file which includes the original illustrations4 ,see #cdjeb-;h4htm or #cdjeb-;h4zip3 7https3_/_/www4gutenberg4org_/cache_/epu b_/#cdjeb_/pg#cdjeb-;images4html7 or 7https3_/_/www4gutenberg4org_/files_/#cd jeb_/cdjeb-;h4zip7 ,images of the original pages are available through ,internet ,archive_/,canadian ,libraries4 ,see http3_/_/www4archive4org_/details_/wirel esstransmis#jjmartuoft ,transcriber's note3 ,a carat character indicates that the following character 7or characters in curly braces7 is a superscript4 ,examples3 ,a#b 7,a raised to the second power71 #aj-g 7#aj raised to the -#g power74 ,text enclosed between underscores was italicized in the original 7italics74 ,a single underscore indicates that the following character or characters is a subscript4 ,example3 ,,cs#b 7the #b is a subscript74 ,numbers enclosed by curly braces within the text are page numbers 7example3 #ajj71 which have been incorporated to enable the reader to use the index4 ,,wireless ,,transmission ,,of ,,photographs ,illustration3 ,,fig4 #aj4 99 99 99 99 99 ,,wireless ,,transmission ,,of ,,photographs by ,,marcus ,j4 ,,martin ,,second ,,edition ,,revised ,,and ,,enlarged #aiai ,the ,wireless ,press1 ,ltd4 #ab-ac ,henrietta ,street1 ,strand ,london1 ,w4,c4 #b v ,,preface ,,to ,,second ,,edition ,although during the last few years very little1 in common with other wireless work1 has been possible in connection with the practical side of the wireless transmission of photographs1 yet1 now that the prospect of experimental work is once again occupying the minds of all wireless workers1 advantage has been taken of a reprint of this little volume to amplify a few points that were insufficiently dealt with in the first edition1 and also to add some fresh matter4 ,to ,chapter ,v4 has been added a short description of the ,nernst lamp1 and also some useful information regarding photographic films1 and a few notes relating to enlarging included in the ,appendix ,b4 ,a fresh appendix dealing with the principles of optical lenses has also been added4 ,this is a subject that plays an important part in any system of wireless photography1 and to those experimenters whose knowledge of optics is limited this section should prove useful4 ,to serious workers engaged on the problem of the wireless transmission of photographs1 attention vi is called to a series of articles which are being published from time to time in the ,wireless ,world1 on the design and construction of wireless photographic apparatus4 ,m4 ,j4 ,m4 ,,maidstone1 #aiai4 vii ,,preface ,in these progressive times it is only reasonable to expect that some attempt would be made to utilise the ether-waves for other purposes than that of telegraphic communication1 and already many clever minds are at work trying to solve the problems of the wireless control of torpedoes and airships1 wireless telephony1 and1 last but not least1 the wireless transmission of photographs4 ,it may seem rather premature to talk about the wireless transmission of photographs at a time when the ordinary systems are not fully developed2 but the prospects of wireless photography are of a very encouraging nature1 especially for long over-water distances1 as there are great difficulties to be overcome in long-distance transmission over ordinary land lines and cables which will be entirely eliminated by wireless methods4 ,from a perusal of ,chapter ,i4 the reader will be able to understand something of the difficulties that are to be encountered in working over long distances1 and he will also be able to appreciate something of the advantages that would be derived viii from a reliable wireless system4 ,apart from the value of such a system for transmitting news pictures1 it would also be of great advantage to transmit to ships at sea photographs of criminals for identification purposes4 ,in such a small volume as this it would be impossible to deal with the working of wireless apparatus and the many systems that have been devised for the transmission of photographs over metallic circuits4 ,the ,author has taken it for granted that other works have been studied in connection with these subjects1 and will therefore only describe such apparatus as is likely to be of use in wireless transmission4 ,at present the transmission of photographs by wireless methods is in a purely experimental stage1 and this book will have served its purpose if it helps to put future experimenters on the right track and prevent them from making expensive and fruitless experiments1 by showing them the right direction in which investigations are being carried out4 ,as there is no claim to originality in respect of a good many pieces of apparatus1 etc41 described1 ,i have not thought it necessary to state the various sources from which the information has been obtained4 ,m4 ,j4 ,m4 ,,ashford1 #aiaf4 99 99 99 99 99 ix ,,contents ,,page ,,preface ,,to ,,second ,,edition v ,,preface vii ,,chapter ,i ,,introductory #a ,foreword--,early experiments--,advantages of ,radio-,photography--,difficulties in ,cable working--,bernochi's ,system--,knudsen's ,system4 ,,chapter ,,ii ,,transmitting ,,apparatus #ac ,wireless ,apparatus--,preparing the ,photographs--,transmitting ,machines--,transmitting ,apparatus--,effects of ,arcing--,spark-,gaps--,contact ,breakers--,complete ,station--,professor ,korn's ,apparatus--,poulsen ,company's ,photographic ,recorder--,comparison of various systems--,practical applications4 ,,chapter ,,iii ,,receiving ,,apparatus #cg ,methods of ,receiving--,author's ,photographic ,receiver--,decohering ,apparatus--,description of ,einthoven ,galvanometer--,use of ,galvanometer in ,receiving--,belin's ,application of ,blondel's ,oscillograph--,description of ,charbonelle's ,receiver--,use of ,telephone ,relay--,description of ,telephone ,relay--,telephotographic ,receiver--,polarisation ,receiver--,kathode-,ray ,receiver--,electrolytic ,receiver--,atmospherics in ,long-,distance working4 x ,,chapter ,,iv ,,synchronising ,,and ,,driving #fc ,driving ,motors--,isochronising the ,electrolytic ,system--,professor ,korn's method--,description of ,hughes ,governor--,author's ,speed ,regulator--,problem of ,synchronising--,methods of ,synchronising--,advances made in ,radio-,photography4 ,,chapter ,v ,,the ,,telephograph #gd ,author's ,system of ,radio-,photography--,requirements--,adv antages--,transmitting machine--,description of ,differential ,relay--,wireless ,receiving ,apparatus--,photo-,telegraphic ,receiving ,apparatus--,circuit ,breaker--,friction ,brake--,magnetic ,clutch--,description of ,isochroniser--,method of working--,types of ,nernst ,lamp--,action of ,nernst ,lamp--,comparison of ,actinic ,value--,inertia of ,photographic ,films--,choosing ,films--,speed of ,films--,standard of ,speed--,comparative ,film ,speeds--,effects of ,minimum ,exposure--,effects of ,maximum ,exposure--,considerations in working and choosing ,films4 ,,appendix ,a ,,selenium ,,cells #aji ,nature of ,selenium--,preparation of ,selenium--,forms of ,selenium ,cells--,action of ,selenium ,cells--,characteristics of ,selenium ,cells--,effects of ,inertia in ,photo-,telegraphy--,methods of counteracting ,inertia--,sensitiveness of ,selenium to ,light--,effect of ,heat on ,selenium4 ,,appendix ,b ,,preparing ,,the ,,metal ,,prints #aae ,outline of ,process--,line ,screens--,choice of ,camera--,fixing ,line ,screen in ,camera--,lenses and ,stops--,taking the ,photograph--,copying ,stands--,choice of ,photographic ,plates--,sources of ,illumination--,metal ,prints--,coating the xi ,metal ,sheets--,sensitising ,solution--,printing ,operations--,developing--,intensifying- -,precautions to be observed in working--,preparing ,sketches on ,metal--,apparatus for ,reducing or ,enlarging--,improvements to ,copying ,board--,lenses for ,copying--,formula for ,copying4 ,,appendix ,c ,,lenses #abf ,action of ,light--,law of ,refraction--,lenses--,prisms--,action of ,lenses--,focal ,length of ,lenses--,formation of ,images--,apparent ,magnitude of ,objects--,real and ,virtual ,images--,formation of ,virtual ,images--,power of ,magnification--,defects of ,lenses--,aberration4 99 99 99 99 99 xiii ,,illustrations ,,fig4 ,,page #a4 ,diagram showing effects of capacity on an intermittent current #e #b4 ,bernochi's wireless apparatus #g #c4 ,knudsen's wireless apparatus #aj #d4 ,wireless transmitting station #ac #e4 ,diagram of experiment illustrating principle of line photograph #af #f4 ,drawing of transmitting machine #ag #g4 ,drawing of transmitting machine #ah #h4 ,drawing of stylus #ah #i4 ,electrical connections of machine #ai #aj4 ,photograph of ,author's experimental machine ,frontispiece #aj;a4 ,end view of ,author's experimental machine facing page #ba #aj;b4 ,view of image broken up by a cross screen #aa4 ,connections of complete transmitting apparatus #bc #ab4 ,drawing of ordinary type of spark-gap #bg #ac4 ,synchronous rotating spark-gap #bh #ad4 ,non-synchronous rotating spark-gap #bh #ae4 ,connections for complete wireless photographic station #cj #af4 ,connections of ,professor ,korn's apparatus #ca #ag4 ,connections of ,poulsen's photographic recorder #cc #ah4 ,author's photographic receiver #ch #ai4 ,enlarged drawing of cone #ci #bj4 ,end view of ,author's photographic receiver #ci #ba4 ,connections of decohering apparatus #da #bb4 ,connections for complete photographic receiver #db xiv #bc4 ,arrangement of ,einthoven galvanometer #de #bd4 ,einthoven galvanometer arranged for receiving #df #be4 ,connection of telephone relay #di #bf4 ,drawing of ,author's improved photographic receiver #ea #bg4 ,diagram giving ratio of vibrating arm #ea #bh4 ,arrangement of polarisation receiver #ec #bi4 ,arrangement of kathode-ray receiver #ed #cj4 ,connections of electrolytic receiver #ef #ca4 ,drawing of improved stylus for receiving #eh #cb4 ,drawing of ,hughes telegraph governor #ff #cc4 ,arrangement of simple speed regulator #fh #cd4 ,diagram of connections of simple speed regulator #fh #ce4 ,author's arrangement for complete radio-photographic station #gg #cf4 ,drawing of transmitting machine and circuit breaker #gh #cg4 ,drawing of special transmitting stylus showing adjusting arrangements #gi #cg;a4 ,end view of transmitting stylus #gi #ch4 ,connections of new type of relay designed by the ,author #hj #ci4 ,arrangement of mercury containers and dipping rods for relay #hb #dj4 ,drawing of ,author's receiver #hd #da4 ,enlarged drawing of diaphragm and steel point #hd #da;a4 ,drawing showing arrangement of bush and counter-weight #hd #db4 ,optical arrangements of receiver #he #dc4 ,optical arrangements of receiver #hf #dd4 ,drawing of circuit breaker #hh #de4 ,drawing of friction brake #hi #df4 ,sectional drawing of magnetic clutch #ij #dg4 ,plan of magnetic clutch #ij #dh4 ,details of ,isochroniser #ib #di4 ,connections of ,isochroniser #id #ej4 ,dial of ,isochroniser #id #ea4 ,diagram of driving mechanism #if xv #eb4 ,diagram showing starting positions of machines #ig #eb;a4 ,arrangement of small type ,nernst lamp #ii #eb;b4 ,ballasting resistances for ,nernst lamps #ajj #eb;c4 ,arrangement of large type ,nernst lamp #aja #ec4 ,connections of selenium cell elements #aaj #ec;a4 ,form of selenium cell used by ,bell and ,tainter #aaj #ed4 ,diagram showing construction of modern cell #aaa #ee4 ,resistance curve of selenium cell #aaa #ee;a4 ,actual curve of selenium cell #aab #ef4 ,diagram of ,professor ,korn's method for counteracting inertia #aac #eg4 ,arrangement of plate sheath and line screen #aag #eh4 ,details of clips to hold line screen #aah #ei4 ,arrangement of apparatus for copying #aai #fj4 ,drawing showing method of arranging camera and copying stand for adjustment #aai #fa4 ,photograph of line screen and metal print facing page #abd #fb4 ,photograph of sketch drawn upon metal foil #fc4 ,method of marking out copying board #abd #fd4 ,diagram illustrating law of refraction #abg #fe4 ,forms of lenses #abh #ff4 ,action of light passed through a prism #abi #fg4 ,diagram illustrating action of a lens #acj #fh4 ,formation of principal focus of a lens #acj #fi4 ,formation of conjugate foci of a lens #aca #gj4 ,apparatus illustrating principle of camera #acb #ga4 ,formation of an image by a lens #acc #gb4 ,diagram illustrating apparent magnitude #acd #gc4 ,formation of virtual image by a convex lens #acg #gd4 ,formation of virtual image by a concave lens #ach #ge4 ,diagram showing spherical aberration #aci #gf4 ,combination of plano-convex lenses #aci #gg4 ,combination of meniscus and convex lenses #aci 99 99 99 99 99 #a ,,radio-,,photography ,,chapter ,i ,,introductory ,those who desire to experiment on radio-photography1 i4e4 transmitting photographs1 drawings1 etc41 from one place to another without the aid of artificial conductors1 must cultivate at least an elementary knowledge of optics1 chemistry1 mechanics1 and electricity2 photo-telegraphy calling for a knowledge of all these sciences4 ,there are1 no doubt1 many wireless workers who are interested in this subject1 but who are deterred from experimenting owing to a lack of knowledge regarding the direction developments are taking1 besides which1 information on this subject is very difficult to obtain1 the science of photo-telegraphy being1 at the present time1 in a purely experimental stage4 ,the wireless transmission of photographs has1 no doubt1 a great commercial value1 but for any system to be commercially practicable1 it must be simple1 rapid1 and reliable1 besides being able to work #b in conjunction with the apparatus already installed for the purpose of ordinary wireless telegraphy4 ,as far back as #ahdg experiments were carried out with a view to solving the problem of transmitting pictures and writing by electrical methods over artificial conductors1 but no great incentive was held forth for development owing to lack of possible application2 but owing to the great public demand for illustrated newspapers that has recently sprung into being1 a large field has been opened up4 ,during the last ten years1 however1 development has been very rapid1 and some excellent results are now being obtained over a considerable length of line4 ,the wireless transmission of photographs is1 on the other hand1 of quite recent growth1 the first practicable attempt being made by ,mr4 ,hans ,knudsen in #aijh4 ,it may seem rather premature to talk about the wireless transmission at a time when the systems for transmitting over ordinary conductors are not perfectly developed1 but everything points to the fact that for long-distance transmission a reliable wireless system will prove to be both cheaper and quicker than transmission over ordinary land lines and cables4 ,the effects of capacity and inductance--properties inherent to all telegraph systems using metallic conductors--have a distinct bearing upon the two questions1 how far and how quickly can #c photographs be transmitted8 ,owing to the small currents received and to prevent interference from earth currents it is necessary to use a complete metallic circuit4 ,if an overhead line could be employed no difficulty would be experienced in working a distance of over #ajjj miles1 but a line of this length is impossible--at least in this country--and if transmission is attempted with any other country1 a certain amount of submarine cable is essential4 ,it has been found that the electrostatic capacity of one mile of submarine cable is equal to the capacity of #bj miles of overhead line1 and as the