitted with metal strip brushes1 frayed ends should be clipped square with a pair of shears1 the ends thoroughly cleaned from any dirt or carbonized oil1 and replaced in their holders4 ,gauze and wire brushes require a little more attention4 ,when their position on the commutator has been well adjusted and looked after1 so that little or no sparking has taken place1 it is generally only necessary to wipe them1 clean the brushes and clip off the fringed edges and corners with the shears1 or a pair of strong scissors4 ,if1 however1 the machine has been sparking1 the faces will be worn or burnt away1 and probably fused4 ,if such be the case1 they will need to be put in the filing clamp1 and filed true4 ,a convenient method of trimming carbon brushes1 or of bedding a complete new set of metal brushes1 is to bind a piece of sandpaper1 face outwards1 around the commutator after the current has been shut off1 and then mount the carbon or metal brushes in the holders1 adjusting the tension of the springs so that the brushes bear with a moderately strong pressure upon the sandpaper4 ,then let the machine run slowly until the ends of the brushes are ground to the proper form4 ,care should be taken1 however1 that the metal dust given off does not get into the commutator connections or armature windings1 or short circuiting will result4 ,if the contact faces of the brushes are very dirty and covered with a coating of carbonized oil1 etc41 it will be necessary to clean them with benzoline or soda solution before replacing4 ,illustration3 ,fig4 #gaj4--;,bissell brush gear4 ,the brushes are held in the brushholders radially and work equally well with armature running in either direction4 ,brushes can be renewed and adjustment made while machine is in operation4 ,ques4 ,describe a filing clamp4 ,ans4 ,as usually constructed1 it consists of two pieces of metal1 both shaped at one end to the correct angle1 to which the brushes must be filed4 ,one of the pieces of metal 7the back part7 has a groove sufficiently large to accommodate the brush1 which is clamped in position by the other piece of metal and a pinching screw4 ,if the clamp be not supplied with the machine a convenient substitute can be made out of two pieces of wood about the same width as the brush4 ,one end of each piece is sawn to the correct angle1 and the brush placed between the two4 ,in filing1 the brush is fixed in the clamp1 with the toe or tip projecting slightly over the edge of the clamp1 and the latter being fixed in a vise1 the brush is filed by single strokes of a smooth file made outwards1 the file being raised from contact with the brush when making the back stroke4 ,illustration3 ,fig4 #gaa4--;,jig for filing brushes to the correct bevel2 used with copper brushes to fit them to the commutator4 ,sparking4--,in all well designed machines there are certain positions upon the commutator for the brushes at which there will be no sparking so long as the commutator is kept clean and in good condition4 ,in other dynamos1 badly designed or constructed1 sparking occurs at all positions1 no matter where the brushes are placed1 and in such dynamos it is therefore impossible to prevent this no matter how well they are adjusted4 ,illustration3 ,fig4 #gab4--;,commutator clamp2 a useful device for holding the segments firmly in position in taking out the end rings of the commutator to repair for internal grounds4 ,it is made of #b #a_/h inch sheet steel1 with a #a_/b inch screw4 ,the illustration clearly shows the adjustable fastening4 ,the notches fit around rivets on one side of each fastening1 which can be moved by removing the two cotters4 ,the clamp is made loose or taut by screwing the bolt in the nut4 ,ques4 ,what two kinds of sparking may be generally distinguished8 ,ans4 ,one kind of sparking is that due to bad adjustment of the brushes1 and a second kind1 that due to bad condition of the commutator4 ,sparks due to bad adjustment of the brushes are generally of a bluish color1 small when near the neutral plane1 and increasing in violence and brilliancy as the brushes recede from the correct positions upon the commutator4 ,when sparks are produced by dirty or neglected state of the commutator1 they are distinguished by a reddish color and a spluttering or hissing4 ,when due to this last mentioned cause1 it is impossible to suppress the sparking until the commutator and brushes have been cleaned4 ,in the former case1 the sparks will disappear as soon as the brushes have been rotated into the neutral points4 ,another class of sparks appear when there is some more or less developed fault1 such as a short circuit1 or break in the armature or commutator4 ,these are similar in character to those produced by bad adjustment of the brushes1 but are distinguished from the latter by their not decreasing in violence when the brushes are rotated towards the neutral plane4 ,having distinguished the classes of sparks which appear at the commutator of a dynamo1 it remains to enumerate the causes which produce them4 ,these are3 #a4 ,bad adjustment of brushes2 #b4 ,bad condition of brushes2 #c4 ,bad condition of commutator2 #d4 ,overload of dynamo2 #e4 ,loose connections1 terminals1 etc42 #f4 ,breaks in armature circuit2 #g4 ,short circuits in armature circuit2 #h4 ,short circuits or breaks in field magnet circuit4 ,bad ,adjustment of ,brushes4--,when sparking is produced by bad adjustment of the brushes1 it may be detected by rotating or shifting the rocker1 by the indication that the sparking will vary with each movement4 ,to obtain good adjustment of the brushes1 it will be necessary to rock them gently backwards and forwards1 until a position is found in which the sparking disappears4 ,ques4 ,if1 in rocking the brushes1 a position cannot be found at which the sparking disappears1 what is the probable cause of the trouble8 ,ans4 ,the brushes may not be set with the proper pitch1 that is they may not be separated a correct distance1 or the neutral plane may not be situated in the true theoretical position upon the commutator through some defect in the winding1 etc4 ,in this last named case1 the brushes may be strictly adjusted to their theoretically correct positions before starting the machine2 then1 when the machine is started and the load put on1 violent sparking occurs1 which cannot be suppressed by shifting the rocker4 ,if1 however1 one set of brushes only be observed1 it will generally be found that1 at a certain position1 the sparking at the set of brushes under observation ceases or is greatly reduced1 while sparking still occurs at the other set4 ,when this position is found1 the rocker should be fixed by the clamping screw1 and the brushes of the other set at which sparking is still occurring adjusted by drawing them back or pushing them forward in their holders until a position is found at which the sparking ceases4 ,correct position of the brushes and the suppression of sparking is a matter of importance1 and any time spent in carefully adjusting the brushes will be amply repaid by the decreased attention and wear of the brushes and commutator4 ,illustration3 ,figs4 #gac to #gae4--;,brushes making bad contact4 ,a brush making a bad contact1 as only at the shaded portion of figs4 #gac and #gad1 will not allow the short circuited coil enough time to reverse1 causing sparking and heating4 ,the latter will also result from bad contact on account of the surface being too small for the current to be carried off4 ,this form of bad contact is worse than that shown in fig4 #gae1 where the area of contact surface only is lessened4 ,if the brushes do not make good contact1 they should be ground down4 ,bad ,condition of ,brushes4--,if the contact faces of the brushes be fused or covered with carbonized oil1 dirt1 etc41 there will be bad contact which is accompanied by heating and sparking4 ,simple examination will generally reveal whether this be the case4 ,the remedy is to remove the brushes1 one at a time if the machine be running1 clean1 file if necessary1 trim1 and readjust4 ,if the brushes be exceedingly dirty1 or saturated with oil1 it will be necessary to clean them with turpentine1 benzoline1 or soda solution1 before replacing4 ,bad ,condition of ,commutator4--,if the surface of the commutator be rough1 worn into grooves1 or eccentric1 or if there be one or more segments loose or set irregularly1 the brushes will be thrown into vibration1 and sparking will result4 ,a simple examination of the commutator will readily detect these defects4 ,a rough and uneven commutator is due to bad adjustment of brushes1 bad construction of commutator1 and to neglect generally4 ,if allowed to continue1 it results in heavy sparking at the brushes1 and the eventful destruction of the commutator4 ,the fault may be remedied by filing or re-turning the commutator4 ,illustration3 ,fig4 #gaf4--,rough and grooved commutator due to improper brush adjustment and failure to keep brushes in proper condition4 ,ques4 ,how is an untrue commutator detected8 ,ans4 ,if the commutator be untrue1 the fact will be indicated when the machine is slowed down by a visible eccentricity1 or by holding the hand1 or a stick in the case of a high tension machine1 against the surface while revolving1 when any irregularity or eccentricity will be apparent by the vibration or movement of the stick4 ,the only remedy for an untrue commutator is to re-turn it in the lathe4 ,ques4 ,what should be done in case of high segments8 ,ans4 ,they should be gently tapped down with a mallet1 and if possible the clamping cones at the commutator end should be tightened4 ,if it be impossible to hammer the segments down1 they should be filed down to the same diameter as the rest of the commutator1 or the commutator re-turned4 ,for low segments1 the only remedy is to pull out the segments1 or turn commutator down to their level4 ,ques4 ,explain the term flats on the commutator4 ,ans4 ,this is the name given to a peculiar fault which develops on one or more segments of the commutator4 ,it is not confined to dynamos of bad design or construction1 but frequently appears on those of the highest class1 and may be recognized as a pitting or flattening of one or more segments4 ,ques4 ,what is the effect of flats on the commutator8 ,ans4 ,sparking at the brushes4 ,ques4 ,what are the causes which produce flats8 ,ans4 ,periodical jumping of the brushes due to a bad state of the commutator1 bad joint in the driving belt1 a flaw1 or a difference in the composition of the metal of the particular bar upon which it appears4 ,but more frequently flats may be traced to a more or less developed fault1 such as a break1 either partial or complete1 in the armature coil4 ,the break may occur either in the coil itself1 or at the point where its ends make connection with the lug of the commutator1 or at the point where the lug is soldered to the segment4 ,ques4 ,what should be done in case of flats8 ,ans4 ,the brushes should be examined to see if any periodical vibration take place4 ,if such be the case1 the cause should be removed1 the flat carefully filed or turned out1 and the brushes readjusted4 ,if it be due to a difference in the composition of the metal of which the segment is made1 the flat will exist as long as the particular segment is in use1 and will need periodic attention4 ,with hard drawn copper or phosphor bronze segments1 this fault is rarely due to this last mentioned cause4 ,it is more frequently due to bad soldering1 of the conductors to the lugs1 or of the lugs to the segments4 ,in all cases of flats1 if the disconnection in the armature circuit be not complete1 and cannot be readily located1 the effect of re-soldering or sweating the ends of the coils into the lugs should be tried4 ,flats may also frequently be cured by drilling and tapping a small hole in the junction between the lug and the segment1 and inserting a small screw1 or bit of screwed copper or brass wire1 afterwards filing down level with the surface of the commutator4 ,illustration3 ,figs4 #gag and #gah4--,method of repairing broken joint between commutator segment and lug4 ,to repair such a break push asbestos in between adjacent bars1 so that heat from the torch will not affect them4 ,asbestos should also be worked in at the back if possible1 for the purpose of keeping solder from places where it would cause trouble4 ,then unsolder the armature leads from the lug