effect of capacity is to retard the current and reduce the speed of working1 it is evident that where there is any great length of cable in the circuit the distance of possible transmission is enormously reduced4 ,if we take for an example the ,london-,paris telephone line with a length of #caa miles and a capacity of #aj4fb microfarads1 we find that about half this capacity1 or #e4i microfarads1#a is contributed by the #bc miles of cable connecting ,england with ,france4 ,in practice the reduction of speed due to capacity has1 to a great extent1 been overcome by means of apparatus known as a line-balancer1 which hastens the slow discharge of the line and #d allows each current sent out from the transmitter--the current in several systems being intermittent--to be recorded separately on the receiver4 ,photographs suitable for press work can now be sent over a line which includes only a short length of cable for a distance of quite #djj miles in about ten minutes1 the time1 of course1 depending upon the size of the photograph4 ,in extending the working to other countries where there is need for a great length of cable1 as between ,england and ,ireland1 or ,america1 the retardation due to capacity is very great4 ,on a cable joining this country with ,america the current is retarded four-tenths of a second4 ,in submarine telegraphy use is made of only one cable with an earth return1 but special means have had to be adopted to overcome interference from earth currents1 as the enormous cost prohibits the laying of a second cable to provide a complete metallic circuit4 ,the current available at the cable ends for receiving is very small1 being only #a_/bjjjjjth part of an ampere1 and this necessitates the use of apparatus of a very sensitive character4 ,one system of photo-telegraphy in use at the present time1 employs what is known as an electrolytic receiver 7see ,chapter ,,iii47 which can record signals over a length of line in which the capacity effects are very slight1 with the marvellous speed of #ab1jjj a minute1 but this speed rapidly decreases with an increase of distance between the #e ,illustration two stations4 ,the effect of capacity upon an intermittent current is clearly shown in ,fig4 #a4 ,if we were to send twenty brief currents in rapid succession over a line of moderate capacity in a given time1 we should find that instead of being recorded separately and distinctly as at a1 each mark would be pointed at both ends and joined together as shown at b1 while only perhaps fifteen could be recorded4 ,if the capacity be still farther increased as at c1 only perhaps half the original number of currents could be recorded in the same time1 owing to the fact that with an increase of resistance1 capacity1 and inductance of the line a longer time is required for it to charge up and discharge1 thereby materially lessening the rate at which it will allow separate signals to pass2 the number of signals that can therefore be recorded in a given time is greatly diminished4 ,if we were to attempt to send the same number of signals over a line of great capacity1 as could be sent1 and recorded separately and distinctly over a line of small capacity--the time limit being of course the same in both instances--we should find that the #f signals would be recorded practically as a continuous line4 ,the two latter cases b1 and c1 ,fig4 #a1 clearly shows the retardation that takes place at the commencement of a current and the prolongation that takes place at the finish4 ,if the photo-telegraphic system previously mentioned could be rendered sensitive enough to work on the ,atlantic cables1 we should find that only about #abjj signals a minute could be recorded1 and this would mean that a photograph which could be transmitted over ordinary land lines in about ten minutes would take at least fifty minutes over the cable4 ,this would be both costly and impracticable1 and time alone will show whether1 for long-distance work1 transmission by wireless will be both cheaper and more rapid than any other method4 ,at present wireless telegraphy has not superseded the ordinary methods of communicating over land1 but there can be no doubt that wireless telegraphy1 if free from ,government restrictions1 would in certain circumstances very quickly supersede land-line telegraphy1 while it has proved a formidable commercial competitor to the cable as a means of connecting this country with ,america4 ,likewise we cannot say that no system of radio-photography will ever come into general use1 but where there is any great distance to be bridged1 especially over water1 wireless transmission is really the only practical solution4 ,from the #g foregoing remarks1 it is evident that a reliable system of radio-photography would secure a great victory in the matter of time and cost alone1 besides which1 the photo-telegraphic apparatus would be merely an accessory to the already existing wireless installation4 ,illustration3 ,,fig4 #b4 ,there have been numerous suggestions put forward for the wireless transmission of photographs1 but they are all more or less impracticable4 ,one of the earliest systems was devised by de' ,bernochi of ,turin1 but his system can only be regarded interesting from an historical point of view1 and as in all probability it could only have been made to work over a distance of a few hundred yards it is of no practical value4 ,fig4 #b will help to explain the apparatus4 ,a glass cylinder ,a' is fastened at one end to a threaded steel shaft1 which runs in two bearings1 one bearing having an internal thread corresponding with that on the #h shaft4 ,round the cylinder is wrapped a transparent film upon which a photograph has been taken and developed4 ,light from a powerful electric lamp ,l1 is focussed by means of the lens1 ,n1 to a point upon the photographic film4 ,as the cylinder is revolved by means of a suitable motor1 it travels upwards simultaneously by reason of the threaded shaft and bearing1 so that the spot of light traces a complete spiral over the surface of the film4 ,the light1 on passing through the film 7the transmission of which varies in intensity according to the density of that portion of the photograph through which it is passing71 is refracted by the prism ,p on to the selenium cell ,s which is in series with a battery ,b and the primary ,x of a form of induction coil4 ,as light of different intensities falls upon the selenium cell1#b the resistance of which alters in proportion1 current is induced in the secondary ,y of the coil and influences the light of an arc lamp of whose circuit it is shunted4 ,this arc lamp ,t is placed at the focus of a parabolic reflector ,r1 from which the light is reflected in a parallel beam to the receiving station4 ,the receiver consists of a similar reflector ,r' with a selenium cell ,e placed at its focus1 whose resistance is altered by the varying light falling upon it from the reflector ,r4 ,the selenium cell #i ,e is in series with a battery ,f and the mirror galvanometer ,h4 ,light falls from a lamp ,d and is reflected by the mirror of the galvanometer on to a graduated aperture ,j and focussed by means of the aplanatic lens ,u upon the receiving drum ,a#b1 which carries a sensitised photographic film4 ,the two cylinders must be revolved synchronously4 ,the above apparatus is very clever1 but cannot be made to work over a distance of more than #bjj yards4 ,a system based on more practical lines was that invented and demonstrated by ,mr4 ,hans ,knudsen1 but the apparatus which he employed for receiving has been discarded in wireless work1 as it is not suitable for working with the highly-tuned systems in use at the present time4 ,knudsen's transmitter1 a diagrammatic representation of which is given in ,fig4 #c1 consists of a flat table to which a horizontal to-and-fro motion is given by means of a clockwork motor4 ,upon this table is fastened a photographic plate which has been prepared in the following manner4 ,the plate upon which the photograph is to be taken has the gelatine film from three to four times thicker than that commonly used in photography4 ,in the camera1 between the lens and this plate1 a single line screen is interposed1 which has the effect of breaking the picture up into parallel lines4 ,upon the plate being developed and before it is #aj ,illustration completely dry1 it is sprinkled over with fine iron dust4 ,with this type of plate the transparent parts dry much quicker than the shaded or dark parts1 and on the iron dust being sprinkled over the plate it adheres to the darker portions of the film to a greater extent than it does to the lighter portions2 a picture partly composed of iron dust is thus obtained4 ,a steel point attached to a flat spring rests upon this plate and is made to travel at right angles to the motion of the table4 ,as the picture is partly composed of iron dust1 and as the steel needle is fastened to a delicate spring it is evident that as the plate passes to and fro under the needle1 both the spring and needle are set in a state of vibration4 ,this vibrating spring makes #aa and breaks the battery circuit of a spark coil1 which in turn sets up sparking in the spark-gap of the wireless apparatus4 ,the receiver consists of a similar table to that used for transmitting1 and carries a glass plate that has been smoked upon one side4 ,a similar spring and needle is placed over this plate1 but is actuated by means of a small electro-magnet in circuit with a battery and a sensitive coherer4 ,as the coherer makes and breaks the battery circuit by means of the intermittent waves sent out from the transmitting aerial1 the needle is made to vibrate upon the smoked glass plate in unison with the needle at the transmitting end4 ,scratches are made upon the smoked plate1 and these reproduce the picture on the original plate4 ,a print can be taken from this scratched plate in a similar manner to an ordinary photographic negative4 ,the two tables are synchronised in the following manner4 ,every time the transmitting table is about to start its forward stroke a powerful spark is produced at the spark-gap4 ,the waves set up by this spark operate an ordinary metal filings coherer at the receiving end which completes the circuit of an electro-magnet4 ,the armature of this magnet on being attracted immediately releases the motor used for driving1 allowing it to operate the table4 ,the time taken to transmit a photograph1 quarter-plate size1 is about fifteen minutes4 #ab ,although very ingenious this system would not be practicable1 as besides speed the quality of the received pictures is a great factor1 especially where they are required for reproduction purposes4 ,the results from the above apparatus are said to be very crude1 as with the method used to prepare the photographs no very small detail could be transmitted4 99 99 99 99 99 #ac ,,chapter ,,ii ,,transmitting ,,apparatus ,let us now consider the requirements necessary for transmitting photographs by means of the wireless apparatus in use at the present time4 ,illustration3 ,,fig4 #d4 ,the connections for an experimental syntonic wireless transmitting station are shown in the diagram ,fig4 #d4 ,a is the aerial2 ,t1 the inductance2 ,e1 earth2 ,l1 hot-wire ammeter4 ,the closed oscillatory circuit consists of an inductance ,f1 spark-gap ,g1 and a block condenser ,c4 ,h is a spark-coil for supplying the energy1 the secondary ,j being connected to the spark-gap4 ,a #ad mercury break ,n and a battery ,b are placed in the primary circuit of the coil4 ,the ,morse key ,k is for completing the battery circuit for signalling purposes4 ,when the key ,k is depressed1 the battery circuit is completed1 and a spark passes between the balls of the spark-gap ,g producing oscillations in the closed circuit1 which are transposed to the aerial circuit by induction4 ,for signalling purposes it is only necessary for the operator by means of the key ,k to send out a long or short train of waves in some pre-arranged order1 to enable the operator at the receiving station to understand the message that is being transmitted4 ,if a photograph could be prepared in such a manner that it would serve the purpose of the key ,k1 and could so arrange matters that a minute portion of the photograph could be transmitted separately but in succession1 and that each portion of the photograph having the same density could be given the same signal1 then it would only be necessary to have apparatus at the receiving station capable of arranging the signals in proper sequence 7each signal recorded being the same size and having the same density as the transmitted portion of the photograph7 in order to receive a facsimile of the picture transmitted4 ,the following method of preparing the photograph#c is one that has been adopted in several #ae systems of photo-telegraphy1 and is the only one at all suitable for wireless transmission4 ,the photograph or picture which is to be transmitted is fastened out perfectly flat upon a copying-board4 ,a strong light is placed on either side of this copying board1 and is concentrated upon the picture by means of reflectors4 ,the camera which is used for copying has a single line screen interposed between the lens and sensitised plate1 and the effect of this screen is to break the picture up into parallel lines4 ,thus a white portion of the photograph would consist of very narrow lines wide apart1 while the dark portion would be made up of wide lines close together2 a black part would appear solid and show no lines at all4 ,from this line negative it will be necessary to take off a print upon a specially prepared sheet of metal4 ,this consists of a sheet of thick lead- or tinfoil1 coated upon one side with a thin film of glue to which bichromate of potash has been added2 the bichromate possessing the property of rendering the glue waterproof when acted upon by light4 ,the print can be taken off by artificial light 7arc lamps being generally used71 but the exact time to allow for printing can only be found by experiment1 as it varies considerably according to the thickness of the film4 ,the printing finished1 the metal print is washed under running water1 when all those parts not acted upon by light1 i4e4 the parts between the lines1 are #af washed away1 leaving the bare metal4 ,we have now an image composed of numerous bands of insulating material 7each band varying in width according to the density of the photograph at any point from which it is prepared7 attached to a metal base1 so that each band of insulating material is separated by a band of conducting material4 ,it is1 of course1 obvious that the lines on the print cannot be wider apart1 centre to centre1 than the lines of the screen used in preparing it4 ,a good screen to use is one having #ej lines to the inch1 but one is perhaps more suitable for experimental work a little coarser1 say #ce lines to the inch4 ,to use a screen having #ej or more lines to the inch1 the transmitting apparatus1 as will be evident later on1 will require to be very nearly perfect4 ,illustration3 ,,fig4 #e4 ,before proceeding further it will perhaps be as well to make an experiment4 ,if we take one of the metal prints or1 more simple1 draw a sketch in insulating ink upon a sheet of metal ,a1 ,fig4 #e1 and connect a battery ,b and the galvanometer ,d as shown1 we shall find on drawing the free end of the wire across the metal plate that all the time the wire is in contact with the lines of insulating material the needle of the galvanometer will remain #ag at zero1 but where it is in contact with the metal plate the needle is deflected4 ,from this experiment it will be seen that we have in our metal line print1 which consists of alternate lines of insulating and conducting material1 a method by which an electric circuit can be very easily made and broken4 ,it is1 of course1 necessary to have some arrangement whereby the whole of the surface of the metal print is utilised for this purpose to the best advantage4 ,one type of transmitting machine used for this purpose is represented by the diagram1 ,fig4 #f4 ,the cylinder ,a is fastened to the steel shaft ,b1 which runs in the two bearings ,d and ,d'1 the bearing ,d' having an internal thread corresponding to that on the shaft4 ,the stylus in this class of machine is a fixture1 the cylinder being given a lateral as well as a revolving movement4 ,as it is impossible to use a rigid drive1 a flexible coupling ,f is employed between the shaft ,b and the motor4 ,illustration3 ,,fig4 #f4 ,another type of machine is shown in ,fig4 #g4 ,the drum in this case is stationary1 the table ,t moving laterally by reason of the screwed shaft #ah ,illustration and half nut ,f4 ,the table1 shown separate in ,fig4 #h1 carries a stiff brass spring ,a1 to which is attached a holder ,b made to take a hardened steel point4 ,the holder is provided with a set screw ,p for securing the steel point ,z4 ,the spring and needle are insulated from the rest of the machine1 as shown in the drawing4 ,in working1 the metal print is wrapped tightly round the cylinder of the machine1 the glue image being1 of course1 uppermost4 ,to fasten the print a little seccotine should be applied to one edge1 and the joint carefully smoothed down with the fingers4 ,illustration ,if there is any tendency on the part of the print to slip round on the drum1 a couple of small spring clips placed over the ends of the drum will act as a preventive4 ,it is necessary to place the print upon the drum in such a manner that the stylus draws away from the edge of the lap and not towards it1 and the metal prints should be of such a size that when placed round the drum of the #ai machine a lap of about #c_/afths of an inch is allowed4 ,illustration3 ,,fig4 #i4 ,the steel point ,z 7ordinary gramophone needles may be used and will be found to answer the purpose admirably7 is made to press lightly upon the metal print1 and while the pressure should be sufficient to make good electrical contact1 it should not be sufficient to cause the needle to scratch the surface of the foil4 ,the pressure is regulated by means of the milled nut ,h4 ,the electrical connections are given in ,fig4 #i4 ,one wire from the battery ,m is taken to the terminal ,t1 and the other wires from ,m and ,f lead to the relay ,r4 ,the current flows from the battery ,m through the spring ,y1 through the drum and metal print1 the stylus ,z1 spring ,a1 down to the relay ,r1 and from ,r back to the battery ,m4 ,as the drum carrying the single line half-tone print is revolved1 the stylus1 by reason of the lateral movement given to the table or cylinder as the case may be1 will trace a spiral path over the entire surface of the print4 ,as the stylus traces over a conducting strip the circuit is completed1 and the tongue of the relay ,r is attracted1 making contact with the stop ,s4 #bj ,on passing over a strip of insulation the circuit is broken and the tongue of the relay ,r returns to its normal position4 ,as already stated1 the conducting and insulating bands on the print vary in width according to the density of the photograph from which it is prepared1 so that the length of time that the tongue of the relay ,r is held against the stop ,s1 is in proportion to the width of the conducting strip which is passing under the stylus at any instant4 ,the function of the transmitter is therefore to send to the relay ,r an intermittent current of varying duration4 ,the two photographs ,figs4 #aj and #aj;a are of a machine designed and used by the writer in his experiments4 ,in this machine the drum is #c4e inches long and #a4e inches in diameter4 ,the lead screw has #cj threads to the inch1 and the reduction between it and the drum is #c3a1 so that the table has a movement of #a_/ijth inch per revolution of the drum4 ,from the brief description of the various types of machines that have been given it will be apparent that in the design of the machine proper there is nothing very complicated1 although the addition of the driving and synchronising apparatus complicates matters rather considerably4 ,the questions of driving and synchronising the machines at the two stations is fully dealt with in ,chapter ,,iv4 ,illustration3 ,,fig4 #aj;a4 ,illustration3 ,,fig4 #aj;b4 ,enlarged view of an image broken up by a cross screen4 #ba ,although the design of the machines is rather simple great attention must be paid both to accuracy of construction and accuracy of working1 and this applies1 not only to the machines 7whether for transmitting or receiving7 but for all the various pieces of apparatus that are used4 ,too much care cannot be bestowed upon this point1 as in the wireless transmission of photographs there is a large number of instruments all requiring careful adjustment1 and which have to work together in perfect unison at a high speed4 ,the machine shown in ,figs4 #aj and #aj;a was designed and used by the writer solely for experimental work4 ,it will be noticed in the description given in the appendix of the method of preparing the metal prints that a #e x #d camera is recommended1 while the machine1 ,fig4 #aj1 is designed to take a print procured from a quarter-plate negative4 ,this size of drum was adopted for several reasons1 and although it will be found quite large enough for general experimental work the writer has come to the conclusion that for practical commercial work a drum to take a print #e x #d will give better results4 ,in making a negative of a picture that is required for reproduction purposes1 the line screen in the camera is replaced by a cross screen1 i4e4 two single line screens placed with their lines at an angle of #ij;deg to one another1 and this breaks the #bb image up into small squares instead of lines4 ,by looking at any ordinary newspaper or book illustration through a powerful magnifying glass the effects of a cross screen will readily be seen4 ,with a cross screen a certain amount of detail is necessarily lost1 but with a single line screen the amount lost is much greater4 ,if there is any very small detail in the picture most of this would be lost in a coarse screen1 hence the necessity of employing as fine a line screen as practicable in order to get as much detail in as possible4 ,it is mainly on this account that a #e x #d print is recommended1 as1 if fairly bold subjects are used for copying1 the small detail 7this is1 of course1 a very vague and indefinable term7 will not be too fine1 and the time required for transmitting reasonable4 ,for obvious reasons it is a great advantage to put the print under pressure to cause the glue image to sink into the soft metal base and leave a perfectly flat and smooth surface4 ,it is essential that the bands on the print lie along the axis of the cylinder1 so that the stylus traces its path across them1 and not with them4 ,we have now an arrangement that is capable of taking the place of the key ,k1 ,fig4 #d1 and the diagram1 ,fig4 #aa1 gives the connections for the complete transmitter4 ,a is the aerial1 ,e earth1 ,t inductance1 ,l ammeter4 ,the closed oscillatory circuit consists of a spark-gap ,g1 inductance ,f1 #bc ,illustration and a condenser ,c4 ,the secondary ,j of the coil ,h is connected to the spark-gap1 and the primary ,p is in circuit with the mercury break ,n1 the battery ,b1 and the local contacts of the relay ,r4 ,the action is as follows4 ,when contact is made between the stylus ,z and the drum ,v by means of the conducting bands on the line print1 the circuit of the relay ,r and the battery ,m is completed4 ,the closing of the local circuit of the relay ,r actuates the second relay ,r'1 allowing the primary circuit of the coil ,h to be closed4 ,as soon as the primary circuit of the coil is completed sparks pass between the electrodes of the spark-gap ,g1 causing waves to radiate from the aerial4 ,the duration of the wave-trains radiated depends upon the duration of contact made by the relays #bd ,r and ,r'1 and this in turn depends upon the width of the conducting strip that is passing under the stylus4 ,the battery ,m should be about #d volts1 and the battery ,d about #b volts4 ,the two-way switch ,x is connected up so that the relay ,r' can be thrown out and the key ,k switched in for ordinary signalling purposes4 ,if any sparking takes place at the point of the stylus1 a small condenser ,c' 7about #a microfarad capacity7 should be connected as shown4 ,in the present instance the condenser should be used more as a preventive than as a cure1 as in all probability the voltage from ,m will not be sufficient to cause destructive 7if any7 sparking2 but1 as most wireless workers know1 anything in the nature of a spark occurring in the neighbourhood of a detector 7this1 of course1 only applies when the receiving apparatus is placed in close proximity to the transmitter7 is liable to destroy the adjustment4 ,in transmitting over ordinary conductors where the initial voltage is fairly high and the self-induction of the circuit very great1 the use of the condenser will be found to be absolutely essential4 ,it has also been noted that the angle which the stylus presents to the drum has a marked effect upon the sparking1 an angle of about #fj;deg being found to give very good results4 ,if the size of the single line print used is #e inches by #d inches1 and a screen having #ej lines #be to the inch is used for preparing it1 then the stylus will have to make #bej contacts during one revolution of the drum4 ,assuming the drum to make one revolution in three seconds1 then the time taken to transmit the complete photograph can be found from the equation ,t w x t x s1 where w is the width of the print1 t the travel of the stylus during one revolution of the drum1 and s the time required for one revolution of the drum4 ,in the present instance this will be ,t #d x #ij x #c #ajhj seconds #ah minutes4 ,the number of contacts made by the stylus per minute is #ejjj1 and in working at this speed the first difficulty is encountered in the use of the two relays4 ,the relay ,r is lightly built1 and capable of working at a fairly high speed1 but ,r' is a heavier pattern1 and consequently works at a slightly lower rate4 ,this relay must necessarily be heavier1 as more substantial contacts are needed in order to pass the heavy current taken by the spark-coil4 ,relays sensitive and accurate enough to work at this speed will in all probability be beyond the reach of the majority of workers1 but there are several types of relays on the market very reasonable in price that will answer very well for experimental work1 although the speed of working will no doubt be slower4 ,for the best results the duration of the wave-trains sent out should be of the same duration as #bf the contact made by ,r1 and therefore equal to the time taken by the stylus to trace over a conducting strip2 but if the duration of the contact made by ,r is t1 then that made by ,r' and consequently the duration of the groups of wave-trains would be t - v where v equals the extra time required by ,r' to complete its local circuit4 ,the difference in time made by the two relays1 although very slight1 will be found to affect very considerably the quality of the received pictures4 ,renewing the platinum contacts is also a great expense1 as they are soon burnt out where a heavy current is passed4 ,if the distance experimented over is short so that the power required to operate the spark-coil is not very heavy1 one relay will be sufficient providing the contacts are massive enough to carry the current safely4 ,it is useless to expect any of the ordinary relays in general use to work satisfactorily at such a high speed1 and in order to compensate for this we must either increase the time of transmitting1 or1 as already suggested1 make use of a coarser line screen in preparing the photographs4 ,for reasons already explained1 all points of make and break should be shunted by a condenser4 ,the effective working speed of an ordinary type of relay may be anything from #ajjj to #bejj dots a minute1 depending upon accuracy of design and construction4 ,in the wireless transmission of photographs it #bg is absolutely essential to use some form of rotary spark-gap1 as where sparks are passed in rapid succession the ordinary type of gap is worse than useless4 ,when a spark passes between the electrodes of an ordinary spark-gap1 ,fig4 #ab1 we find that for a fraction of a second after the first spark has passed1 the normally high resistance of the gap has been lowered to less than one ohm4 ,if the column of hot gas which constitutes the spark is not instantly dispersed1 but remains between the electrodes1 it will provide an easy path for any further discharges1 and if sparks are passed at all rapidly1 what was at first a disruptive and oscillatory discharge will degenerate into a hot1 non-oscillatory arc4#d ,illustration3 ,,fig4 #ab4 ,two forms of rotating spark-gaps are shown in ,figs4 #ac and #ad1 and are known as synchronous and non-synchronous gaps respectively4 ,in the synchronous gap the cog-wheel is mounted on the shaft of the alternator1 and a cog comes opposite the fixed electrode when the maximum of potential is reached in the condenser1 thus ensuring a discharge at every alternation of current4 ,with this type of gap a spark of pure tone is obtained which #bh ,illustration ,illustration is of great value where the signals are received by means of a telephone1 but where the signals are to be mechanically recorded the tone of the spark is of little consequence4 ,in a non-synchronous gap a separate motor is used for driving the toothed wheel1 and can either be mounted on the motor shaft or driven by means of a band1 there being no regard given to synchronism with the alternator4 ,the fixed electrode is best made long enough to cover about two of the teeth1 as this ensures regular sparking and a uniform sparking distance2 the #bi spark length is double the length of the spark-gap4 ,the toothed wheel should revolve at a high speed1 anything from #ejjj to #hjjj revolutions per minute1 or even more being required4 ,the shaft of the toothed wheel is preferably mounted in ball-bearings4 ,owing to the large number of sparks that are required per minute in order to transmit a photograph at even an ordinary speed1 it is necessary that the contact breaker be capable of working at a very high speed indeed4 ,the best break to use is what is known as a mercury jet interrupter1 the frequency of the interruptions being in some cases as high as #gj1jjj per second4 ,no description of these breaks will be given1 as the working of them is generally well understood4 ,in some cases an alternator is used in place of the battery ,b1 ,fig4 #d1 and when this is done the break ,m can be dispensed with4 ,in larger stations the coil ,h is replaced with a special transformer4 ,the writer has designed an improved relay which will respond to currents lasting only #a_/ajjth part