and remove the latter4 ,next1 with specially made cape chisels1 cut in a slot in the commutator bar for a new lug4 ,care and skill are required not to destroy the mica insulation between the segments4 ,the slot should be cut one-quarter to three-eighths inch deep4 ,the connector is then soldered in place4 ,with care a satisfactory connection can be made in this way1 which will last well4 ,if it do not last1 the trouble in almost every case is due to poor soldering4 ,short circuits sometimes occur after this operation1 because of solder falling in at the back and lodging on lower connections4 ,in large machines1 the excessive current flowing is quite likely to melt this solder1 and the machine may buck1 throwing out the melted solder1 after which it may be all right again4 ,while the bar connector is out1 however1 asbestos should be packed in back of it to prevent this occurrence1 which may be a serious affair4 ,all surplus solder and the asbestos packing should be removed after the connection is finished1 and the connections cleaned with compressed air4 ,the armature should be turned over slowly1 air being applied all the while4 ,segments ,loose or ,knocked ,in4--,when the segments are loose1 it is an indication that the clamping ring or cone has worked loose4 ,this should therefore be tightened up1 and the commutator re-turned if necessary4 ,ques4 ,how should low commutator segments be treated8 ,ans4 ,the commutator surface may be turned down to the level of the low segment1 or the latter may be pulled out again to its former level1 this latter being the preferable method1 if it can possibly be effected4 ,ques4 ,how is a commutator segment pulled out to its correct position8 ,ans4 ,a hand vise is firmly clamped to the lug1 or a loop of copper wire is passed round the conductor where it joins the commutator4 ,a bar of iron1 to act as a lever1 is supported on a fulcrum over the commutator1 and one end of the bar is passed through the loop or vise4 ,pressure is applied to the other end which will generally bury the segment up to its proper position4 ,how to ,re-turn a ,commutator4--,in re-turning the commutator1 the armature should first be carefully taken out of the armature chamber1 avoiding knocks or blows of any kind4 ,the whole of the winding should then be wrapped in calico or canvas before the armature is put into the lathe1 to prevent any particles of metal becoming attached to the surface of the armature at the time the commutator is being turned4 ,the armature should on no account be rolled upon the floor1 or subjected to blows or knocks while being put into the lathe4 ,in re-turning the commutator1 a sharp pointed tool should be used with a very fine feed4 ,a broad nosed tool ought not to be used1 as it is liable to burr over the segments4 ,after turning1 the commutator should be lightly filed with a dead smooth file1 and finally polished with coarse and fine sandpaper4 ,after the commutator has been turned and polished1 the insulation between the segments should be lightly scraped with the tang of a small file to remove any particles of metal or burrs which might short circuit the commutator4 ,the points where the armature wires are soldered to the lugs should also be carefully cleaned with a brush1 and should then receive a coat or two of shellac varnish4 ,while the commutator is being turned1 care should be taken that the setting marks for the adjustment of the brushes are not turned out if these be present4 ,the same care should be used in putting the armature back into the armature chamber as was used in taking it out1 otherwise the insulation may be damaged4 ,illustration3 ,figs4 #gai and #gbj4--;,bissell commutators4 ,the segments are of hard drawn copper and are insulated from each other and from the shell by mica4 ,ques4 ,should the commutator be run without any lubricant8 ,ans4 ,in most cases it will be found that a little lubricant is needed in order to prevent cutting the brushes1 cutting the commutator2 this is especially the case when hard strip brushes are used4 ,the quantity of oil applied should be very small2 a few drops smeared upon a piece of clean rag1 and applied to the commutator while running1 being quite sufficient4 ,ques4 ,what kind of oil should be used on the commutator8 ,ans4 ,mineral oil1 such as vaseline1 or any other hydrocarbon4 ,animal or vegetable oils should be avoided1 as they have a tendency to carbonize1 and thus cause short circuiting of the commutator1 with attendant sparking4 ,illustration3 ,figs4 #gba to #gbc4--,method of repairing a large hole burned in two adjacent bars of a commutator4 ,fig4 #gba shows the hole4 ,the first operation is to clean carefully and tin the surface of the hole4 ,the two bars are then wedged apart and mica strips1 ,a ,b1 fig #gbb1 of the requisite size and thickness forced in4 ,the commutator must now be warmed up as much as possible by means of soldering irons1 and strips of mica1 ,c ,d1 ,e ,f1 fig4 #gbc1 placed at the front and back of the hole1 being kept in position by pieces of wood ,w1 solder is poured into the hole from a ladle1 using a rough mica funnel to guide it4 ,overload of ,dynamo4--,it may happen1 through some cause or other that a greater output is taken from the machine than it can safely carry4 ,when this is the case1 the fact is indicated by excessive sparking at the brushes1 great heating of the armature and other parts of the dynamo1 and possibly by the slipping of the belt 7if it be a belt driven machine71 resulting in a noise4 ,the causes most likely to produce overload are3 #a4 ,excessive voltage2 #b4 ,excessive current2 #c4 ,reversal of polarity of dynamo2 #d4 ,short circuits or grounds in dynamo1 or external circuits4 ,ques4 ,what is the indication of excessive voltage8 ,ans4 ,it is indicated by the voltmeter1 or by the brilliancy of the pilot lamp4 ,illustration3 ,fig4 #gbd4--,method of smoothing commutator with a stone4 ,the proper stone to use is made out of white sandstone similar to that used for grindstones1 but a trifle softer4 ,it is dove-tailed into a holder1 as shown in the illustration1 and held in place by a set screw4 ,when being used1 one knob is grasped in one hand and the other knob in the other hand1 the stone being moved back and forth along the length of the commutator4 ,as the stone will become coated with copper at first1 it must be cleaned frequently by means of coarse sandpaper4 ,the fine dust from the stone will get under the brushes and wear them to a very close fit4 ,after using the stone1 finish with fine sandpaper4 ,ques4 ,what are the causes of excessive voltage8 ,ans4 ,over excitation of the field magnet or too high speed4 ,in the former case1 resistance should be introduced into the field circuit to diminish the current flowing therein if a shunt machine2 or if a series machine1 a portion of the current should be shunted across the field coils by means of a resistance arranged in parallel with the series coils2 or the same effect may be produced in both cases by reducing the speed of the armature if this be possible4 ,if due to excessive speed1 which will be indicated by a speed indicator1 the natural remedy is to reduce the speed of the engine driving the dynamo1 or1 if this be not easily done1 insert resistance into the dynamo circuit1 as described above4 ,ques4 ,what are the causes of excessive current8 ,ans4 ,if the dynamo be supplying arc lamps1 the excessive current may possibly be caused by the bad feeding of the lamps4 ,if this be the case1 the fact will be indicated by the oscillations of the ammeter needle1 and the unsteadiness of the light4 ,if incandescent lamps be in the circuit1 the fault may be caused by there being more lamps in circuit than the dynamo is designed to carry4 ,under such circumstances1 another dynamo should be switched into circuit in parallel1 or1 if this be not possible1 lamps should be switched off until the defect is remedied4 ,when motors are in the circuit1 sparking frequently results at the dynamo commutator1 owing to the fluctuating load4 ,in such cases the brushes should be adjusted to a position at which the least sparking occurs with the average load4 ,ques4 ,what may be said with respect to reversal of polarity of dynamos8 ,ans4 ,when compound or series wound dynamos are running in parallel1 their polarity is occasionally reversed while stopping by the current from the machines at work4 ,loose ,connections1 ,terminals1 etc4--,when any of the connecting cables1 terminal screws1 etc41 securing the different circuits are loose1 sparking at the brushes1 as a rule1 results1 for the reason that the vibration of the machine tends to continually alter the resistance of the various circuits to which they are connected4 ,when the connections are excessively loose1 sparking also results at their points of contact1 and by this indication the faulty connections may be readily detected4 ,when this sparking at the contacts is absent1 the whole of the connections should be carefully examined and tested4 ,breaks in ,armature ,circuit4--,if there be a broken circuit in the armature1 as sometimes happens through a fracture of the armature connections1 etc41 there will be serious flashing or sparking at the brushes1 which cannot be suppressed by adjusting the rocker4 ,as a rule it results in the production of flats upon one or more bars of the commutator4 ,illustration3 ,fig4 #gbe4--,sandpaper holder for commutator4 ,the sandpaper is made fast on top by a clamp and screw4 ,the two face blocks are pivoted and adjust themselves to the commutator1 and will fit any size of commutator4 ,if it have four brushes1 the lower block will go in between the brush-holders4 ,ques4 ,how may such sparking be reduced without stopping the machine8 ,ans4 ,by placing one of the brushes of each set a little in advance of the others1 so as to bridge the gap4 ,short ,circuits in ,armature ,circuit4--,this fault is indicated by sparking at the commutator1 and in bad cases by an excessive heating of the armature1 dimming of the light and slipping of the belt1 and in the case of a drum armature1 by a sudden cessation of the current4 ,illustration3 ,fig4 #gbf4--,sandpaper block4 ,it is made to fit the surface of the commutator4 ,at ,s is a saw cut into which the ends are pushed after being wrapped around the block4 ,the latter should be cut down on the dotted lines to form a handle4 ,short ,circuits or ,breaks in ,field ,magnet ,circuit4--,either of these faults is liable to give rise to sparking at the commutator4 ,if one of the coils be short circuited1 the fact will be indicated by the faulty coil remaining cool while the perfect coil is overheated4 ,the fault may arise through some of the connections to the coils making contact with the frame of the machine or with each other4 ,to ascertain this1 examine all the connections1 and test with a battery and galvanometer4 ,a total break in one or more of the field coils may readily be detected by means of the battery and galvanometer4 ,a partial break is not1 however1 so readily discovered1 for the reason that the coil wires may be in sufficiently close contact to give a deflection of the galvanometer needle4 ,the only methods of detecting this fault is by measuring the resistance of the coils with an ohmmeter or ,wheatstone bridge1 or by placing an ammeter in circuit with each coil in turn1 and comparing the amount of current flowing in each4 ,if the partial break be not accessible1 the only way to remedy the fault is to rewind the coil1 and the same applies to a break in the interior of the coil4 ,short ,circuits in ,commutator4--,these are of frequent occurrence1 and result in heating the armature and sparking at the brushes4 ,they are caused either by metallic dust or particles lodging in the insulation between the segments1 or by the deterioration of the commutator insulation4 ,to remedy1 the insulation between the segments should be carefully examined1 and any metallic dust1 filings1 or burrs cleaned or scraped out4 ,when the commutator is insulated with asbestos or pasteboard 7as is oftentimes the case in dynamos of ,european make71 short circuits very frequently occur through the insulation absorbing moisture or oil1 which is subsequently carbonized by the sparking at the brushes4 ,in faults of this description the only remedy is to expel all moisture from the commutator insulation by means of heat1 and scrape out all metallic dust which may be embedded in the surface of the insulation4 ,if this do not effect a cure1 it will be necessary to dig out the insulation1 as far as possible1 with a sharp tool1 and drive in new insulation4 ,oil should not be used on commutators insulated with these materials1 but only asbestos dust or ,french chalk4 ,,chapter ,,xxxiv ,,heating ,the excessive heating of the parts of dynamos and motors is probably the most frequent and annoying fault which arises in operation4 ,when the machine heats1 it is a common mistake to suppose that any part found to be hot is the seat of the trouble4 ,hot bearings may cause the armature or commutator to heat1 or vice versa4 ,all parts of the machine should be tested to ascertain which is the hottest1 since heat generated in one part is rapidly diffused4 ,this is best done by starting with the machine cold2 any serious trouble from heating is usually perceptible after a run of a few minutes at full speed with the field magnets excited4 ,heating may be due to various electrical or mechanical causes1 and it may occur in the different parts of the machine1 as in3 #a4 ,the connections2 #b4 ,the brushes and commutator2 #c4 ,the armature2 #d4 ,the field magnet2 #e4 ,the bearing4 ,ques4 ,how is heating detected8 ,ans4 ,by applying the hand to the different parts of the machine if low tension1 or a thermometer if high tension1 and also by a smell of overheated insulation1 paint1 or varnish4 ,ques4 ,what should be done if the odor of overheated insulation1 paint or varnish be noticeable8 ,ans4 ,it is advisable to stop the machine at once1 otherwise the insulation is liable to be destroyed4 ,ques4 ,what is the allowable rise of temperature in a well designed machine8 ,ans4 ,it should not exceed #hj ,fahr41 above the surrounding air1 and in the case of the bearings1 this temperature ought not to be reached under normal conditions of working4 ,if this limit be exceeded after a run of six hours or less1 it indicates a machine either badly designed and probably with the material cut down to the lowest possible limit with a view to cheapness1 or some fault or other which should be searched for and remedied as early as possible1 otherwise the machine will probably be destroyed4 ,ques4 ,how should the rise of temperature be measured8 ,ans4 ,it is not sufficient to feel the machine with the hand1 but special thermometers must be placed on the armature winding1 immediately on stopping the machine1 covering them with cotton or wool to prevent cooling4 ,readings must be taken at short intervals1 and continued till no further rise of temperature is indicated4 ,heating of ,connections4--,a rise of temperature of the connections may be due to either excessive current1 or bad contacts1 or both4 ,the terminals and connections will be excessively heated if a larger current pass through them than they are designed to carry4 ,this nearly always proceeds from an overload of the dynamo1 and if this be rectified1 the heating will disappear4 ,if the contacts of the different connections of the dynamo be not kept thoroughly clean and free from all grit1 oil1 etc41 and the connections themselves be not tightly screwed up1 heating will result1 and the connections may even become unsoldered4 ,heating of ,brushes1 ,commutator and ,armature4--,when heating occurs in these parts1 it may be due to any of the following causes3 #a1 excessive current2 #b1 hot bearings2 #c1 short circuits in armature or commutator2 #d1 moisture in armature coils2 #e1 breaks in armature coils2 #f1 eddy currents in armature core or conductor4 ,illustration3 ,fig4 #gbg4--,ventilated commutator2 sectional view showing air ducts4 ,air is frequently circulated through a commutator in order to maintain it at a sufficiently low temperature1 suitable openings being provided for this purpose as shown4 ,ques4 ,what may be said with respect to excessive current8 ,ans4 ,when a dynamo is overloaded1 the temperature of the armature will rise to a dangerous extent1 depending upon the degree to which the safe capacity of the machine is exceeded1 and heavy sparking of the brushes will also result4 ,if the overload be not removed1 the insulation of the armature may be destroyed4 ,ques4 ,state some causes of hot bearings4 ,ans4 ,lack of oil2 presence of grit or other foreign matter in the bearings2 belt too tight2 armature not centred with respect to pole pieces2 bearings too tight2 bearings not in line2 shaft rough or cut4 ,illustration3 ,fig4 #gbh4--,self-oiling and self-aligning bearing4 ,the self-oiling feature consists of rings which revolve with the shaft1 and feed the latter with oil continually1 which they bring up from the reservoir below4 ,the dirt settles to the bottom1 and the upper portion of the oil remains sufficiently clean for a long time1 after which it is drawn off1 and a fresh supply poured in through holes provided in the top4 ,these latter are often located directly over the slots in which the rings are placed1 so that the bearings can be lubricated immediately by means of an oil cup if the rings fail to act or the reservoir become exhausted4 ,the bearing is made self-aligning by providing the bearing proper with an enlarged central portion of spherical shape1 held in a spherical seat formed in the pedestal by turning1 milling1 or by casting ,babbitt or other fusible metal around it1 thus allowing the bearing to adjust itself to the exact direction of the shaft4 ,the upper half of the box can be taken off to facilitate renewal1 etc41 and to permit the armature to be removed4 ,ques4 ,what is the effect of hot bearings8 ,ans4 ,besides giving trouble themselves1 the heat may be conducted along the armature shaft and core1 thus giving rise to excessive heating of the armature4 ,,points ,,relating ,,to ,,hot ,,bearings #a4 ,use good oil2 #b4 ,see that oil cups or reservoirs are full and all oil passages clear2 #c4 ,in self-oiling and splash systems where the oil is used over again1 it should be kept in clean condition by frequent straining2 #d4 ,keep bearings clean and properly adjusted2 #e4 ,maintain bearings in good alignment2 #f4 ,avoid tight belts2 #g4 ,examine the air gap or clearance between armature and pole faces and see that they are uniform4 ,ques4 ,what troubles are encountered with short circuits in the armature or commutator8 ,ans4 ,this results in sparking at the brushes1 and in the heating of one or more of the armature coils1 and even in the burning up of the latter if a bad case4 ,when the armature is overheated1 and the defect does not proceed from an overload or the causes mentioned below1 the dynamo should be immediately stopped and tested for this fault4 ,ques4 ,what will happen with an overheated commutator8 ,ans4 ,it will decompose carbon brushes and cover the commutator with a black film1 which offers resistance and increases the heat4 ,ques4 ,what should be done if carbon brushes become hotter than the other parts8 ,ans4 ,use higher conductivity carbon4 ,reduce length of brush by adjusting holder to grip brush nearer the commutator4 ,reinforce brushes with copper gauze1 sheet copper or wires1 or use some form of combined metal and carbon brush4 ,increase size or number of brush if necessary1 so the current does not exceed #cj amperes per square inch of contact4 ,brushes heat sometimes due to too much friction4 ,they should not press against the commutator more than is necessary for good contact4 ,ques4 ,give some causes for heating of armature4 ,ans4 ,eddy currents2 moisture2 short circuits2 unequal strength of magnetic poles2 operation above rated voltage1 and below normal speed4 ,ques4 ,what trouble is encountered with eddy currents8 ,ans4 ,considerable heating of the whole of the armature results1 which may even extend to the bearings4 ,illustration3 ,fig4 #gbi4--;,eck ,manchester type motor4 ,it is a very small size unit and is designed for special purposes where very little room is available4 ,the motor occupies a space of #b-a_/b #d-c_/h between bearings and develops #a_/af horse power at #b1jjj ,r4 ,p4 ,m4 ,the frame of this motor is made of high permeability steel so as to reduce the weight to a minimum4 ,the armature is of the hand wound bipolar type built up of thin punchings4 ,the armature1 after being wound1 is baked at high temperature for a prolonged period and then dipped while hot in insulating varnish4 ,pulley is one inch in diameter and takes a #a_/d inch round belt4 ,weight of motor #e-a_/b pounds4 ,ques4 ,how can this be overcome8 ,ans4 ,there is no remedy for eddy currents other than the purchase of a new armature1 or reconstruction4 ,the fault may be detected by exciting the field magnets and running the machine on open circuit1 with the brushes raised off the commutator for some time1 when the armature will be found to be excessively heated4 ,ques4 ,how does moisture in the armature coils affect the armature8 ,ans4 ,the effect of this fault being to practically short circuit the armature1 a heating of the latter results4 ,in bad cases1 steam or vapor is given off4 ,ques4 ,what is the effect of short circuits in the armature8 ,ans4 ,it produces overheating4 ,ques4 ,what trouble is likely to occur when the armature is not centered in the armature chamber8 ,ans4 ,a heating of the bearings is liable to be occasioned through the attractive forces developed by the center of the armature core not being parallel with the centre of the armature chamber or bore1 or through the core being nearer one pole piece than the other4 ,this may result from unequal wearing of the bearings1 and therefore the bearings should either be relined or the bolt holes of the bearings readjusted1 or the bearings packed up until the armature is correctly centered4 ,ques4 ,what happens in case of breaks in the armature coils8 ,ans4 ,this fault results in local heating of the armature1 for the reason that resistance is interposed in the path of the current at the fracture4 ,it always results in sparking at the brushes1 and the heating being confined to the neighborhood of the break4 ,ques4 ,what are the effects of operation above the rated voltage and below normal speed8 ,ans4 ,voltage above normal is a possible cause of heating1 and operation below normal speed calls for an increase of field strength and reduces the effective ventilation1 thus tending to cause heating4 ,illustration3 ,fig4 #gcj4--;,forced system of lubrication as applied to engine of the generating set shown in fig4 #ddc4 ,in engines employing the forced system of lubrication the crank pit1 which is formed by the columns1 is accessible through doors in the front and back of the engine4 ,the base of the engine forms an oil tank to which is attached a small plunger pump driven by an eccentric on the shaft4 ,the lubricant is carried under pressure to the various parts of the engine by the mechanism shown in the accompanying diagram4 ,the oil is forced by a pump to a groove in the main bearing1 and a drilled hole in the shaft connects this groove with the crank pin4 ,from the crank pin box the oil is further forced to the wrist pin through the pipe running along the side of the connecting rod4 ,the passage in the crosshead allows the oil to be forced from the wrist pin to the guides4 ,as the oil is forced from one bearing to another1 it is quite important that the bearing caps be set tight1 otherwise the oil will escape before reaching the last bearing4 ,after passing through the bearings1 the oil is collected in the base1 strained and used again4 ,the oil should be free from foreign substances1 and to guard against the introduction of any foreign matter1 a strainer1 which may be taken out for examination or cleaning1 is attached to the suction valve of the pump4 ,an oil pressure of from #aj to #bj lbs4 should be maintained1 and may be regulated by adjusting the set screw on the relief valve of the oiling system4 ,the pressure gauge need not remain in the circuit continuously4 ,only mineral oils should be used for lubrication4 ,a heavy oil gives better results and prevents knocking more effectively than thin oil4 ,an oil which has been found to give good results1 consists of two-thirds red engine oil and one-third heavy cylinder oil4 ,as the oil passes through the bearings repeatedly1 it gradually loses its lubricating properties1 becoming thick and gritty1 and should be occasionally run through a filter and mixed with new oil4 ,the frequency