of a second1 and capable of dealing with rather large currents in the local circuit4#e ,this relay has not yet been tried1 but if it is successful the two relays ,r and ,r' can be dispensed with1 and the result will be more accurate and effective transmission4 #cj ,illustration3 ,,fig4 #ae4 ,the connections for a complete experimental station1 transmitting and receiving apparatus combined1 are given in ,fig4 #ae4 ,the terminals ,w1 ,w are for connecting to the photo-telegraphic receiving apparatus ,q1 being a double pole two-way switch for throwing either the transmitting or receiving apparatus in circuit4 ,there is another system of transmitting devised by ,professor ,korn1 which employs an entirely different method from the foregoing4 ,by using the apparatus just described1 the waves generated are what are known as damped waves1 and by using these damped waves1 tuning1 which is so essential to good commercial working1 can be made to reach a fairly high degree of efficiency4 #ca ,the question of damped versus undamped waves is a somewhat burning one1 and no attempt will be made here to deal with the merits or demerits of the claims made for the respective systems4 ,a series of articles describing the production of undamped waves and their efficiency in working compared with damped waves will be found in the ,wireless ,world1 ,nos4 #c and #d1 #aiac1 and are well worth reading by any one interested in the subject4 ,illustration3 ,,fig4 #af4 ,a diagrammatic representation of the apparatus as arranged by ,professor ,korn is given in ,fig4 #af4 ,the undamped or continuous waves are generated by means of a high-frequency alternator or ,poulsen arc4 ,in ,fig4 #af1 ,x is the generator1 ,f inductance1 ,c condenser2 the aerial inductance ,t is connected by the aerial ,a and earth ,e4 ,by this means the waves are tuned to a certain period4 #cb ,a metal print1 similar to what has already been described1 is wrapped round the drum ,d of the machine1 and when the stylus ,z traces over an insulating strip the waves generated are in tune with the receiving station1 but when it traces over a conducting strip1 a portion of the inductance ,t is short-circuited1 the period of the oscillations is altered1 and the two stations are thrown out of tune4 ,the receiving station is provided with an aperiodic circuit1 which consists of an inductance ,f'1 condenser ,c'1 and a thermodetector ,n4 ,a string galvanometer ,h 7described in ,chapter ,,iii471 and the self-induction coils ,b1 ,b' are connected as shown1 the coils ,b1 ,b' preventing the high-frequency currents1 which change their direction1 from flowing through the galvanometer4 ,the manner in which the string galvanometer is arranged to reproduce a transmitted picture is shown in ,fig4 #bd4 ,the connections adopted by the ,poulsen ,company for photographically recording wireless messages are given in ,fig4 #ag1 a string galvanometer of the ,einthoven type being used4 ,the two self-induction coils ,s and ,s' are in circuit with the detector ,d and the galvanometer ,g4 ,the condenser ,c' prevents the continuous current produced by the detector from flowing through the high frequency circuit2 ,p is the primary of the aerial #cc inductance and ,f the secondary4 ,the method of transmitting adopted by ,professor ,korn appears to be a simple and reliable arrangement1 provided that an equally reliable method of producing the undamped waves can be found4 ,owing to the absence of mechanical inertia it should be capable of working at a good speed1 while the absence of a number of pieces of delicate apparatus all requiring careful adjustment add greatly to its reliability4 ,illustration3 ,,fig4 #ag4 ,in any spark system with a properly designed aerial a coil taking ten amperes is capable of transmitting signals over a distance of thirty to fifty miles1 but where the number of interruptions of the break required per second is very high1 as in radio-photography1 it must be remembered that a much higher voltage is needed to drive the requisite amount of current through the primary winding of the coil than would be the case if the interruptions were slower4 ,it is possible to use platinum #cd contacts for the relays1 for currents up to ten amperes1 but for heavier currents than this some arrangement where contact is made with mercury will be found to be more economical and reliable4 ,in the transmitter already described and given in ,fig4 #aa1 the best results would be obtained by finding the speed at which the relay ,r' works best1 and regulating the number of contacts made by the stylus accordingly4 ,the method employed by ,de' ,bernochi 7see ,chapter ,i47 of varying the intensity of a beam of light by passing it through a photographic film1 which in turn alters the resistance of a selenium cell1 has been very successfully employed in at least one system of photo-telegraphy4 ,its application has also been suggested for wireless transmission1 and although with any system using continuous waves this would not be very difficult1 it could hardly be adapted to work with the ordinary spark system4 ,the apparatus for receiving from this type of transmitter would1 on the other hand1 necessarily be more elaborate than the methods that are described in the next chapter1 and as far as the writer's experience goes1 experiments along these lines would not prove very profitable1 as simplicity is the keynote of success in any radio-photographic system4 ,it has been suggested that in order to decrease the time of transmission a cylinder capable of #ce taking a print #g inches by #e inches be employed1 the print being prepared from rather a coarse line screen--say #ce to the inch--and a traverse of about #a_/ej inch given to the stylus1 thus reducing the time of transmission to about twelve minutes4 ,it is questionable1 however1 whether the increase in speed would compensate for the loss of detail1 as only very bold subjects could be transmitted4 ,as already pointed out1 wireless transmission would only be employed for fairly long distances1 and the extra time and expense required to receive a fairly good detailed picture is negligible when compared with the enormous time it would take to receive the original photograph by any ordinary means of transit4 ,the public much prefer to have passable pictorial illustrations of current events than wait several days for a more perfect picture--the original1 and the advantage of any newspaper being able to publish photographs several days before its rivals is obvious4 ,there can also be no doubt but that a system of radio-photography1 if fairly reliable and capable of working over a distance of say thirty miles1 would be of great military use for transmitting maps and written matter with a great saving of time and even life4 ,written matter could be transmitted with even greater safety than messages which are sent in the ordinary way in ,morse ,code1 as the signals received in the receiver #cf of an hostile installation would be but a meaningless jumble of sounds1 and even were they possessed of radio-photographic apparatus the received message would be unintelligible1 unless they knew the exact speed at which the machines were running and could synchronise accurately4 99 99 99 99 99 #cg ,,chapter ,,iii ,,receiving ,,apparatus ,there are only two methods available at present for receiving the photographs1 and both have been used in ordinary photo-telegraphic work with great success4 ,they have disadvantages when applied to wireless work1 however1 but these will no doubt be overcome with future improvements4 ,the two methods are 7#a7 by means of an ordinary photographic process1 and 7#b7 by means of an electrolytic receiver4 ,in several photo-telegraphic systems the machine used for transmitting has the cylinder twice the size of the receiving cylinder1 thus making the area of the received picture one-quarter the area of the picture transmitted4 ,the extra quality of the received picture does not compensate for the disadvantage of having to provide two machines at each station1 and in the writer's opinion results1 quite good enough for all practical purposes1 can be obtained by using a moderate size cylinder so that one machine answers for both transmitting #ch and receiving1 and using as fine a line screen as possible for preparing the photographs4 ,illustration3 ,,fig4 #ah4 ,the writer1 when first experimenting in photo-telegraphy1 endeavoured to make the receiving apparatus self-contained1 and one idea which was worked out is given in ,fig4 #ah4 ,the electric lamp ,l is about #h c4p41 and is placed just within the focus of a lens which has a focal length of #c_/d inch4 ,when a source of light is placed at some point between a lens and its principal focus1 the light rays are not converged1 but are transmitted in a parallel beam the same size as the lens4 ,it has been found that this arrangement gives a sharper line on the drum than would be the case were the light focussed direct upon the hole in the cone ,a4 ,an enlarged drawing of the cone is given in ,fig4 #ai4 ,the hole in the tip of the cone ,a is a bare #a_/ij inch in diameter--the size of this hole depends upon the travel per revolution of the drum or table of the machine used--and in working1 the cone is run as close as possible to the #ci drum without being in actual contact4 ,the magnet ,m is wound full with ,no4 #dj ,s4,c4,c4 wire1 and the armature is made as light as possible4 ,the spring to which the armature is attached should be of such a length that its natural period of vibration is equal to the number of contacts made by the transmitting stylus4 ,the spring must be stiff enough to bring the armature back with a fairly crisp movement4 ,the spring and armature is shown separate in ,fig4 #bj4 ,illustration3 ,,fig4 #ai4 ,illustration3 ,,fig4 #bj4 ,the shutter ,c is about #a_/d inch square and made from thin aluminium4 ,the hole in the centre is #a_/af x #a_/h inch1 and the movement of the armature is limited to about #c_/cb inch4 ,in all arrangements of this kind there is a tendency for the armature spring to vibrate1 as it were1 sinusoidally1 if the coil is magnetised and demagnetised at a higher rate than the natural period of vibration of the spring4 #dj ,this causes an irregularity in the rate of the vibrations which affects the received image very considerably4 ,a photographic film is wrapped round the drum of the machine1 being fastened by means of a little celluloid cement smeared along one edge4 ,this device1 although it will work well over artificial conductors1 is not suitable for wireless work1 as it is too coarse in its action2 it can be made sensitive enough to work at a speed of #ajjj to #aejj contacts per minute1 with a current of 4#e milliampere4 ,it is impossible to obtain a current of this magnitude from the majority of the detectors in use1 so that if any attempt is made to use this device for radio-photography it will be necessary to employ a ,marconi coherer 7filings71 as this is practically the only coherer from which so large a current can be obtained4 ,there have been many attempts made to receive with an ordinary filings coherer1 but as was pointed out in ,chapter ,i4 these have now been discarded in serious wireless work1 being only used in small amateur stations or experimental sets4 ,as the reasons for this are well known to the majority of wireless workers there is no need to enumerate them here4 ,a method whereby a filings coherer can be decohered1 the act of decohering closing a local circuit which contains the photographic #da receiving apparatus1 is given in the diagram ,fig4 #ba4 ,illustration3 ,,fig4 #ba4 ,in the figure1 the coherer ,c is fixed in rigid supports1 one support being provided with a platinum pin ,f4 ,to the coherer is connected the sensitive electro-magnet ,m1 which becomes magnetised as soon as the incoming waves act upon the coherer4 ,to the armature ,b is attached a light aluminium arm ,s1 pivoted at ,k1 and carrying at the other end the striker ,g1 which is fitted with a platinum contact4 ,when the armature ,b is attracted the coherer is decohered by the force of the impact between the contacts ,f and ,g4 ,to prevent damage to the coherer the force of the blow is taken off by the ability of the striker to work back through a hole in the arm ,s1 the spring #db ,n keeping it normally in a fixed position4 ,t and ,p are adjusting screws1 and the terminals ,j are for connecting to the receiving apparatus4 ,with this arrangement a very short wave-train causes only one tap of the contacts1 so that only one mark is registered on the receiving drum for every contact made on the transmitter4 ,illustration3 ,,fig4 #bb4 ,the drawing1 ,fig4 #bb1 gives a diagrammatic representation of apparatus arranged for another photographic method of receiving4 ,the machine shown in ,fig4 #f is used in this case4 ,a is the aerial1 ,e earth1 ,p primary of oscillation-transformer1 ,s secondary of transformer1 ,c variable condenser1 ,c' block condenser1 ,d detector1 ,x two-way switch1 ,t telephone4 ,a ,de' ,arsonval galvanometer ,h is also connected to the switch ,x1 so that either the telephone or the galvanometer can be switched in4 ,the #dc galvanometer can be made sensitive enough to work with a current as small as #aj-g of an ampere1 with a period of about #a_/aejth of a second4 ,the screen ,j has a small hole about #a_/h inch diameter drilled in the centre4 ,under the influence of the brief currents which pass through the detector every time a group of waves is received1 the mirror of the galvanometer swings to-and-fro in front of the screen ,j1 and allows the light reflected from the source of light ,m to pass through the aperture in the screen1 on to the lens ,n4 ,round the drum ,v of the machine is wrapped a sensitive photographic film1 and this records the movements of the mirror which correspond to the contacts on the half-tone print used in transmitting4 ,every time current passes through the galvanometer1 the light that is received from ,m1#f passes through the aperture in the screen ,j1 and is focussed by the lens ,n to a point upon the revolving film4 ,as soon as the current ceases1 the mirror swings back to its original position1 and the film is again in darkness4 ,upon being developed a photograph1 similar to the negative used for preparing the metal print is obtained4 ,if desired the apparatus can be so arranged that the received picture is a positive instead of a negative4 #dd ,the detector used should be a ,lodge wheel-coherer or a ,marconi valve-receiver1 as these are the only detectors that can be used with a recording instrument4 ,if the swing of the galvanometer mirror is too great1 a small battery with a regulating resistance can be inserted in order to limit the movement of the mirror to a very short range2 the current of course flowing in an opposite direction to the current flowing through the coherer4 ,in this1 as in all other methods of receiving1 the results obtained depend upon the fineness of the line screen used in preparing the metal prints2 and as already shown the fineness of the screen that can be used is dependent upon the mechanical efficiency of the entire apparatus4 ,another system1 and one that has been tried as a possible means of recording wireless messages1 is as follows4 ,the wireless arrangements consist of apparatus similar to that shown in ,fig4 #bb1 but instead of a ,lodge coherer a ,marconi valve is used1 and an ,einthoven galvanometer is substituted