of this change depends on the oil1 as well as the number of hours the engine is in operation1 and can easily be determined by observation4 ,the oil in the reservoir should stand about #b inches over the suction and discharge valves1 and no water should be allowed to mix with it4 ,should any water accumulate in the base1 it should be drawn off by the cock provided for the purpose before starting the engine4 ,ques4 ,how may the field magnets become heated8 ,ans4 ,by excessive field current2 eddy current in pole pieces2 moisture2 short circuits4 ,ques4 ,what may be said with respect to excessive field current8 ,ans4 ,when heating results from this cause1 all the exciting coils will be heated equally4 ,it may be due to excessive voltage1 in the case of shunt dynamos2 or to an overload in the case of compound and series dynamos4 ,in either case it may be remedied by reducing the voltage or overload4 ,if due to the coils being incorrectly coupled up1 that is1 coupled up in parallel instead of in series1 it will be necessary to rectify the connections or insert a resistance in series4 ,ques4 ,state the causes of eddy currents in the pole pieces8 ,ans4 ,this fault may be due to defective design or construction of the armature4 ,slotted armatures are particularly liable to cause this fault1 if the teeth and air gap be not properly proportioned4 ,the defect may also be occasioned by variation in the strength of the exciting current4 ,if due to this latter cause1 it will be accompanied by sparking at the brushes4 ,if a shunt dynamo1 insert an ammeter into the shunt circuit1 and note if the deflection be steady4 ,if this be not the case1 the variation in the current most probably proceeds from imperfect contacts thrown into vibration4 ,ques4 ,how is the insulation affected by moisture8 ,ans4 ,moisture tends to decrease the insulation resistance1 thus in effect producing a short circuit with its attendant heating4 ,ques4 ,how is moisture in the field coils detected8 ,ans4 ,it is easily detected by applying the hand to the coils1 when they will be found to be damp1 and in addition steam or vapor will be given off where the machine is working4 ,the fault may be remedied by drying and varnishing the coils4 ,ques4 ,what is the indication of short circuits in the field coils8 ,ans4 ,this fault is characterized by an unequal heating of the field coils4 ,if the coils be connected in series1 the faulty coil will be heated to a less extent than the perfect coils2 if connected in parallel1 the faulty coil will be heated to a greater extent than the perfect coils4 ,the former can thus be easily located4 ,,chapter ,,xxxv ,,operation ,,of ,,motors ,in operating motors of any considerable size1 whether connected to the public supply mains of a central generating station for combined lighting and power service1 or to power service mains only1 there are certain precautions to be observed in starting1 stopping1 and regulating the motor1 in order that the efficiency of the supply1 and indirectly the working of other motors and lamps connected to the mains in the immediate neighborhood1 may not be affected by abnormal variations of pressure4 ,these precautions should be observed also to prevent any danger of the motor itself being subjected to detrimental mechanical shocks and excessive temperatures in the working parts4 ,before ,starting a ,motor4--,the general instructions relating to inspection and adjustment1 lubrication1 etc41 which have already been given1 should be carefully followed preparatory to starting,e4 ,e ,,note4--,in starting a motor1 first see that the bearings contain sufficient oil and that the brushes bear evenly on the commutator4 ,if a circuit breaker be used1 close it2 then close the main switch4 ,rotate slowly the handle of the starting rheostat as far as it will go4 ,care should be taken1 in starting the motor1 that the handle of the rheostat be not rotated too fast4 ,to stop a motor1 open the circuit breaker or switch1 which will cut in the resistance of the starting box4 ,never attempt to stop a motor by forcibly pulling open the starting box1 ,disregard of these instructions may cause burning out of the field coils4 ,starting a ,motor4--,in starting a motor1 resistance must be put in series with the armature because1 since there is no reverse electromotive force to counteract the applied voltage when the motor is at rest1 the switching of the latter direct to the motor would result in an abnormal rush of current4 ,this1 in addition to being uneconomical and productive of a drop of voltage in the mains1 would injure all except the smallest motors4 ,hence motors above two horse power usually require a rheostat4 ,ques4 ,describe a rheostat or starting box4 ,ans4 ,it consists essentially of a suitable resistance to be inserted at starting to reduce the initial rush of current1 and which can be cut out in sections by successive movements of a lever as the speed increases4 ,illustration3 ,fig4 #gca4--,press for forcing on and removing a commutator4 ,small commutators are pressed on to the shaft by a hand press4 ,all of the larger commutators are pressed on by means of a power press4 ,in the above figure is shown a hand press4 ,the plate ,b is used in removing old commutators4 ,it is placed back of the commutator as at x y with the slot ,c over the shaft4 ,bolts a b are passed through the holes in the plate and secured by nuts4 ,the commutator can then be forced off the shaft4 ,in pressing on a commutator1 a sleeve is placed over the shaft at ,o1 and against the commutator4 ,the rear end of the shaft is secured so it will withstand the pressure1 and the commutator is forced on4 ,the power presses are built on the principle of a hydraulic press4 ,in pressing on a commutator a piece of babbitt metal or soft brass should be used against the end of the shaft4 ,the shaft should be painted with white lead before having the commutator pressed on1 in order to lubricate the shaft so that the commutator will press on easily4 ,the wiper rings are pressed on after the commutator and then the armature is ready to be connected4 ,ques4 ,describe what occurs in starting a motor4 ,ans4 ,when the lever of the starting box is moved to the first contact some of the resistance is cut out of the circuit and current flows through the motor4 ,this produces a torque and starts the armature rotating4 ,the movement of the armature induces a reverse voltage1 which1 as the speed increases1 gradually reduces the applied current4 ,with this reduction of current1 the torque is reduced and the speed not accelerated as quickly as at first4 ,when the applied current has been reduced to a certain value by the increasing reverse current1 the handle of the starting box is moved to the next contact1 and so on till all the resistance in the starting box has been cut out1 the motor then attaining its normal speed4 ,illustration3 ,figs4 #gcb to #gce4--,various starting resistances4 ,the type of resistance used in motor starting rheostats of small size consists usually of tinned iron wire wound on asbestos tubes1 as shown in fig4 #gcb1 the tubes being firmly supported by porcelain nipples1 the ends of which fit into holes in the top and bottom of the enclosing case4 ,in starters of larger size1 cast metal grids1 as shown in fig4 #gcc1 are used4 ,in addition to these types of resistance1 some forms of starter are equipped with what is known as unit type resistance4 ,in this form1 the resistance is built up of a number of separate sections1 or units1 which are connected to form the complete starting or regulating resistance as the case may be4 ,a single unit consists of a moulded core of vitreous material upon which is wound the resistance wire1 as shown in fig4 #gcd4 ,the surface of the unit is then coated with a special cement and baked4 ,by this method the resistance material is protected from mechanical injury and is also made proof against moisture and other conditions which sometimes affect the ordinary type of resistance4 ,in addition to units coated with cement only1 there are still other types of units1 as in fig4 #gce1 which are provided with a sheet metal covering around the cement1 as a further precaution against injury4 ,each of the various types of resistance described possesses certain characteristics not shared by the others1 the use of any particular type being largely governed by conditions of service4 ,ques4 ,what is the difference between a starting box and a speed regulator8 ,ans4 ,motor starting rheostats or starting boxes1 are designed to start a motor and bring it gradually from rest to full speed4 ,they are not intended to regulate the speed and must not be used for such purpose4 ,failure to observe this caution will result in burning out the resistance which1 in a motor starter1 is sufficient to carry the current for a limited time only1 whereas in the case of speed regulators sufficient resistance is provided to carry the full load current continuously4 ,illustration3 ,fig4 #gcf4--,view of ,cutler-,hammer starter with slate front removed showing open wire coil resistance4 ,the type of resistance here used consists of tinned iron wire wound on asbestos tubes4 ,the bottom of the casing is perforated to secure ventilation4 ,ques4 ,for what kinds of service are speed regulators used8 ,ans4 ,in cases when the speed must be varied1 as in traction motors1 organ blowers1 machine tool drive1 etc4 ,ques4 ,how long does it take to start a motor8 ,ans4 ,usually from five to ten seconds4 ,ques4 ,how is the starting lever operated8 ,ans4 ,it is moved progressively from contact to contact1 pausing long enough on each contact for the motor to accelerate its speed before passing to the next4 ,ques4 ,what are the conditions at starting in a series motor8 ,ans4 ,there is a rush of current1 the magnitude of which depends on the amount of resistance cut out at each movement of the starting lever4 ,illustration3 ,figs4 #gcg and #gch4--,sliding contact starters4 ,fig4 #gcg1 starter with button contacts2 fig4 #gch1 starter with renewable contacts4 ,motor starters in which the successive steps of resistance are cut out by a pivoted lever carrying a contact shoe which slides over button contacts or over contact segments1 are known as sliding contact starters4 ,button contacts are usually furnished with motor starting rheostats of small size while contact segments are used on those of greater capacity4 ,the contact segment being held in position by two screws1 is readily renewable when worn by long service or damaged by arcing4 ,the fixed button contact is not so easily renewed but being used only on small size starters is never likely to be subjected to severe service4 ,some starters1 however1 have renewable button contacts4 ,ques4 ,how are small series motors started on battery circuits8 ,ans4 ,by simply closing a switch to complete the circuit1 the resistance of the battery being sufficient to prevent a great rush of current while starting4 ,ques4 ,how is a shunt motor started8 ,ans4 ,in starting a shunt motor1 no trouble is likely to occur in connecting the field coils to the circuit4 ,since the resistance of the armature is very low1 it is necessary on constant voltage circuits to use a starting rheostat in series with the armature4 ,the necessary connections are shown in fig4 #gef4 ,the switch is first closed thus sending current through the field coils1 before any passes through the armature4 ,the rheostat lever ,p is then moved to the first contact to allow a moderate amount of current to pass through the armature4 ,the resistance of the rheostat is gradually cut out by further movement of the lever ,p1 thus bringing the motor up to speed4 ,illustration3 ,figs4 #gci and #gdj4--,multiple switch starters4 ,fig4 #gci1 starter with no voltage release2 fig4 #gdj1 starter with no voltage release and circuit breaker4 ,the multiple switch type of starter is designed to overcome the arcing on sliding contacts which1 in the case of large motors would be very severe4 ,the cutting out of each step of resistance is accomplished in the multiple switch starter by a separate carbon contact switch which breaks the circuit with a quick snappy action4 ,ques4 ,how does the reverse voltage affect the starting of a motor8 ,ans4 ,when a motor is standing still1 there is no reverse voltage1 and the current taken at first is governed principally by the resistance of the circuit4 ,if the motor be series wound1 there is a momentary reverse voltage1 due to self-induction while the field is building up4 ,if the motor be shunt wound1 self-induction delays the current through the field coils1 but that through the armature is not impeded by such cause4 ,when the armature begins to revolve1 reverse voltage is developed which increases with the speed4 ,the resistance of the starting box may be gradually cut out as the armature comes to speed4 ,thus the reverse voltage gradually replaces ohmic drop in limiting the current as the motor comes to speed4 ,illustration3 ,fig4 #gda4--,starting rheostat with no voltage and overload release4 ,the no voltage release permits the starting lever to fly to the off position should the voltage fail momentarily1 thus protecting the motor against damage should the voltage suddenly return to the line4 ,the movement of the lever is due to a spring4 ,the overload device causes the lever to back to the off position should the current exceed a predetermined maximum for which the release is adjusted4 ,fig4 #gdb4--;,compound starter4 ,rheostats designed for the double duty of starting a motor and regulating its speed are commonly known as compound starters1 the resistance provided being a combination of armature resistance for starting duty and shunt field resistance for speed regulation4 ,failure to ,start4--,this fault1 which is liable to occur in a motor of any description1 is similar to failure to excite in a dynamo1 and is liable to be produced by any of the causes mentioned in connection with the latter fault1 excluding insufficient speed1 and insufficient residual magnetism4 ,when a motor fails to start1 it should first be ascertained if a supply of electrical energy be available in the mains4 ,this may readily be discovered by means of a voltmeter1 or if low tension service1 by means of the fingers bridging across the main terminals4 ,if the supply of energy be present1 the contact arm of the starter should be moved into such position that all resistance is inserted into circuit with the motor4 ,this is important1 as the motor may start suddenly while trying to ascertain the cause of the stoppage4 ,illustration3 ,fig4 #gdc4--,starting panel4 ,in installing any kind of motor starting rheostat1 it is necessary to provide main line knife switch and fuses in addition to the starting box4 ,the appearance of the installation can be much improved by mounting all of these upon one panel4 ,having closed the switch1 if the motor fail to start1 it will be advisable to remove the load if possible1 as the failure may arise from an overload of the machine4 ,this being effected and the motor not starting1 the terminals of the latter should be tested by the means already described for voltage4 ,if no voltage be generated1 a broken circuit or a defective contact may be looked for in the main fuse1 switch1 or starting box4 ,the resistance coils of the latter1 through the heat developed1 frequently break in positions out of sight4 ,if a defective contact of this nature cannot readily be seen1 the contact arm should be moved slowly over the contacts1 as it is possible the broken coil may be cut out of circuit by this means4 ,illustration3 ,figs4 #gdd to #gdf4--;,cutler-,hammer motor starting rheostats with no voltage and overload release4 ,fig4 #gdd1 starter with fixed button contact1 fig4 #gde1 with renewable button contact1 and fig4 #gdf1 with contact segments4 ,in construction the resistance is enclosed in a pressed steel box on which is mounted a marbleized slate panel carrying the starting lever1 contacts and protective devices4 ,by means of a calibrated scale1 the overload release 7shown in the lower left hand corner1 figs4 #gdd and #gde1 and in the lower right hand corner fig4 #gdf7 can be set to break the circuit on any overload not exceeding #ej per cent4 of the rated capacity of the motor4 ,this calibrated scale can also be used for determining1 with a fair degree of accuracy1 the amount of current being consumed by the motor4 ,if a difference of pressure exist between the motor terminals1 the field magnets will1 if shunt or compound wound and in good order1 be excited1 which may be ascertained by means of a bar of iron4 ,if no magnetism be present1 it will of course1 indicate a broken or bad connection1 either between the terminals of the field coils1 or one or more of the coils themselves4 ,if the bar pull strongly1 the position of the brushes upon the commutator in regard to the neutral points should be ascertained1 and the rocker adjusted1 if necessary1 to bring them into their correct positions4 ,if this fail to start the motor1 the connecting leads from the motor terminals to the brushes and the brushes themselves should be carefully examined for broken or bad connections1 and defective contact of the brushes with the commutator4 ,in the latter case1 it may arise from a dirty state of the commutator1 or from the brushes not being fed properly4 ,if due to these causes1 pressing the brushes down upon the commutator with the fingers will probably start the motor4 ,if the failure to start arise from none of these causes1 it is probably due to the field coils acting in opposition1 or to a short circuited armature4 ,this latter remark applies more especially to motors provided with drum armatures4 ,illustration3 ,fig4 #gdg4--;,allen-,bradley compression type resistance units4 ,the contact resistance between the discs composing the resistance column is subject to variations of pressure thereby producing proportionate resistance changes in the column as a whole4 ,in the complete resistance unit1 the resistances column is encased in a drawn steel tube1 which is lined with a highly refractory cement1 for purpose of insulation1 affording the column both mechanical and electrical protection and excluding the air which effectually prevents any combustion should the column become red hot due to overload4 ,the ends of the tube are closed by means of caps through which pass electrodes for making connections between the discs and exterior conductors4 ,the steel tube1 when necessary1 is provided with ribs or fins for the dissipation of acquired heat4 ,illustration3 ,fig4 #gdh4--;,allen-,bradley type ,z automatic motor starter4 ,the operation of this machine is as follows3 ,when the main switch is closed1 the motor circuit is made through the fuses1 resistance unit1 current relay1 and the motor armature4 ,at the same time1 the solenoid circuit is closed 7this is connected directly across the line1 and takes a current which is a small fraction of an ampere71 and the plunger of the solenoid is drawn up1 which produces a pressure on the resistance unit1 and increases the current in the motor circuit to the predetermined value at which the current relay is set4 ,when this value is reached the current relay operates and opens the solenoid circuit1 which reduces the magnetic pull and allows the solenoid plunger to drop back slightly4 ,this action increases the resistance in the motor circuit1 which decreases the current sufficiently to allow the relay to close again4 ,similar cycles of operation are repeated as the motor accelerates1 and each time the plunger is drawn a little farther into the solenoid1 until the short circuiting contacts on the top are pushed together1 which short circuits the current relay and resistance unit1 making them inoperative1 and completing the operation of starting the motor4 ,it will be noted that in starting a motor with this device the current is always held down to a certain predetermined value1 and it is impossible to overload the motor by too rapid starting4 ,the current relay is calibrated in amperes1 and may be set to suit existing conditions4 ,the action of the starter being controlled by a current relay and not by an oil or air dash pot1 the motor will start rapidly when under a light load1 and slowly when more heavily loaded4 ,the fuses or circuit breakers may be set at a value that will furnish protection to the motor under running conditions4 ,precautions with ,shunt ,motors4--,with motors of this type1 because of the large amount of self-induction in the shunt windings1 it is important to note3 #a1 that in switching on the field magnet1 the current may take an appreciable time to grow to its normal value1 and #b1 that in switching off1 especially with quick break switches1 high voltages are induced in the windings1 which may break down the insulation4 ,illustration3 ,fig4 #gdi4--,monitor starter giving automatic start with knife switch control2 designed for use with printing presses4 ,it consists of a set of solenoids connected in series and so interoperating as to cut resistance out of the armature circuit of the motor as the apparatus it is driving comes up to speed4 ,this type is for small motors or where no need arises for speed regulation2 there is1 therefore1 no adjustment of speed possible aside from an actual change in motor conditions4 ,at full speed the motor is directly across the main supply line4 ,fig4 #gej4--,monitor automatic starter1 equipped with relay for push button control4 ,ques4 ,what provision is made so that the magnetizing current will have time to reach its normal value8 ,ans4 ,the field connections are generally separated from the actual starter1 and taken to the main switch1 so that wherever the main switch is closed1 the current flows through the field coils1 before the starting lever is moved4 ,ques4 ,how are the connections arranged to avoid excessive voltage in the windings due to self-induction8 ,ans4 ,generally the armature and field magnet circuits are placed in a closed circuit that is never opened4 ,in other cases1 in order that the rise of voltage may not injure the insulation when the shunt is opened1 a special form of main switch is sometimes used which1 before breaking from the supply1 puts a non-inductive resistance across the shunt of the motor4 ,this is known as a flashing resistance4 ,illustration3 ,figs4 #gea to #gec4--,monitor control switches4 ,fig4 #gea1 push button start and stop switch2 fig4 #geb1 safety lever control switch with slow and fast buttons for rotary printing presses4 ,this device will upon pressure of the start button1 set the machine in motion and bring it up to the predetermined speed1 either as previously set by the starter limits or by the setting of the rheostat arm4 ,the stop button projects some distance beyond any other portion of the device1 in order that in case of emergency the operator may stop the machine merely by hitting the face of the switch with his open hand4 ,the lever control switch1 fig4 #gec1 is similar in its action to the push button switch but combines two other features3 locking point1 and visual indication of the station from whence the press has been stopped4 ,with the lever at the downward position1 the press is locked and cannot be started from any other station4 ,in order to make the press ready to start the lever must be raised to the central position4 ,thus a man may safely enter the press without delay by setting his station to the locked position1 knowing that it cannot be started except by some one coming to that station and realizing that the press has been purposely locked4 ,also1 by looking along the press1 it is possible to tell from which station it has been locked1 and proper action can be immediately taken4 ,the safety control station is usually combined for use on large rotary presses with the slow and fast push buttons as shown in fig4 #geb4 ,a pressure upon the fast or slow buttons will cause the press speed to be correspondingly accelerated or retarded1 and this action will continue so long as the button is pressed4 ,the press continues to run at the speed attained at the instant of releasing the button4 ,any speed may1 therefore1 be selected or changed to suit momentary requirements4 ,this gives complete control excepting reversal which is not required of such a press4 ,illustration3 ,fig4 #ged4--,wiring diagram of the standard form of ,monitor controller4 ,a set of solenoids are connected in series and so interoperating as to cut resistance out of the armature circuit of the motor as the apparatus it is driving comes up to speed4 ,the controller is designed to be used on all classes of work4 ,in its simplest form1 a single copper and graphite contact1 is controlled by two magnets1 so proportioned as to cut out the entire