for the reflecting galvanometer4 ,the ,einthoven galvanometer consists of a very powerful electro-magnet1 the pole pieces of which converge almost to points4 ,a very fine silvered quartz thread is stretched between the pole pieces1 as shown in ,fig4 #bc1 the tension being adjustable4 ,the period of swing is about #a_/bejth of a second4 ,a hole is bored through the poles1 and one of them is fitted #de ,illustration with a sliding tube which carries a short focus lens ,n4 ,the light from ,m passes through the magnets1 and a magnified image of the quartz thread is thrown upon the ebonite screen ,j4 ,this screen is provided with a fine slit1 and when the galvanometer is at rest the shadow of the thread just covers the slit in the screen and prevents any light from ,m reaching the photographic film4 ,upon signals being received the shadow of the thread moves to one side for a long or short period1 uncovering the slit1 and allowing light to pass through4 ,the lens ,r concentrates the collected light to a point upon the revolving film4 ,the connections for the complete receiver are given in ,fig4 #bd4 ,the modified form of the ,einthoven galvanometer1 as arranged by ,professor ,korn for use with his selenium machines for photo-telegraphy over ordinary land lines1 consists of two fine silver wires which are displaced in a lateral direction between the pole pieces when traversed by a current2 the current passing through both wires in the same #df direction4 ,a small shutter of aluminium foil is attached to the wires at the optical centre4 ,the silver wires used are #a_/ajjj inch in diameter1 with a natural period of about #a_/abjth of a second2 the length of wires free to swing being usually about #e cm4 ,illustration3 ,,fig4 #bd4 ,the period of the wires depends to a great extent upon their length and diameter1 and also upon their tension4 ,by using short fine wires the period can be made much smaller1 but a greater current is required to produce a similar displacement4 ,where the current available1 as in wireless telegraphy1 is very small1 and a definite displacement of the wires is required1 it is at once apparent that with wires of a given diameter there is a limit to their length and therefore to the period4 ,finer wires can be used1 but here again there is a practical limit to their fineness1 although galvanometers have been constructed with a single silvered quartz thread #a_/abjjjth of an inch diameter1 which1 when placed in a powerful field1 will give a good displacement with a current as small as #aj-h ampere4 #dg ,with the apparatus arranged by the ,poulsen ,company1 given in the diagram1 ,fig4 #ag1 for photographically recording wireless signals1 the current required to operate the galvanometer for signals transmitted at the rate of #aejj a minute is #a x #aj-f ampere1 while for signals up to #bejj a minute a current about #e x #aj-f ampere is necessary4 ,another very sensitive instrument1 employed by ,m4 ,belin1 and known as ,blondel's oscillograph1 consists of two fine wires stretched between the poles of a powerful electro-magnet1 a small and very light mirror being attached to the centre of the wires4 ,the current passes down one wire and up the other1 and the wires1 together with the mirror1 are twisted to a degree depending upon the strength of the received current4 ,in order to render the instrument dead-beat the moving parts are arranged to work in oil4 ,the light reflected from the mirror is made use of in a manner similar to that shown in ,fig4 #bb4 ,in all photographic methods of receiving1 the apparatus must be enclosed in some way to prevent any extraneous light from reaching the film1 or better still placed in a room lighted only by means of a ruby light4 ,the following method is given more as a suggestion than anything else1 as ,i do not think it has been tried for wireless receiving1 although it is stated to have given some good results over #dh ordinary land lines4 ,it is the invention of ,charbonelle1 a ,french engineer1 and is quite an original idea4 ,his method consists of placing a sheet of carbon paper between two sheets of thin white paper1 and wrapping the whole tightly round the drum of the machine4 ,a hardened steel point is fastened to the diaphragm of a telephone receiver1 and this receiver is placed so that the steel point presses against the sheets of paper4 ,as the diaphragm and steel point vibrates under the influence of the received currents marks are made by the carbon sheet on the bottom paper4 ,over a line where a fair amount of current is available at the receiver1 the diaphragm would have sufficient movement to mark the paper1 but the movement would be very small with the current received from a detector4 ,this difficulty could no doubt be overcome to a certain extent by making a special telephone receiver1 with a large and very flexible diaphragm1 and wound for a very high resistance4 ,the movement of an ordinary telephone diaphragm for a barely audible sound is1 measured at the centre1 about #aj-f of a c4m4 ,with a unit current the movement at the centre is about #a_/gjjth of an inch4 ,greater movement of the diaphragm could be obtained by connecting a ,telephone relay to the detector1 and using the magnified current from the relay to operate the telephone4 #di ,illustration3 ,,fig4 #be4 ,the telephone relay consists of a microphone ,c1 ,fig4 #be1 formed of the two pieces of osmium iridium alloy4 ,the contact is separated to a minute degree partly by the action of the local current from ,f1 which flows through it and also through the winding ,w of the two magnet coils4 ,the local current from ,f assists in forming the microphone by rendering the space between the contacts conductive4 ,the vibrating reed ,p is fastened to the metal frame 7not shown7 which carries a micrometer screw by which the distance between the contacts can be accurately regulated4 ,it will be seen from ,fig4 #be that the local circuit consists of a battery ,f 7about #a4e volts71 the microphone contacts ,c1 the windings ,w1 milliampere meter ,b1 and the terminals ,t1 for connecting to the galvanometer or telephone1 all in #ej series4 ,on the top of the magnet cores ,n1 ,s is a smaller magnet ,d1 wound with fine wire for a resistance of about #dice ohms1 the free ends of the coils being connected to the detector terminals4 ,the working is as follows4 ,supposing the current from the detector flows through ,d in such a way that its magnetism is increased1 the reed ,p will be attracted1 the contacts opened1 and their resistance increased4 ,it will be seen that the current from ,f is passed through the coils ,w1 in such a way as to increase the magnetism of the permanent magnet1 so that any opening of the microphone contact increases their resistance1 causes the current to fall1 and weakens the magnets to such an extent that the reed ,p can spring back to its normal position4 ,on the other hand1 if the detector current flows through ,d in such a direction as to decrease the magnetism in the permanent magnets1 the reed ,p will rise and make better contact owing to the removal of the force opposing the stiffness of the reed4 ,owing to the decrease in the resistance of the microphone1 the strength of the local current will be increased1 the magnets strengthened1 and the reed ,p will be pulled back to its original position4 ,this relay gives a greatly magnified current when properly adjusted1 the current being easily increased from #aj-d to #aj-b amperes4 ,it is also very sensitive1 but needs careful adjustment in order that the best results may #ea be obtained4 ,a greater range of magnification can be obtained by placing two or more relays in series4 ,illustration3 ,,fig4 #bf4 ,a very sensitive receiver designed by the writer is given in the figures #bf and #bg4 ,to the centre of a telephone diaphragm is fastened a light steel point ,p1 and the movement of this point is communicated to the aluminium arm ,d1 which is pivoted at ,c4 ,as will be seen the telephone receiver is of special construction1 it containing only one coil and therefore only one core2 by this means the movement of the diaphragm is centralised4 ,the coil is wound for a resistance of about #bjj ohms1 and the diaphragm should be fairly thin but very resillient4 ,illustration3 ,,fig4 #bg4 ,to the free end of ,d is fastened the mirror ,t1 made from thin diaphragm glass about #a-a_/b centimetres diameter1 and having a focal length of #dj inches4 ,light from the lamp ,l is transmitted by the lens ,n in a parallel beam to the mirror which #eb concentrates it to a point upon a hole #a_/ajjth of an inch in diameter in the screen ,j4 ,as the telephone diaphragm vibrates under the influence of the received signals the arm1 and consequently the mirror1 vibrates also1 and the hole in the screen ,j is constantly being covered and uncovered by the spot of light4 ,it will be seen from ,fig4 #bg that the ratio between the centre of the mirror and the pivot ,c1 and ,c and the steel point ,p is #aj3a1 so that if a movement of #a_/bjjjjth of an inch is obtained at the centre of the diaphragm the mirror will move #a_/bjjjth of an inch2 and as the focal length of the mirror is #dj inches a movement of #a_/ejth inch is given to the spot of light4 ,this receiver is capable of working at a fairly high speed1 as the inertia of the moving parts is practically negligible2 the weight of the arm and mirror being less than #bj grains4 ,the hole in the screen is made slightly less in diameter than the traverse of the revolving cylinder1 the slight distance between the cylinder and the screen allowing the light to disperse sufficiently to produce a line on the film of about the right thickness4 ,there are two other possible means of photographically receiving the picture that upon investigation may yield some results2 but it is doubtful whether the current available1 even that obtained from a telephone relay1 will be sufficient to produce the desired magnetic effect1 and the #ec insertion of a second relay would detract greatly from the efficiency by decreasing the speed of working4 ,if rays of monochromatic light from a lamp ,l1 ,fig4 #bh1 pass through a ,nicol prism ,p 7polarising prism71 then through a tube containing ,,cs#b 7carbon bisulphide71 afterwards passing through the second prism ,p' 7analysing prism71 and if the two ,nicol prisms are set at the polarising angle1 no light from ,l would reach the photographic film wrapped round the drum ,v of the machine4 ,upon the tube being subjected to a field produced by a current passing through the coil ,c1 the refractive index of the liquid will be changed1 and light from ,l will reach the photographic film4#g ,illustration3 ,,fig4 #bh4 ,the second method is rather more complicated1 and is based upon the fact that the kathode rays in a ,crookes' tube can be deflected from their course by means of a magnet4 ,in ,fig4 #bi the kathode ,k of the ,x-ray tube sends a kathode ray discharge through an aperture in the anode ,a1 through a small aperture in the ebonite screen ,j #ed on to the drum ,v of the machine1 round which is wrapped a photographic film2 ,a and ,k being connected to suitable electrical apparatus4 ,upon the coil ,m being energised1 the kathode-ray is deflected from its straight-line course1 and the drum ,v is left in darkness4 ,illustration3 ,,fig4 #bi4 ,the method which is now going to be described is very ingenious1 as it makes use of what is known as an electrolytic receiver4 ,this method of receiving has proved to be the most practical and simple of all the photo-telegraphic systems that have been devised4 ,the application of this system to wireless reception is as follows4 ,the aerial ,a1 and the earth ,e1 are joined to the primary ,p of a transformer1 the secondary ,s being connected to a ,marconi valve receiver ,c4 ,the valve receiver is connected to the battery ,b and silvered quartz thread ,k of an ,einthoven galvanometer 7already described74 ,the thread is #a_/abjjjth of an inch in diameter1 and will respond to currents as small as #aj-h of #ee an ampere4 ,the light from ,m throws an enlarged shadow of the thread over a slit in the screen ,j1 and as the thread moves to one side under the influence of a current1 the slit in ,j is uncovered1 and the light from ,m is thrown upon a small selenium cell ,r4 ,in the dark the selenium cell has a very high resistance1 and therefore no current can flow from the battery ,d to the relay ,f4 ,when the string of the galvanometer moves to one side and uncovers the slit in the screen ,j1 a certain amount of light is thrown upon the selenium cell lowering its resistance1 allowing sufficient current to pass through to operate the relay4 ,round the drum of the machine 7shown in ,fig4 #g7 is wrapped a sheet of paper that has been soaked in certain chemicals that are decomposed on the passage of an electric current through them4 ,as soon as the local circuit of the relay is closed1 the current from the battery ,z 7about #ab volts7 flows through the paper and produces a coloured mark4 ,the picture1 therefore1 is composed of long or short marks which correspond to the varying strips of conducting material on the single line print4 ,in order to render the marks short and crisp1 a small battery ,y1 and regulating resistance ,l1 is placed across the drum and stylus4 ,the diagram1 ,fig4 #cj1 gives the connections for the complete receiver4 #ef ,the paper used is soaked in a solution consisting of ,ferrocyanide of potassium #a_/d oz4 ,ammoniac ,nitrate #a_/b oz4 ,distilled water#h #d oz4 ,illustration3 ,,fig4 #cj4 ,the paper has to be very carefully chosen1 as besides being absorbent enough to remain moist during the whole of the receiving1 the surface must also remain fairly smooth1 as with a rough paper the grain shows very distinctly1 and if there is an excess of solution the electrolytic marks are inclined to spread and so cause a blurred image4 ,the writer tried numerous specimens of paper before one could be found that gave really satisfactory results4 ,it was also found that when working in a warm room the paper became nearly #eg dry before the receiving was finished1 and the resistance of the paper being greatly increased 7this may be anything up to #ajjj ohms71 the marking became very faint4 ,a sponge moistened with the solution and applied to the undecomposed portion of the paper1 while still revolving1 was found to help matters considerably4 ,another experience which happened during the writer's early experiments1 the cause of which ,i am still unable to explain1 occurred in connection with the stylus4 ,the stylus used consisted of a sharply pointed steel needle1 and after working for about three minutes it was noticed that the lines were becoming gradually wider1 finally running into each other4 ,upon examination it was found that the point of the needle had worn away considerably1 becoming in fact1 almost a chisel point4 ,almost every needle tried acted in a similar manner1 and to overcome this difficulty the stylus shown in ,fig4 #ca was devised4 ,it will be seen that it consists of a holder ,a1 somewhat resembling a drill chuck1 fastened to the flat spring ,b in such a manner that the angle the stylus makes to the drum can be altered4 ,the needle consists of a length of #cf-;gauge steel wire1 and as this wears away slowly the jaws of the holder can be loosened and a fresh length pushed through4 ,the wire should not project beyond the face of the holder more than #a_/hth inch4 ,the gauge #eh of wire chosen would not suit every machine1 the best gauge to use being found by trial1 but in the writer's machine the pitch of the decomposition marks is much finer than of those made by the commercial machines1 and this gauge1 with the slight but unavoidable spreading of the marks1 will produce a mark of just the right thickness4 ,as already mentioned1 no explanation of this peculiarity on the part of the stylus can be given1 as there is nothing very corrosive in the solution used1 and the pressure of the stylus upon the paper is so slight as to be almost negligible4 ,illustration3 ,,fig4 #ca4 ,no special means are required for fastening the paper to the drum1 the moist paper adhering quite firmly4 ,care should be taken1 however1 to fasten the paper--which should be long enough to allow for a lap of about #a_/d inch--in such a manner that when working the stylus draws away from the edge of the lap and not towards it4 ,the current required to produce electrolysis is very small1 about one milliampere being sufficient4 #ei ,providing that the voltage is sufficiently high1 decomposition will take place with practically no current1 it being possible to decompose the solution with the discharge from a small induction coil4 ,the quantity of an element liberated is by weight the product of time1 current1 and the electro-chemical equivalent of that element1 and is given by the equation ,w zct1 where ,w quantity of element liberated in grammes4 z electro-chemical equivalent1 c current in amperes1 t time in seconds4 ,the chemical action that takes place is therefore very small1 as the intermittent current sent out from the transmitter in some cases only lasts from #a_/ejth to #a_/ajjth a second4 ,the decomposed marks on the paper are blue1 and1 as photographers know1 blue is reproduced in a photograph as a white1 so that a photograph taken of our electrolytic picture1 which will of course be a blue image upon a white ground1 will be reproduced almost like a blank sheet of paper4 ,if1 however1 a yellow contrast filter is placed in front of the camera lens1 and an orthochromatic plate used1 the blue will be reproduced in the photograph as a dead black4 ,there is one other point that requires attention4 ,it will be noticed that the metal print used for #fj transmitting is a positive1 since it is prepared from a negative4 ,the received picture will therefore be a negative1 making the final reproduction1 if it is to be used for newspaper work1 a negative also4 ,obviously this is no good4 ,the final reproduction must be a positive1 therefore the received picture must be also a positive4 ,to overcome this difficulty matters must be so arranged at the receiving station that in the cases of ,figs4 #ag1 #ah1 #bb1 and #bd1 the film is kept permanently illuminated while the stylus on the transmitter is tracing over an insulating strip1 and in darkness when tracing over a conducting strip4 ,in ,fig4 #cj the relay ,f should allow a continuous current from ,z to flow through the electrolytic paper1 and only broken when the resistance of the selenium cell is sufficiently reduced to allow the current from ,d to operate the relay4 ,the author has endeavoured to make direct positives on glass of the picture to be transmitted1 so that a negative metal print could be prepared4 ,the results obtained were not very satisfactory1 but the method tried is given1 as it may perhaps be of interest4 ,the plate used in the camera has to be exposed three or four times longer than is required for an ordinary negative4 ,the exposed plate is then placed in a solution of protoxalate of iron 7ferrous oxalate7 and left until the image shows plainly through the back of the plate4 ,it #fa is then washed in water and placed in a solution consisting of ,distilled water #ajjj cc4 ,nitric acid #b cc4 ,sulphuric acid #c cc4 ,bichromate of potash #aje grammes4 ,alum #hj ,after being in this bath for about fifteen minutes the plate is again well washed in water1 and developed in the ordinary way4 ,the first two operations should be performed in the dark room1 but the remaining operations can be performed in daylight1 once the plate has been placed in the bichromate bath4 ,as already stated1 the results obtained were not very satisfactory1 and such a method is not now worth following up1 as it is comparatively easy so to arrange matters at the receiving station that a positive or negative image can be received at will4 ,it is necessary to connect the stylus of the receiving machine to the positive pole of the battery ,z1 otherwise the marks will be made on the underside of the paper4 ,the electrolytic receiver1 owing to the absence of mechanical and electro-magnetic inertia1 is capable of recording signals at a very high speed indeed4 ,atmospherics1 which are such a serious nuisance in long-distance wireless telegraphy1 will also prove a nuisance in wireless photography1 #fb but their effects will not be so serious in a photographic method of receiving as they would be in the electrolytic system4 ,in a photographic receiver where the film is1 under normal conditions1 constantly illuminated1 the received signals 7both the transmitted signals and the atmospheric disturbances7 will be recorded1 after development1 as transparent marks upon the film1 the remainder of the film being1 of course1 perfectly opaque4 ,by careful retouching the marks due to the disturbances can be eradicated1 a print upon sensitised paper having been first obtained to act as a guide during the process4 99 99 99 99 99 #fc ,,chapter ,,iv ,,synchronising ,,and ,,driving ,clockwork and electro-motors are the source of driving power that are most suitable for photo-telegraphic work1 and each has its superior claims depending on the type of machine that is being used4 ,for general experimental work1 however1 an electro-motor is perhaps the most convenient1 as the speed can be regulated within very wide limits4 ,for a constant and accurate drive a falling weight has no equal1 but the apparatus required is very cumbersome and the work of winding both tedious and heavy4 ,this method of driving was at one time universally employed with the ,hughes printing telegraph1 but it has now been discarded in favour of electro-motors1 which are more compact1 besides being cheaper to instal in the first instance4 ,synchronising and isochronising the two machines are the most difficult problems that require solving in connection with wireless photography1 and as previously mentioned1 the #fd synchronising of the two stations must be very nearly perfect in order to obtain intelligible results4 ,the limit of error in synchronising must be about #a in #ejj in order to obtain results suitable for publication4 ,the electrolytic system is perhaps the easiest to isochronise1 as the received picture is visible4 ,on the metal print used for transmitting1 and at the commencing edge a datum line is drawn across in insulating ink4 ,the reproduction of this line is carefully observed by the operator in charge of the receiving instrument1 and the speed of the motor is regulated until this line lies close against a line drawn across the electrolytic paper4 ,although this may seem an ideal method there are one or two considerations to be taken into account4 ,unless the decomposition marks are made the correct length and are properly spaced1 however good the isochronising may be1 the result will be a blurred image4 ,any one who has worked with a selenium cell1 will know that it cannot change from its state of high resistance to that of low resistance with infinite rapidity1 and the effects of this inertia1 or fatigue as it has been called1 are more pronounced when working at a high speed4 ,in working1 the effects of this inertia would be to increase the time of contact of the relay ,f 7,fig4 #cj7 as the current from ,d would flow for a slightly longer period through ,r to ,f than the period of #fe illumination allowed by ,k4 ,this1 of course1 would mean a lengthening of the marks on the paper2 results would also differ greatly with different selenium cells4 ,there is a method of compensation by which the inertia of a cell can almost entirely be overcome1 but it would add greatly to the complicacy of the receiving apparatus4 ,in using an electro-motor with any optical method of receiving there are two methods available4 ,the first is an arrangement similar to that used by ,professor ,korn in his early experiments with his selenium machines4 ,the motor used for driving has several coils in the armature connected with slip rings1 from which an alternating current may be tapped off2 the motor acting partially as a generator1 besides doing good work as a motor in driving the machine4 ,this alternating current is conducted to a frequency meter1 which consists of a powerful electro-magnet1 over which are placed magnetised steel springs1 having different natural periods of vibration4 ,by means of a regulating resistance the motor is run until the spring which has the same period as the desired armature speed vibrates freely4 ,the speed of the motors at both stations can thus be adjusted with a fair amount of accuracy4 ,another method is to make use of a governor similar to those employed in the ,hughes printing telegraph system4 ,a drawing of the governor is given in ,fig4 #cb4 ,it consists of a ,illustration #fg metal frame which supports an upright steel bar ,s1 whose ends turn on pivots4 ,this bar is rectangular in section4 ,the gear-wheel ,g is fastened near the bottom of this rod and gears with a similar wheel on the shaft of the driving motor 7not shown74 ,suspended from the broader sides of ,s are the two flexible arms ,d1 each carrying a brass ball ,t4 ,these balls are not fastened to the arms1 but can slide up and down1 being held in position by the wire springs ,m1 one end of each spring being fastened to the screws ,c4 ,these screws work in a slot cut in the upper part of ,s1 and are connected to the adjusting screw ,e4 ,when ,e is turned the screws are raised or lowered accordingly1 and also the balls on the arms ,d4 ,fastened to the arms are two brushes of tow ,b1 and these revolve inside but just clearing the inner surface of the steel ring ,z4 ,upon the motor speed increasing above the normal the arms ,d1 and consequently the balls ,t1 swing out1 making a larger circle1 causing the brushes ,b to press against the steel ring ,z1 setting up friction which1 however1 is reduced as soon as the motor regains its ordinary working speed4 ,by careful adjustment the speed of the motors can be kept perfectly constant4 ,the object of having the balls ,t adjustable on ,d1 is to provide a means of altering the motor speed1 as the lower the balls on ,d the slower the mechanism runs1 and vice versa4 #fh ,illustration ,a simple and effective speed regulator devised by the writer is given in drawings #cc and #cd4 ,it comprises two parts1 ,a and ,b1 the part ,a being connected to the driving motor1 and the part ,b working independently4 ,the independent portion ,b consists of an ordinary clock movement ,m1 a steel spindle ,j being geared to one of the slower moving wheels1 so that it makes just one revolution in two seconds4 ,this spindle1 which runs in two coned bearings1 carries at its outer end a light ,illustration pointer ,d1 about two inches long1 to the underside of which is fastened the thin brass contact spring ,s1 which presses lightly upon the ebonite ring ,n4 #fi ,the portion ,a comprises a spindle1 pointer1 and contact spring similar to those employed in ,b1 the spindle ,j' being geared to the driving motor by means of ,f1 so that the pointer ,d' makes a little more than one revolution in two seconds4 ,by means of a special form of brake on the driving motor1 the speed is reduced1 so that both pointers travel at the same rate1 viz4 one revolution in two seconds4 ,by careful adjustment the two pointers can be made to revolve in synchronism1#i and when this is obtained the contact springs ,s1 ,s'1 pass over the contacts ,c1 ,c'1 completing the circuit of the battery ,b and lamp ,l4 ,when working properly the lamp ,l lights up regularly once every second4 ,this regulator is an excellent one to use for experimental work1 although it depends a great deal upon the skill of the operator1 but good adjustment should be obtained in about two minutes4 ,it is a good plan to insert a clutch of some description between the driving motor and the machine1 so that the regulator can be adjusted prior to the act of receiving or transmitting1 the machine being prevented from revolving by means of a catch4 ,the motor used should be powerful enough to take up the work of driving the machine without any reduction in speed4 ,the clocks ,m can be regulated so that they only gain or lose a few seconds in #gj twenty-four hours1 which gives an accuracy in working sufficient for all practical purposes4 ,connection is made with the contact springs ,s1 ,s'1 by means of the springs ,t1 ,t'1 which press against the spindles ,j1 ,j'4 ,another important point is the correct placing of the picture upon the receiving drum4 ,it is necessary that the two machines besides revolving in perfect isochronism should synchronise as well1 i4e4 begin to transmit and record at exactly the same position on the cylinders1 viz4 at the edge of the lap1 so that the component parts of the received image shall occupy the same position on the paper or film as they do on the metal print4 ,if the receiving cylinder had1 let us suppose1 completed a quarter of a revolution before it started to reproduce1 the reproduction when removed from the machine and opened out will be found to be incorrectly placed2 the bottom portion of the picture being joined to the top portion1 or vice versa1 and this means that perhaps an important piece of the picture would be rendered useless even if the whole is not spoilt4 ,it is evident1 therefore1 that some arrangement must be employed whereby synchronism1 as well as isochronism of the two instruments can be maintained4 ,there are several methods of synchronising that are in constant use in high-speed telegraphy1 in which the limit of error is reduced to a minimum1 #ga and some modification of these methods will perhaps solve the problem1 but it must be remembered that synchronism is far easier to obtain where the two stations are connected by a length of line than where the two stations are running independently4 ,in one system of ordinary photo-telegraphy synchronism is obtained in the following manner4 ,the receiving cylinder travels at a speed slightly in excess of the transmitting cylinder1 and as its revolution is finished first is prevented from revolving by a check1 and when in this position the receiving apparatus is thrown out of circuit and an electro-magnet which operates the check is switched in4 ,when the transmitting cylinder has completed its revolution 7about #a_/ajjth of a second later7 the transmitting apparatus1 by means of a special arrangement1 is thrown out of circuit for a period1 just long enough for a powerful current to be sent through the line4 ,this current actuates the electro-magnet4 ,the check is withdrawn and the receiving cylinder commences a fresh revolution in perfect synchronism with the transmitting cylinder4 ,as soon as the check is withdrawn the receiving apparatus is again placed in circuit until another revolution is completed4 ,as the receiver cannot stop and start abruptly at the end of each revolution a spring