starting resistance when the armature current falls to a predetermined value4 ,in the larger sizes1 the number of steps controlling the resistance is increased and arranged to produce the correct acceleration4 ,in every case the regulation of the starting resistance is effected entirely by the current passing to the motor without the use of dash pot1 mechanical governor or delicate cut outs4 ,the time element is thus directly proportioned to the load and the motor brought up to speed in the shortest time consistent with the load1 but always with safe limitation of the armature current4 ,the distinction between the current controlled starter and the starter with dash pot governor should be noted4 ,the starter here shown limits the starting current to a fixed value throughout the starting operation1 which is an ideal condition and prevents blowing fuses in starting4 ,ques4 ,how can shunt motors be controlled from a distant point8 ,ans4 ,the starter and switch are placed at the desired point and the two main wires and the field wires run from that point to the motor4 ,this requires additional wire which increases the cost and line loss4 ,regulation of ,motor ,speed4--,motors are generally run on constant voltage circuits4 ,under these conditions1 the speed of series motors varies with the load and at light loads becomes excessive4 ,shunt motors run at nearly constant speeds4 ,for many purposes1 particularly for traction1 and for driving tools1 it is desirable to have speed regulation1 so that motors running on constant voltage circuits may be made to run at different speeds4 ,the following two methods are generally used for regulating the speed of motors operated on constant voltage circuits3 #a4 ,by inserting resistance in the armature circuit of a shunt wound motor2 #b4 ,by varying the field strength of series motors by switching sections of the field coils in or out of circuit4 ,ques4 ,describe the first method4 ,ans4 ,this method is illustrated in fig4 #gef4 ,when the main switch is closed1 the field becomes excited1 then by moving the lever ,p of the starting rheostat the various contacts 7#a1 #b1 #c1 #d1 #e71 more or less of the rheostat resistance is cut out of the armature circuit1 thus varying the speed correspondingly4 ,this is the same as the method of starting a motor1 that is1 by variation of resistance in armature circuit1 but it should be noted that when this method is used for speed regulation1 a speed regulating rheostat should be used instead of the ordinary starting box1 because the latter1 not being designed for the purpose1 will overheat and probably burn out4 ,illustration3 ,fig4 #gee4--,monitor printing press controller4 ,it provides variable speed and other control features required in the operation of large rotary presses1 such as those used for printing newspapers4 ,from any one of various stations similar to the one illustrated in fig4 #gec1 located at all desirable places about the press1 the latter may be started1 stopped1 accelerated1 slowed down or locked4 ,it differs from other types of printing press controller in that the solenoid has an overall maximum pull of less than one inch and does not actuate the main line current directly but through pilot circuits1 which in turn1 operate flapper switches2 there are no sliding contacts4 ,at the control stations1 the operator can distinguish the accelerating button from the retarding button by the sense of touch and obviously he can in the same manner ascertain the position of the lever4 ,the position of the lever whether at start1 stop or safety1 can be readily observed at a distance4 ,when the lever of either control station is placed at stop1 the current is disconnected from the motor and a powerful dynamic brake brings the press to rest without delay and without shock or harmful strain4 ,the start will always be made with all resistance in the armature circuit1 and with full field1 and should the current supply fail1 the controller will release and open the circuit to the motor4 ,this controller will give a speed range as low as #aj of normal speed by armature resistance and1 by field control1 any increase within the speed of the motor4 ,ques4 ,describe the second method4 ,ans4 ,this method of regulating the speed of a series motor is shown in fig4 #geg4 ,the current through the armature will flow through all the field windings when the position of the switch lever ,s1 is on contact #d1 and the strength of the field will be the maximum4 ,by moving the arm to contact #c1 #b1 etc41 sections of the field winding are cut out1 thus reducing the strength of field and varying the speed4 ,illustration3 ,fig4 #gef4--,speed regulation of shunt motor by variable resistance in the armature circuit4 ,ques4 ,how does the speed vary with respect to variation of field strength8 ,ans4 ,decreasing the field strength of a motor increases its speed1 while increasing the field strength decreases the speed4 ,under the conditions of maximum field strength1 as with switch ,s on point #a1 the torque will be greatest for any given current strength and the reverse voltage also greatest at any given speed4 ,the current through the armature of the motor1 to perform any given work1 will thus be a minimum1 as well as the speed at which the motor has to run1 in order to develop sufficient reverse voltage to permit this current to flow4 ,regulation of speed by varying the field strength is limited in range of action1 since the field saturation point is soon reached1 moreover1 with too low a field strength1 armature reaction produces excessive field distortion1 sparking1 etc4 ,illustration3 ,fig4 #geg4--,speed regulation of series motor by cutting out sections of the field winding4 ,in this method the field winding is tapped at several points1 dividing the coil into sections and the leads from these points are connected a multi-point switch of the type that would be used on a rheostat4 ,by moving the lever ,s1 to the left or right1 the current will flow through one or more sections of the field winding1 thus decreasing or increasing the ampere turns and thereby providing means of regulation4 ,illustration3 ,fig4 #geh4--,speed regulation of a series motor by the method of short circuiting sections of the field winding4 ,it will be seen that there are seven different positions for the contact springs on the barrel contacts4 ,a4 represents the armature and brushes1 little ,a1 ,b1 and ,c1 the divided field magnet coils1 ,l the line connection1 and ,g the earth connection4 ,the diagram shows the connections for trolley car operation4 ,ques4 ,how is the speed of shunt and compound motors varied with respect to the normal speed in the two methods8 ,ans4 ,the first method 7variable resistance in armature circuit7 reduces the speed below the normal or rated speed of the machine1 while the second method increases the speed above the normal4 ,in the first method the amount of speed reduction depends partly upon the amount of resistance introduced into the armature circuit1 and partly upon the load4 ,in the second method the amount of speed increase depends entirely upon the amount of resistance placed in the shunt winding circuit4 ,eighty-five per cent4 is about the maximum speed reduction obtainable by armature resistance but so great a reduction is seldom satisfactory since comparatively slight increases in the load will cause the motor to stall4 ,shunt field regulation may be obtained up to any point for which the motor is suited1 the only limitation in this case being the maximum speed at which the motor may be safely operated4 ,it should be remembered1 however1 that speed increase by shunt field weakening increases the current in proportion to the increase in speed1 and care should be taken not to overload the armature4 ,,note4--,a compound motor may be made to run at constant speed1 if the current in the series winding of the field be arranged to act in opposition to that of the shunt winding4 ,in such case1 an increase of load will weaken the fields and allow more current to flow through the armature without decreasing the speed of the armature1 as would be necessary in a shunt motor4 ,such motors1 however1 are not very often used1 since an overload would weaken the fields too much and cause trouble4 ,if the current in the series field act in the same direction as that in the shunt fields1 the motor will slow up some when a heavy load comes on1 but will take care of the load without much trouble4 ,,note4--,motors have much the same faults as dynamos1 but they make themselves manifest in a different way4 ,an open field circuit will prevent the motor starting1 and will cause the melting of fuses or burning out of the armature4 ,a short circuit in the fields1 if it cut out only a part of the winding1 will cause the motor to run faster and very likely spark badly4 ,if the brushes be not set exactly opposite each other1 there will also be bad sparking4 ,if they be not at the neutral point1 the motor will spark badly4 ,brushes should always be set at the point of least sparking4 ,if it become necessary to open the field circuit1 it should be done slowly1 letting the arc gradually die out4 ,a quick break of a circuit in connection with any dynamo1 or motor is not advisable1 as it is very likely to break down the insulation of the machine4 ,the ordinary starting box for motors is wound with comparatively fine wire and will get very hot if left in circuit long4 ,the movement of the arm from the first to the last point should not occupy more than thirty seconds and if the armature do not begin to move at the first point1 the arm should be thrown back and the trouble located4 ,illustration3 ,fig4 #gei4--;,cutler-,hammer multiple switch starter with no voltage release2 for use with large motors1 or with motors of medium size where the starting conditions are severe or when more than fifteen seconds are required to accelerate the motor4 ,in operation1 the cutting out of each step of resistance is accomplished by a separate lever and the levers themselves are so interlocked as to prevent closing switches except in proper order1 beginning with the lever on the left4 ,the last switch 7the one on right hand side7 is held by an electro-magnet when closed1 each of the other switches being held in the closed position by a latching device on the switch next to it4 ,in front of each switch is placed a metal stop1 so arranged as to prevent any switch being operated until the one next to it on the left has been closed4 ,these metal stops constitute the interlocking mechanism and prevent the starting of the motor in any way except by closing the switches in regular rotation1 thus insuring proper resistance in the circuit and protecting the motor from excessive starting currents4 ,when the current is interrupted1 the electro-magnet releases the last switch1 which1 on opening1 releases the latch on the switch next to it1 allowing that switch to open1 and this in turn releases the next latch and so on1 the switches opening automatically one after another4 ,in starting the motor1 each switch should be closed quickly and firmly1 pausing a second or two before closing the next switch to give the motor time to accelerate4 ,illustration3 ,fig4 #gfj4--;,cutler-,hammer speed regulator with no voltage release1 regulation by armature resistance only1 reducing speed of motor below normal4 ,no resistance in the armature circuit4 ,no provision is made in regulators of this type for increasing the speed of the motor4 ,the maximum speed obtainable when these regulators are used is1 therefore1 the normal speed at which the motor is designed to operate with no resistance in circuit4 ,with all resistance in circuit and the motor taking normal current these regulators will reduce the speed of the motor #ej per cent4 ,if the motor be taking less than normal current the percentage of speed reduction obtainable will be correspondingly less4 ,the notched fan tail extension on the lower end of the lever engages with a magnetically operated pawl to hold the lever squarely on any contact so long as the no voltage release magnet is energized4 ,ques4 ,how is a wide range of speed regulation secured8 ,ans4 ,by a combination of the two methods4 ,regulation by ,armature ,resistance4--,speed regulators for this method of regulation1 are designed to carry the normal current on any contact without overheating and when all the resistance is in the circuit1 they will reduce the speed of the motor about #ej per cent4 provided the motor be taking the normal current4 ,when operating without resistance in the armature circuit1 shunt wound and compound wound motors will regulate to approximately constant speed regardless of load4 ,this characteristic of inherent regulation is lost1 however1 when armature resistance is employed to reduce the speed of the motor1 fluctuations in load resulting in fluctuations in speed1 which become more noticeable as the amount of resistance inserted in the armature circuit is increased4 ,accordingly1 it becomes necessary to move the lever of the speed regulator forward or backward to again obtain the speed at which the machine was operating before the load changed4 ,illustration3 ,fig4 #gfa4--;,cutler-,hammer compound starter with no voltage and overload release4 ,this is a starting rheostat and field regulator combined4 ,in operation1 two levers are employed1 both being mounted on the same hub post and one lying directly under the other4 ,the upper lever only is provided with a handle1 but when moving from the off position to the starting position 7that is to say1 from left to right7 the lower1 or starting1 lever is carried along by the upper1 or speed regulating1 lever until it comes in contact with the no voltage release magnet where it is held fast by the attraction of the magnet1 leaving the upper lever free to be moved backward over the field contacts1 thus weakening the shunt field of the motor little by little until the desired speed is attained4 ,during the operation of starting the motor1 the field resistance is short circuited by an auxiliary contact 7the slotted metal strip shown near center of rheostat7 but as soon as the starting lever touches the no voltage release magnet or1 in other words1 when the motor has been accelerated to normal speed1 this short circuit is removed1 and the field resistance becomes effective for speed regulation4 ,the motor is accelerated from rest to normal speed by moving both levers from left to right1 while increases in speed above normal are obtained by moving the upper lever from right to left4 ,only the lower1 or starting lever comes into contact with the no voltage release magnet4 ,this lever is provided with a strong spiral spring which tends always to throw the lever back to the off position4 ,hence should the voltage fail1 the no voltage release magnet releases the starting lever and this1 in flying back to the off position1 opens the armature circuit of the motor and carries the speed regulating lever with it to the off position4 ,the upper1 or speed regulating lever1 not being influenced by the spring1 though mounted on the same hub post as the starting lever1 may be moved back and forth at will or left indefinitely in the position which gives the speed desired4 ,when the speed of a motor driving a constant torque machine is reduced by inserting resistance in the armature circuit there is no corresponding reduction in current consumed4 ,the motor runs more slowly simply because a part of the energy impelling it is shunted into the resistance and there dissipated in the form of heat4 ,hence1 whether the motor be operating at full speed or half speed1 the amount of current consumed is the same2 the only difference being that in the one case all the energy taken from the line is expended in driving the motor while in the other case only one half is utilized for power1 the other half being dissipated in the resistance4 ,speed regulation by armature resistance only is therefore open to two objections3 #a1 the difficulty of maintaining constant speed under varying load conditions1 and #b1 the necessity of wasting energy to secure speed reduction4 ,these objections are1 in part1 offset by the fact that speed reduction by armature resistance may be applied to any motor of standard design and requires nothing more than the simplest and least expensive speed regulating rheostat4 ,in cases where the motor will be operated nearly always at full speed1 the difference in first cost of the installation may justify the use of the armature resistance method of control4 ,as a rule1 speed regulation by shunt field resistance is preferable4 ,illustration3 ,fig4 #gfb4--;,cutler-,hammer compound speed regulator with no voltage and overload release2 regulation by combined armature and shunt field resistance1 designed to both decrease and increase the speed of a motor4 ,speed reduction is accomplished by inserting resistance in the armature circuit1 the maximum amount of speed reduction obtainable with these controllers being #ej per cent4 below normal4 ,speed increase is obtained by inserting resistance in the shunt field circuit1 the maximum amount of speed increase obtainable with these controllers being #be per cent4 above normal4 ,regulation by ,shunt ,field ,resistance4--,since regulation by this method is for speeds above normal1 a starter must be used to bring the motor up to its rated speed4 ,usually the starter is combined with the regulator1 as shown in fig4 #gfa1 the device being called a compound starter4 ,illustration3 ,figs4 #gfc to #gfe4--;,holzer-,cabot shunt wound motor2 diagrams showing connections and positions of index point for forward and reverse rotation4 ,,location ,,and ,,setting4--,the motor should be placed in as cool1 clean and well ventilated a location as possible1 away from acid or other fumes which would attack the metal parts or insulation1 and should be easily accessible for cleaning and oiling4 ,do not put it in some corner where care of motor will be neglected because of the trouble of getting at it4 ,the motor should be set so that the shaft is level and parallel with the shaft it is to drive so that the belt will run in the middle of the pulleys4 ,do not use a belt which is much too heavy or too light for the work it has to do1 as it will materially reduce the output of the motor4 ,the belt should be about one-half inch narrower than the pulley4 ,,rotation4--,in order to reverse the direction of rotation1 interchange leads ,a and ,b1 and shift brush ring as shown in the diagram above4 ,,suspended ,,motors4--,motors with ring oil bearings may be used on the wall or ceiling by taking off end caps and revolving #ij or #ahj degrees until the oil wells come directly below the bearings4 ,,starting4--,before starting the motor see that the armature revolves freely1 that the bearings are full of oil1 and the oil rings are in place and free to turn4 ,examine connections carefully to see that they are according to above diagram1 after which proceed as follows3 #a4 ,close the main knife switch4 ,this action should not allow any current to pass through the motor 7see ,note #b72 #b4 ,move the lever of the starting rheostat quickly and squarely onto the first segment1 and hold it there for about a second2 #c4 ,move the lever to the second segment and hold it there for about a second2 #d4 ,move the lever to the third segment and hold it there for about a second1 and so on from one segment to the next until the lever has been moved over all the segments to the short circuit position1 where it should be held firmly by the retaining magnet4 ,if the motor do not start when the lever of the starting rheostat is on the third segment1 open the main knife switch and look for the trouble4 ,this may consist of any of the following3 a4 ,wrong connections2 b4 ,too great a load on the motor2 c4 ,the motor brushes not in proper position2 d4 ,an open circuit of some kind2 e4 ,a short circuit of some kind4 ,,note #a4--;,it is always advisable1 in case of trouble1 to make sure that the fields of the motor are magnetized4 ,this test is easily made by first closing the main knife switch1 then moving the lever of the starting rheostat to the first segment1 and finally having an assistant place a screw driver or other piece of iron against the pole pieces of the motor4 ,if the fields be magnetized1 a heavy pull on the iron should result4 ,,note #b4--;,any possibility of arcing on the first contact of the starting rheostat when starting can be obviated by first moving the lever onto the initial contact1 holding it there1 and then closing the main line switch1 after which proceed as per paragraphs #c and #d4 ,,to ,,stop ,,the ,,motor4--,open the main knife switch and let the starting rheostat take care of itself4 ,the lever will not fly back immediately1 but will hold until the motor has slowed down considerably4 ,,note4--,the above directions apply only to starters of the sliding contact type4 ,,temperatures4--,if located as instructed above1 these motors will carry full load as indicated on the name plate on the motor with a temperature rise of not over #dj degrees ,centigrade1 or #ge degrees ,fahrenheit above the surrounding air4 ,this will feel hot to the hand but is far below the danger point4 ,if the motor feel too hot1 get a thermometer and measure the temperature4 ,to do this1 place the bulb of the thermometer for #aj minutes against the frame1 cover with a cloth or piece of waste1 and note temperature as compared with that of room4 ,if the motor run in a small1 enclosed space with no ventilation1 the temperatures will be somewhat higher than those given above4 ,,oiling4--,fill the oil wells to the overflow before starting and keep them full4 ,use good dynamo oil4 ,be sure that the oil rings turn freely while the motor is running4 ,if in a dirty place1 draw off the old fluid and fill with new every two or three months4 ,,care ,,of ,,motor4--,the motor must be kept clean4 ,if the commutator become rough1 smooth it up with ,no4 #jj sandpaper moistened with oil4 ,when fitting new brushes or changing them1 always sandpaper them down until they fit the commutator perfectly1 by passing to and fro beneath the brush a strip of sandpaper1 having the rough side toward the brush4 ,brushes must always be renewed before the metal of the holder comes in contact with the commutator4 ,don't use anything on commutator except good mineral machine oil1 or kerosene1 and this only in very small quantities applied with a cloth having no lint or threads4 ,illustration3 ,fig4 #gff4--,sectional view showing principal parts of ,reliance adjustable speed motor3 #a1 lever fulcrum pin2 #b1 lever2 #c1 sliding thrust bearing box2 #d1 ball bearing2 #e1 armature shaft end nut2 #f1 cap2 #g1 commutator end yoke2 #h1 lever rod2 #i1 compression spring2 #aj1 steel frame2 #aa1 speed adjustment nut2 #ab1 thrust collars and pins2 #ac1 hand wheel rod2 #ad1 hand wheel2 #ae1 sleeve nut2 #af1 oil well cover2 #ag1 bearing bushing2 #ah1 sleeve2 #ai1 oil ring2 #bj1 pinion end yoke2 #ba1 rocker arm2 #bb1 brush holder stud2 #bc1 brush2 #bd1 commutator2 #be1 armature2 #bf1 armature laminations2 #bg1 armature coils2 #bh1 armature end plate2 #bi1 armature shaft2 #cj1 leads2 #ca1 axial position of commutating pole2 #cb1 axial position of main field pole2 #cc1 slide rail screws2 #cd1 end yoke cap screws2 #ce1 slide rails2 #cf1 commutating coil2 #cg1 commutating pole2 #ch1 main field pole2 #ci1 main field coil4 ,illustration3 ,fig4 #gfg4--;,cutler-,hammer reversible starter with no voltage release1 adapted to start and operate motor at full speed in either direction1 such for instance as motors driving auxiliary motions on lathes1 planers and other machine tools which may rotate in either direction but always at constant speed4 ,they are not designed to reduce the speed of the motor1 but merely to start it and bring it smoothly up to full speed in either direction4 ,two no voltage release latching devices are provided so that the lever will be held in the full speed position in either direction so long as the voltage of the line remains constant4 ,on failure of voltage a strong centering spring attached to the hub-post of the lever throws the latter to the central1 or off position4 ,the shunt field circuit is not opened by starters of this type4 ,the weakening of the shunt field of a motor by the insertion of resistance in the shunt field circuit causes the armature to revolve more rapidly4 ,one advantage of this method of control is that the motor will inherently regulate to approximately constant speed under widely varying load conditions4 ,another advantage is found in the fact that all of the current taken from the line is utilized for power1 the changes in speed being obtained not by dissipating a portion of the effective energy in the resistance 7as in the case of the armature resistance method of control7 but by weakening the reverse voltage by inserting resistance in the shunt field circuit4 ,speed increase by shunt field weakening is limited1 however1 to about #aj to #ae per cent4 above the normal speed in motors of standard construction4 ,greater ranges of speed can be obtained from motors especially designed for shunt field control but should not be attempted with motors of standard design without first ascertaining from the manufacturer the maximum safe speed4 ,combined ,armature and ,shunt ,field ,control4--,regulation by combined armature and shunt field resistance is by far the easiest way of obtaining a wide range of speeds4 ,rheostats embodying these methods are known as compound speed regulators1 one form being shown in fig4 #gfb4 ,standard regulators can be obtained giving a wide range of speed variation1 and special regulators may be constructed giving practically any desired range4 ,selection of ,starters and ,regulators4--,unsatisfactory operation of these devices is1 in nearly all cases1 due to lack of precaution in selecting the proper piece of apparatus for the work to be done4 ,one of the commonest errors is to select a rheostat of insufficient capacity4 ,if the current required to operate the motor at full speed with no resistance in circuit be greater than the rated capacity of the rheostat1 overheating of the resistance will result4 ,an increase in temperature even to a point where the hand cannot be held on the enclosing case need cause no apprehension1 but should the resistance become red hot it indicates that the apparatus is being worked far beyond its capacity1 and the load on the motor should be reduced or a regulator of greater capacity substituted4 ,if the current required to operate the motor at full speed with no resistance in circuit be less than the rated capacity of the rheostat no overheating will occur1 but it will not be possible to secure the full #ej per cent4 speed reduction the rheostat is designed to give with all resistance in circuit4 ,illustration3 ,fig4 #gfh4--,various sizes of ,watson commutator4 ,the segments are punched from hard drawn copper strip and are insulated from each other and the core by amber mica1 of hardness corresponding to that of the copper in order that the wear of mica and copper may be uniform4 ,the segments are assembled in a ring under great pressure and are repeatedly heated and tightened1 being finally secured and rigidly locked together4 ,in ordering a starter or regulator1 the manufacturer should be furnished with the following information3 #a4 ,horse power of motor with which speed regulator will be used2 #b4 ,voltage of motor2 #c4 ,winding of motor1 whether series1 shunt1 or compound wound2 #d4 ,nature of the machine which motor is to operate2 #e4 ,normal rated speed of motor to be used2 #f4 ,maximum speed at which it is desired to operate the motor2 #g4 ,minimum speed at which it is desired to operate the motor2 #h4 ,whether controller will ever be required to reverse direction of motor or to operate it in one direction only2 #i4 ,if reversible controller be desired1 whether or not full range of speed control is required in both directions2 #aj4 ,whether the regulator shall be equipped with any of the following devices3 no voltage release1 overload release1 knife switch1 fuses2 #aa4 ,whether button contacts or renewable contact segments are preferred2 #ab4 ,giving1 also1 if possible1 the resistance of the shunt field cold1 and the shunt field current at the maximum speed required4 ,if this cannot be ascertained1 give horse power1 voltage1 normal speed1 maximum speed required1 serial number of motor and name of manufacturer4 ,illustration3 ,fig4 #gfi4--,organ blower speed regulator2 diagram showing operation and method of installing4 ,a cord running from the top of the organ bellows passes over two pulleys and is then made fast to the chain furnished with the regulator4 ,this chain passes around a sheave which turns on a post projecting from the center of the slate panel4 ,attached to the lower end of the chain is a weight1 also furnished with the regulator4 ,as the air is exhausted from the bellows the latter slowly collapses1 drawing the rope down with it1 and in so doing turns the sheave from left to right1 thus cutting resistance out of circuit and increasing the speed of the motor which pumps air into the bellows4 ,responding to the inrush of air1 the bellows expands1 relaxing the tension on the rope which is now pulled in the opposite direction by the weight1 thus turning the sheave from right to left1 cutting resistance into circuit once more and slowing down the motor4 ,the speed of the motor is thus automatically regulated by the bellows1 with the result that a practically uniform pressure is maintained at all times4 ,in connection with an organ blower regulator it is necessary to install a separate starting rheostat4 ,this is required for the reason that all organ bellows leak4 ,during the intermissions in the musical part of the service1 or at other times when the blower is not operating1 the air gradually escapes and the bellows settles down1 moving the rheostat arm to the right and cutting out resistance4 ,with the motor at rest and the bellows empty all the blower regulator resistance would be short circuited and it is therefore necessary to avoid throwing the motor directly across the line when starting again4 ,a starting rheostat with no voltage release is suitable for this purpose1 and should be installed within easy reach of the organist1 so that a moment or two before beginning to play he can move the lever of the starting box and get the motor into operation4 ,where remote control is desirable a self starter can be substituted for the manually operated starting box1 in which case the entire installation can be controlled by a push button1 or single throw knife switch4 ,illustration3 ,fig4 #ggj4--;,general ,electric type ,k#g controller with cover open showing construction4 ,the mechanism consists of a long spindle1 carrying a number of heavy brass or gun metal segments1 making contact for a longer or shorter time with a corresponding number of spring contacts4 ,the spindle is provided at its upper end with a handle1 and the various contacts are made by turning it through an arc of about #aej4 ,for this method a moderate amount of resistance is employed4 ,the first contact joins both motors and the full amount of resistance in series across the line1 and as the motors are standing still1 maximum current flows so that they exert their full torque4 ,the moment they start to revolve1 the current tends to fall1 due to the generation of a reverse voltage2 to prevent this and maintain a heavy current for some time1 thus obtaining rapid acceleration1 the resistance is arranged so that it can be gradually reduced1 until at about the fourth notch the two motors are in series without resistance across the line4 ,to increase still further the speed in the above type of controller1 the series fields may be shunted1 and then the next steps place the motors in parallel with the resistance4 ,speed ,regulation of ,traction ,motors4--,the speed regulator for motors of this class is called a controller1 and being located in an exposed place is enclosed in a metal casing4 ,controllers are designed to be used for starting1 stopping1 reversing1 and regulating the speed of motors where one or more of these operations have to be frequently repeated4 ,the controller used with a single motor equipment is practically the same as any other single motor starting box1 excepting that the resistance has sufficient carrying capacity to be left in the circuit some time4 ,when the motor is to operate at full speed all the resistance is cut out4 ,to reverse1 a reversing notch is placed in the armature or field circuit1 but not in both4 ,ques4 ,what provision is made to overcome the arc when the circuit is opened8 ,ans4 ,a magnetic field is used with such polarity that it blows out the arc4 ,illustration3 ,fig4 #gga4--;,controller of the ,rauch and ,lang electric vehicles4 ,it is of the flat radial type4 ,two movable copper leaf contacts of ample size make all commutations necessary to obtain the various speeds4 ,five speeds forward and reverse are provided4 ,magnetic blow out coils are used on all controllers designed for #ejj volt circuits1 and on types designed for lower voltages requiring more than #fj amperes normal capacity4 ,the coils are wound with either copper wire or flat strips of sufficient capacity to carry full load current continuously without undue heating1 and after being wound they are treated with an insulating compound making them moisture proof4 ,ques4 ,what provision is made to prevent reversal before bringing the controller lever to the off position8 ,ans4 ,controllers having separate reversing cylinders are fitted with mechanical interlocks making it necessary to place lever in off position before reversing4 ,illustration3 ,figs4 #ggb to #ghb4--;,diagram of controller connections1 illustrating the series parallel method of two motor control4 ,two ,motor ,regulation4--,with a two motor equipment1 the controller becomes more complicated because it must be arranged to switch the motors in series or in parallel1 so as to secure economy at half and full speed4 ,the various connections of series-parallel regulation are shown in figs4 #ggb to #ghb4 ,from these diagrams it is seen that the motors are first operated in series until all the resistance is cut out by the controller 7figs4 #ggb to #ggg74 ,the next point on the controller puts the two motors in parallel with some resistance in the circuit 7fig4 #ggh71 which resistance is gradually short circuited on the remaining controller points1 until at full speed all the resistance is cut out1 the two motors remaining in parallel 7figs4 #ggh to #ghb74 ,stopping a ,motor4--,if it be desired to stop a motor1 the main switch is opened4 ,as the armature of the motor continues to operate1 due to its inertia1 it generates an electromotive force which sends a current through the shunt connected field circuit and helps to maintain the field excitation4 ,when the speed of the motor has decreased sufficiently so as not to endanger the motor should the main switch be thrown1 the current in the series magnet becomes weakened1 and the spring throws back the starting box arm4 ,it should be noted that in stopping a motor having a starting box provided with a no voltage release simply open the main switch and do not touch the lever because otherwise1 the self induced voltage of the field circuit may puncture the field winding or the insulation of the adjoining wires in the starting box4 ,,hawkins ,,practical ,,library ,,of ,,electricity ,,in ,,handy ,,pocket ,,form ,,price #a ,,each ,they are not only the best1 but the cheapest work published on ,electricity4 ,each number being complete in itself4 ,separate numbers sent postpaid to any address on receipt of price4 ,they are guaranteed in every way or your money will be returned4 ,complete catalog of series will be mailed free on request4 ,,electrical ,,guide1 ,,no4 #a ,containing the principles of ,elementary ,electricity1 ,magnetism1 ,induction1 ,experiments1 ,dynamos1 ,electric ,machinery4 ,,electrical ,,guide1 ,,no4 #b ,the construction of ,dynamos1 ,motors1 ,armatures1 ,armature ,windings1 ,installing of ,dynamos4 ,,electrical ,,guide1 ,,no4 #c ,electrical ,instruments1 ,testing1 ,practical ,management of ,dynamos and ,motors4 ,,electrical ,,guide1 ,,no4 #d ,distribution ,systems1 ,wiring1 ,wiring ,diagrams1 ,sign ,flashers1 ,storage ,batteries4 ,,electrical ,,guide1 ,,no4 #e ,principles of ,alternating ,currents and ,alternators4 ,,electrical ,,guide1 ,,no4 #f ,alternating ,current ,motors1 ,transformers1 ,converters1 ,rectifiers4 ,,electrical ,,guide1 ,,no4 #g ,alternating ,current ,systems1 ,circuit ,breakers1 ,measuring ,instruments4 ,,electrical ,,guide1 ,,no4 #h ,alternating ,current ,switch ,boards1 ,wiring1 ,power ,stations1 ,installation and ,operation4 ,,electrical ,,guide1 ,,no4 #i ,telephone1 ,telegraph1 ,wireless1 ,bells1 ,lighting1 ,railways4 ,,electrical ,,guide1 ,,no4 #aj ,modern ,practical ,applications of ,electricity and ,ready ,reference ,index of the #aj ,numbers4 ,theo4 ,audel ,co4 ,publishers #gb ,,fifth ,,avenue1 ,,new ,,york