clutch is inserted between the driving motor and the machine4 #gb ,although a method of synchronising similar to this may later on be devised for wireless photography1 the writer1 from the result of his own experiments1 is led to believe that results good enough for all practical purposes can be obtained by fitting a synchronising device whereby the two machines are started work at the same instant1 and relying upon the perfect regulation of the speed of the motors for correct working4 ,the method of isochronism must1 however1 be nearly perfect in its action1 as it is easy to see that with only a very slight difference in the speed of either machine this error will1 when multiplied by #dj or #ej revolutions1 completely destroy the received picture for practical purposes4 ,from what has been written in this and in the preceding chapters it will be evident that the successful solution of transmitting photographs by wireless methods will necessitate the use of a great many pieces of apparatus all requiring delicate adjustment1 and depending largely upon each other for efficient working4 ,as previously stated1 there is at present no real system of wireless photography1 the whole science being in a purely experimental stage1 but already ,professor ,korn has succeeded in transmitting photographs between ,berlin and ,paris1 a distance of over #gjj miles4 ,if such a distance could be worked over successfully1 there is no reason to doubt that before long #gc we shall be able to receive pictures from ,america with as great reliability and precision as we now receive messages4 ,in nearly all wireless photographic systems devised up to the present the chief portion of the receiver consists of a very sensitive galvanometer1 and although very good results have been obtained by their use they are more or less a nuisance1 as the extreme delicacy of their construction renders them liable to a lot of unnecessary movement caused by external disturbances4 ,a galvanometer of the ,de' ,arsonval pattern1 used by the writer1 was constantly being disturbed by merely walking about the room1 although placed upon a fairly substantial table2 and for the same reason it was impossible to attempt to place the driving motor of the machine on the same table as the galvanometer4 ,for ship-board work it will be evident that the use of such a sensitive instrument presents a great difficulty to successful working1 and a good opening exists for some piece of apparatus--to take the place of the galvanometer--that will be as sensitive in its action but more robust in its construction4 99 99 99 99 99 #gd ,,chapter ,v ,,the ,,telephograph ,in the present chapter it is proposed to give a brief description of a system of radio-photography devised by the author1 and which includes a greatly improved method of transmitting and receiving1 as well as an ingenious arrangement for synchronising the two stations2 the whole being an attempt to produce a system that would be capable of working commercially over fairly long distances4 ,the system about to be described1 and which ,i have designated the telephograph1 is the outcome of several years' original experimental work1 many difficulties that were manifest in the working of the earlier systems having been overcome by apparatus that has been expressly designed for the purpose4 ,in any practical system of radio-photography the following points are of great importance3 7#a7 the speed of transmission2 7#b7 the quality of the received picture2 7#c7 the method of synchronising #ge the two machines so that transmission and reception begin simultaneously2 7#d7 the correct regulation of the speed of the driving motors2 7#e7 the simplicity and reliability of the entire arrangement4 ,points #a and #b are dependent upon several factors2 the number of contacts made by the stylus per minute2 the size of the metal print used2 the number of lines per inch on the screen used in preparing the print2 and the accurate and harmonious working of the various pieces of apparatus employed4 ,in the system under discussion the size of the metal print used is #e inches by #g inches1 and a screen having #ej lines to the inch is used for preparing it4 ,with the drum of the machine making one revolution in four seconds1 the stylus makes #hg contacts per second1 or #ebbj a minute1 the time for complete transmission being twenty-five minutes4 ,by the use of ordinary relays not more than #bjjj contacts a minute can be obtained1 and in the present system it is only by means of a specially designed relay that such a high rate of working has been made possible4 ,similarly1 too1 with the receiving of such a large number of signals transmitted at such a high speed1 a special instrument has been devised that can record this number of signals without any trouble1 and could even record up to #hjjj signals a minute1 provided that a suitable transmitter could be designed4 #gf ,in the present system the writer does not claim to have completely solved the problem of the wireless transmission of photographs1 but it is a great advance on any system previously described1 and the following advantages are put forward for recognition3 7#a7 a greatly improved method of transmitting and receiving2 7#b7 a simple method of regulating the speed of the driving motors and maintaining isochronism with a limit of error of less than #a in #hjj2 7#c7 an arrangement for synchronising the two machines whereby transmitting and receiving begin simultaneously2 7#d7 the use of one machine only at each station4 ,,transmitting ,,apparatus ,a diagrammatic representation of the apparatus required for a complete station1 transmitting and receiving combined1 is given in ,fig4 #ce1 the usual wireless equipment having been omitted from the diagram to avoid confusion4 ,the ,machine4--,this1 as will be seen from ,fig4 #cf1 consists of a base-plate ,m1 to which are attached the two bearings ,b and ,b'4 ,the bearing ,b' is fitted with an internal thread to correspond with the threaded portion of the shaft ,d4 ,the drum ,v is a brass casting1 being fastened to the shaft by set screws4 ,the shaft is threaded #ge to the inch4 ,the bearings are preferably of the concentric type4 ,the circuit breaker ,c is so arranged that when #gg the drum has traversed the required distance1 the end of the shaft pushes back the spring ,m1 breaking the circuit of the driving gear and stopping the machine4 ,the machine is connected to the driving gear by the flexible coupling ,a4 ,illustration3 ,,fig4 #ce4 ,m1 motor2 ,y1 isochroniser2 ,f1 clutch2 ,a1 machine2 ,r1 stylus2 ,s1 relay2 ,x1 gearing2 ,o1 circuit breaker2 ,t1 receiver2 ,c1 condenser2 ,u1 telephone relay2 ,k1 polarised relay2 ,l1 contact breaker2 ,d1 ,d#a1 ,d#b1 ,d#c1 batteries2 ,p1 friction brake2 ,b1 ,b#a1 double-pole two-way switches2 ,n1 ,n#a1 ,n#b1 single switches2 ,w1 key2 ,e1 electric clock2 ,j1 telephones4 ,the drum measures #e inches long by #b-a_/h inches diameter1 and this takes a metal print #e inches by #g inches1 which allows for a lap of about #a_/d inch4 ,in working1 the print is wrapped tightly round the drum1 being secured by means of a little seccotine smeared along one edge4 ,care must be taken that the edge of the lap draws away from the point of #gh the stylus and not towards it4 ,a margin of bare foil1 about #a_/h inch wide1 should be left on the print at the commencing edge1 the purpose of which will be explained later4 ,illustration3 ,,fig4 #cf4 ,the ,stylus4--,as the drum of the machine travels laterally1 by reason of the threaded shaft and bearing1 the stylus must necessarily be a fixture4 ,it consists of a holder ,b1 drilled to take a hardened steel point ,s1 attached to the spring ,m4 ,the spring is arranged to work in the guide ,f1 which is provided with an adjusting screw ,w for regulating the pressure of the stylus upon the print2 the pressure being sufficient to enable good contact to be made1 but must not be heavy enough to scratch the soft foil4 ,the needle should present an angle of about #fj;deg to the surface of the print1 as this angle has been found to give the best results in working4 ,to eliminate any sparking that may take place at the point of make and break1 due to the self-induction of the relay coils1 a condenser ,c1 about #a microfarad capacity1 should be connected across #gi the drum and stylus4 ,the complete stylus is given in the drawings1 ,figs4 #cg1 #cg;a1 and also in the diagrams ,figs4 #h and #i4 ,illustration3 ,,fig4 #cg4 ,showing the arrangement for sliding the stylus to or from the machine4 ,illustration3 ,,fig4 #cg;a4 ,the ,relay4--,as will be seen from the diagram1 ,fig4 #ch1 this consists of two electro-magnets having very soft iron cores1 the magnet ,m being wound in the usual manner1 while the magnet ,n is wound differentially4 ,the armature ,a is made as light as possible1 and is pivoted at ,p1 and when there is no current flowing through any of the coils1 is held midway between the magnet cores by the two spiral springs ,s and ,t1 which are under slight but equal tension4 ,the connections are as follows4 ,the wires from the winding on ,m are connected directly to the relay terminals ,f and ,h1 as are also the wires from one winding on ,n4 ,the other winding on ,n is connected in series with the battery ,c1 ammeter ,b1 and regulating resistance ,r4 #hj ,illustration3 ,,fig4 #ch4 ,when the circuit of the battery ,c is completed1 the coil of ,n1 to which it is connected1 is energised1 and the armature ,a is attracted against the stop ,v4 ,when in this position the tension of the spring ,s is released1 while the tension of the spring ,t is increased4 ,as soon as the circuit of the battery ,d is completed by means of the metal line print on the transmitting machine1 the current divides at the terminals ,f and ,h1 a portion flowing through the magnet coil ,m1 and a portion through the remaining winding on ,n4 ,the current which flows through the winding on ,n produces a magnetising effect equal to that caused by the other winding on ,n1 but since the two windings are of equal length and resistance1 and since the current flowing through the two windings is of equal strength but in opposite directions1 the result is to neutralise #ha the magnetising effects produced by each winding1 and consequently no magnetism is produced in the cores4 ,the other portion of the current from ,d flows through the coil ,m1 and it becomes magnetised at the same time that the coil ,n becomes demagnetised4 ,the armature ,a is attracted by ,m against the stop ,x1 and this attraction is assisted by the spring ,t1 which was under increased tension4 ,the conditions of the springs are now reversed1 the spring ,s being under increased tension1 while the tension of the spring ,t is released4 ,as soon as the current from ,d is broken1 the magnetism disappears from ,m1 the neutralising current in ,n ceases1 and ,n once more becomes magnetised1 owing to the current which still flows through one winding from ,c2 the armature is therefore again attracted by ,n1 assisted by the spring ,s4 ,the current flowing through the two windings of ,n must be perfectly equal1 and the regulating resistance ,r1 and ammeters ,b and ,b'1 are inserted for purposes of adjustment4 ,the current from ,c must flow in a direction opposite to that which flows from ,d4 ,illustration3 ,,fig4 #ci4 ,h1 ,h'1 containers2 ,m1 mercury2 ,e1 paraffin oil2 ,t1 ,t'1 terminals2 ,c1 suspending rod2 ,d1 base2 ,f1 ,f'1 dipping rods4 ,the local circuit of the relay is completed by means of a copper dipper in mercury1 somewhat resembling an ordinary mercury break1 but modified to suit the present requirements4 ,the arrangement will be seen from ,fig4 #ci4 ,the whole of the #hb moving parts are made as light as possible1 and for this reason the rod ,c and the dippers ,f1 ,f' should be made as short as convenient4 ,the containers ,h1 ,h' are separate1 of cast iron1 and rectangular in shape4 ,the dipper is of very thin copper tube--an advantage where alternating current is to be used--and is made adjustable for height on the suspending rod ,c4 ,the leg ,f is of such a length that permanent contact is made with the mercury in the container ,h1 while the leg ,f' clears the surface of the mercury by about #a_/d inch1 when the armature of the relay is in its normal position4 ,to prevent undue churning of the mercury1 which would necessarily take place if the dipper entered and left the mercury at each movement of the armature1 a pointed ebonite plug is inserted in the end of the tube4 ,this will be found to give good results at a high speed1 the mercury being practically undisturbed1 and the production of sludge reduced to a minimum4 ,to prevent oxidation of the mercury1 and to prevent arcing1 the surface is covered with paraffin oil4 ,if this is not sufficient to prevent arcing a condenser should be shunted across the #hc containers4 ,the volume of mercury1 and the area of the dippers1 should be sufficient to carry the current used for a considerable period without heating up to any extent4 ,an adjustable weight ,j is provided in order to balance the armature and dipping rod4 ,the remaining transmitting apparatus consists of the battery ,d#b and the usual wireless apparatus4 ,the double-pole two-way switch ,b' is to enable the photo-telegraphic set to be switched out and the hand key ,w switched in for ordinary signalling purposes4 ,the battery ,d#b should be about #ab volts4 ,,receiving ,,apparatus ,the wireless portion of the receiver is similar to that given in ,fig4 #bb1 is of the usual syntonic type1 and comprises an oscillation transformer1 ,s being the secondary1 and ,p the primary2 ,c' is a block condenser1 and ,c a variable condenser4 ,the detector ,d is of the carborundum crystal or electrolytic pattern4 ,a two-way switch ,b is provided so that the relay ,u can be switched out and the telephones ,j switched in for ordinary receiving purposes4 ,the relay ,u is a ,brown's telephone relay4 ,illustration3 ,,fig4 #dj4 ,the ,receiver4--,the magnified current from the relay ,u is taken to a special telephone receiver1 the construction of which is given in ,fig4 #dj4 ,the diaphragm ,f is about #b-a_/b inches diameter1 and should be fairly thin but very resilient4 ,only one #hd ,illustration ,illustration coil is provided1 and this should be wound with ,no4 #dg ,s4,s4,c4 copper wire for a resistance of about #bjjj ohms4 ,by using only one coil and therefore only one core1 the movement of the diaphragm is centralised4 ,to the centre of the diaphragm a light steel point is fastened1 about #a_/b inch long1 and provided with a projecting hook ,h4 ,an enlarged view of this pin is given in ,fig4 #da4 ,the movement of the diaphragm and consequently of the steel point ,p is communicated to a pivoted rod ,r1 which is of special construction4 ,a piece of aluminium tube #c-c_/d inches long1 and of the section given at ,b1 is bushed at one end with a piece of brass of the shape shown in ,fig4 #da;a4 ,a stiff steel wire ,t about #a inch long 7#bj gauge7 is screwed into the end of ,z1 and carries a counterbalance weight ,c4 ,a hardened #he steel spindle1 pointed at both ends1 is fastened at ,d1 and runs between two coned bearings1 one of which is adjustable4 ,the underside of ,z is flattened1 and a small coned depression is made for the reception of the pointed end of the pin4 ,by means of the spring ,j the two pieces1 ,z and ,p1 are held firmly together1 at the same time allowing perfect freedom of movement4 ,the bridge ,g is made from a piece of sheet aluminium placed in a slot cut in the tube ,r1 the end of the tube being pressed tight upon ,g1 and secured by means of a small rivet4 ,the optical arrangements are as follows4 ,by means of the ,nernst lamp ,l1 and the lenses ,b and ,b'1 ,figs4 #db and #dc1 a magnified shadow of ,g is thrown upon the screen ,j4 ,when the shutter ,g is in its normal position 7i4e4 at rest71 its shadow is just above the small hole in ,j1 and light from ,l reaches the photographic film wrapped round the drum ,v of the machine4 ,illustration3 ,,fig4 #db4 ,j1 screen2 ,l1 ,nernst lamp2 ,g1 shutter2 ,b1 condensing lens2 ,b#a1 focussing lens4 ,when1 however1 signals are sent out from the transmitting apparatus1 the magnified current from the relay ,u energises the coil of the special telephone ,s1 attracting the diaphragm ,f1 and consequently giving movement to the pivoted rod ,r4 ,as by means of the optical arrangements a #hf magnified movement as well as a magnified image of ,g is thrown upon the screen ,j1 the shadow of ,g will1 when the telephone ,s is actuated1 cover the hole in the screen1 and prevent any light from reaching the film on ,v1 until current from the relay ,u ceases to flow4 ,therefore1 when the stylus of the transmitter traces over a conducting strip on the metal print1 no light reaches the film on ,v1 but when tracing over an insulating strip the shadow of ,g on the screen ,j rises1 and the light from ,l reaches the film4 ,by this means a positive picture is received1 which is a great advantage where the photographs are required for reproduction4 ,atmospherics would be represented by irregular transparent marks on the film after development1 and these can be easily eradicated by retouching4 ,illustration3 ,,fig4 #dc4 ,e1 ebonite screen2 ,f1 focussing lens2 ,g1 shutter2 ,o1 condensing lens2 ,l1 ,nernst lamp4 ,the drum of the machine moves laterally #a_/geth of an inch per revolution1 and the hole in the screen is #a_/ijth of an inch in diameter4 ,as the screen ,j is not in direct contact with the film1 the slight diffusion of the light that takes place will produce #hg a mark of about the right thickness4 ,with a movement of the diaphragm of only #a_/djjjjth of an inch1 the actual movement of ,g will be #a_/djjjth of an inch4 ,if the optical arrangements have a magnifying power of #ajj1 then the movement of the shadow upon the screen will be #a_/djth of an inch1 which will be ample to cover the aperture4 ,the aluminium rod ,r1 minus the counter-weight1 can be made to weigh not more than #ab grains4 ,it is necessary to enclose the optical parts in a light tight box1 indicated by the dotted lines in ,fig4 #dc1 in order to prevent any extraneous light from reaching the film4 ,the ,contact ,breaker4--,the contact breaker 7,l1 ,fig4 #ce71 as will be seen from ,fig4 #dd1 consists of an electro-magnet ,n1 the windings of which are connected with the battery ,b and the polarised relay ,k4 ,the armature which is supported by the spring ,g carries a contact arm ,a1 which in its normal position makes permanent contact with the contact screw ,t1 and completes the circuit between the relay ,k and the telephone relay ,u 7,fig4 #ce74 ,as soon as the transmitter sends out the first signal1 the magnified current from the telephone relay actuates the relay ,k1 which in turn completes the circuit of the contact breaker4 ,directly the armature ,m has been attracted1 the contact with ,t is broken1 and ,a makes fresh contact with the screw ,h1 by means of the spring ,z #hh fastened to the underside of ,a4 ,the armature1 once it has been attracted1 is held in permanent contact with ,h by the catch ,s1 independent of the magnets ,n4 ,as soon as contact is made with ,h1 the clutch 7,f1 ,fig4 #ce7 circuit is completed1 and the circuit of the relay ,k is broken4 ,when the circuit of the clutch ,f is broken by means of the circuit breaker ,c on the machine 7,fig4 #cf71 the stop ,s is pulled back by hand1 allowing the contact arm ,a to rise1 and again make fresh contact with the contact screw ,t4 ,illustration3 ,,fig4 #dd4 ,,driving ,,apparatus ,the ,friction ,brake4--,this consists of a steel disc ,a1 ,fig4 #de1 about #b-a_/b inches diameter and #c_/h inch or #a_/b inch wide on the face1 secured to the main shaft of the driving motor4 ,the arm ,h1 pivoted at ,c1 carries at one end the curved block ,b1 which is faced with a pad of tow ,f4 ,the other extremity is pivoted to the steel rod ,p1 which slides #hi ,illustration in holes bored in the standards ,j4 ,one end of the rod ,p is screwed with a fine thread1 about #ge to the inch1 and is fitted with a regulating wheel ,t1 by means of which the block ,b can be made to press upon the disc ,a with any required degree of pressure4 ,a fairly stiff steel spring ,r is placed upon the rod ,p1 between one standard ,j and the collar ,n4 ,as the speed of the driving motor is slightly in excess of that required by the machine1 the block ,b1 by means of the wheel1 is made to press upon the disc ,a1 setting up friction which reduces the motor speed until the isochroniser indicates that the correct working speed has been attained4 ,the ,clutch4--,the details of this will be seen from ,figs4 #df and #dg4 ,it consists of a steel shaft coned at both ends running between two countersunk bearings1 one of which is adjustable4 ,this shaft carries the two portions of the clutch ,a and ,b1 the portion ,a being a fixture on the shaft1 and the portion ,b running free upon it4 ,the portion ,b is a gun-metal casting bored to run accurately upon the steel shaft4 ,a soft iron annular ring is fastened to the face4 ,illustration3 ,,fig4 #df4 ,e1 spindle2 ,r1 bobbins2 ,p1 iron cores2 ,d1 copper rings2 ,t1 brushes2 ,n1 back plate2 ,v1 front plate2 ,j1 gearing2 ,s1 spring2 ,h1 collar2 ,z1 iron ring2 ,f1 fixed bearing2 ,c1 insulating bush4 ,the portion ,a consists of a gun-metal casting #ij ,illustration bored a tight fit for the shaft ,e1 secured by means of a set screw4 ,the two magnet cores ,p are screwed into the front plate ,v1 which is also of gun-metal1 and after the bobbins ,r have been slipped on1 the shanks of the cores are passed through holes drilled in the flange ,n of the main casting and held in place with nuts4 ,the faces of both ,a and ,b must be turned perfectly square with the shaft1 so that they run accurately together4 ,the portion ,b is #ia kept in contact with ,a by means of a spring ,s1 the pressure being regulated by the collar ,h4 ,current is taken to the magnets by means of the two insulated copper rings ,d mounted upon the body of ,a4 ,the gear-wheels on both portions have teeth of very fine pitch1 the number of teeth on each being regulated by the speed of the driving motor and the required machine speed4 ,connection with the circuit breaker ,l and the battery ,b#b is made with the collecting rings ,d by the brushes ,t4 ,the complete connections are given in the diagram ,fig4 #ea4 ,the ,isochroniser4--,this is a device for ensuring the correct speed regulation of the driving motors1 and is shown in detail in ,fig4 #dh4 ,it comprises two portions1 one portion being rotated at a definite speed by electrical means1 and the other portion rotated by the driving motor4 ,the main portion consists of a metal tube ,n1 bushed at both ends1 the bottom end of the tube being arranged to work on ball-bearings4 ,an ebonite bush ,c carries three copper rings ,t1 ,t#a1 ,t#b1 and the brushes ,r1 ,r#a1 ,r#b are in electrical contact with them4 ,the ebonite plate ,j1 #c-a_/b inches diameter1 is secured to the top end of ,n1 and carries a contact piece ,q1 shown separate at ,e4 ,as will be seen this is a block of ebonite with three contacts arranged on the top surface4 ,the middle contact ,p is #a_/fdth of an inch wide1 and the contacts ,p#a #ib and ,p#b are placed on either side at a distance of #a_/af inch2 the contact strips ,p#a1 ,p#b carry the brass pins ,d1 which are about #a_/af inch diameter1 and spaced #c_/h inch apart4 ,a connecting wire is carried from the contact ,p to the copper ring ,t1 another from ,p#a to ,t#a1 and one from ,p#b to ,t#b4 ,illustration3 ,,fig4 #dh4 ,n1 brass tube2 ,s1 bushes2 ,g1 ball-bearing2 ,h1 gear-wheel2 ,t1 ,t#a1 ,t#b1 copper rings2 ,c1 insulating block2 ,r1 ,r#a1 ,r#b1 brushes2 ,j1 ebonite disc2 ,q1 contact block2 ,d1 metal pins2 ,o1 pulley1 ,p1 ,p#a1 ,p#b1 contact plates2 ,k1 needle2 ,z1 spring2 ,w1 steel rod2 ,e1 countersunk bearing4 ,the bushes ,s are bored a running fit for the steel rod ,w 7shown separate at ,a71 which is coned at both ends1 and runs between two countersunk bearings1 the bottom bearing ,e being fixed while #ic the top bearing 7not shown7 is adjustable4 ,a needle ,k is fastened near the end of the rod ,w1 and attached to this needle is the spring ,z1 which presses lightly but firmly upon the contact block ,q4 ,to provide a level surface for ,z to work over1 the spaces between the contact pieces are filled in with an insulating material1 and the whole surface finished off perfectly smooth4 ,the spring ,z is #a_/h inch wide for portion of its length1 but at the point where it presses upon ,q it is reduced in width to #a_/fdth of an inch 7see ,fig4 #dh74 ,the driving arrangements are as follows4 ,a counter-shaft ,q1 ,fig4 #ea1 fitted with a grooved pulley1 is run in bearings parallel with the shaft ,w1 and is connected by suitable gearing to the shaft of the driving motor1 so that the needle ,k makes one revolution in about #b-a_/b seconds4 ,a belt passing over the pulleys connects the two shafts1 and the tension of the belt is regulated by means of an adjustable jockey pulley4 ,the tube ,n1 carrying the disc ,j1 must be rotated at a fixed speed1 and this is accomplished in the following manner4 ,an ordinary electric clock impulse dial1 actuated from a master clock1 is connected by suitable gearing ,h1 so that the tube ,n makes exactly one revolution in #b seconds2 it being possible to adjust an electric clock of the ,synchronome type1 so that it only gains or loses about #a second in #bd hours1 and this provides #id an accuracy sufficient for all practical purposes4 ,the connections are given in ,fig4 #di1 and the face of the instrument in ,fig4 #ej4 ,it will be seen that a connecting wire is run from the steel spindle ,w to one terminal each of the lamps ,l1 ,l#a1 ,l#b1 and from the other terminal of the lamps to one terminal of the batteries ,j1 the battery comprising a set of three #d-volt accumulators4 ,the other terminals of the batteries are joined one to each of the brushes ,r1 ,r#a1 ,r#b4 ,illustration3 ,,fig4 #di4 ,illustration3 ,,fig4 #ej4 ,m1 terminals for connecting to electric clock2 ,l1 white lamp2 ,l#a1 blue lamp2 ,l#b1 red lamp4 ,the lamps are coloured1 the lamp ,l being white1 and the lamps ,l#a and ,l#b blue and red respectively1 and care must be taken in connecting up that when the needle ,k makes contact with the stud ,p the white lamp ,l is in circuit4 ,when the machines are working1 the operator1 by means of the brake 7already described71 reduces the speed of the driving motor until the needle ,k travels in unison with the disc ,j1 making permanent contact with ,p on the contact #ie block ,q1 which is evidenced by the lamp ,l remaining alight4 ,if1 however1 the needle travels faster than the disc ,j1 contact with ,p is broken and fresh contact is made with ,p#b1 the lamp ,l is extinguished and the red lamp ,l#b lights up1 and remains alight until the operator reduces the speed4 ,similarly1 too1 if the needle travels slower than ,j1 contact is made with ,p#a1 and the circuit of the blue lamp ,l#a is completed4 ,when the speed is either above or below the normal1 the needle ,k engages with one or the other of the pins ,d1 and as the tension of the driving belt is only such as is required to drive the needle1 the belt slips on the pulleys until the normal speed is regained4 ,,method ,,of ,,working ,the clockwork motor ,m1 ,fig4 #ea1 should be capable of running for several hours with one winding1 and powerful enough to take up the work of driving the machine without any appreciable effort4 ,the main spindle of the motor is so arranged that it makes one revolution in two minutes1 and the reduction in speed between the motor shaft and the shaft to which the coupling ,a is attached is #cj3a4 ,the metal line print having been wrapped round the drum of the machine1 the stylus is put into position1 at the edge of the lap1 and with the needle resting about half-way on #if the margin of the bare foil left at the commencing edge of the print4 ,now1 when the two stations are in perfect readiness for work1 the motors are started and the speed adjusted2 the speed of the machine being just under one revolution in four seconds4 ,illustration3 ,,fig4 #ea4 ,m1 clockwork motor2 ,s1 isochroniser2 ,e1 friction break2 ,t1 brushes2 ,f1 electric clutch2 ,x1 gearing2 ,d1 ,d#a1 switches2 ,a1 flexible coupling2 ,k1 polarised relay2 ,l1 circuit breaker2 ,b#a1 ,b#b1 ,b#c1 batteries2 ,p1 electric clock2 ,w1 terminals for connection to telephone relay2 ,h1 terminals for connection to terminals ,j1 on transmitting machine4 ,the switch ,d is then closed1 and the arm of the switch ,d#a placed on the contact stud 7#a71 at the transmitting station only4 ,as soon as the switches are closed the clutch ,f comes into action1 and the transmitting machine begins to revolve4 ,when the whole of the line print wrapped round the drum of the machine has passed under the stylus1 the end of the shaft ,d1 ,fig4 #cf1 engages #ig with the spring m1 breaking the clutch circuit and allowing the motor to run free4 ,as soon as the machine stops1 the switch ,d is opened and the machine run back to its starting position by hand4 ,at the receiving station the switch ,d is also closed1 and the arm of the switch ,d#a placed on the contact stud 7#b74 ,the closing of these switches does not bring the clutch ,f into operation until current from the telephone relay ,u connected to the wireless receiving apparatus works the sensitive polarised relay ,k1 which in turn completes the circuit of the circuit-breaker ,l4 ,when the armature of ,l is attracted1 the circuit of the relay ,k is broken1 the circuit of the clutch ,f is completed1 and the machine starts revolving4 ,illustration3 ,,fig4 #eb4 ,the current from the relay ,u1 due to the transmitting stylus passing over one contact strip on the metal print1 is too brief to actuate the heavier mechanism of the relay ,k1 hence the need of the margin of bare foil at the commencing edge of the metal print1 so that a practically continuous current will flow to the relay ,k until the armature is attracted4 ,as1 however1 the relay is not actuated at the receipt of the first signal1 and as it is necessary for the machine to start recording at a certain point on the film1 viz4 #ih at the edge of the lap--the reason for this was given in ,chapter ,,iv4--the starting position of the receiving drum will be similar to that given in the diagram ,fig4 #eb1 where ,x indicates the lap of the photographic film1 and the arrow the direction of rotation4 ,it is1 of course1 obvious that a somewhat similar adjustment must be made with regard to the position of the stylus on the metal print at the transmitting machine4 ,in the present system1 as in almost every photographic method of receiving that has been described1 the ,nernst lamp is invariably mentioned as the source of illumination4 ,since the advent of the high-voltage metal-filament lamps the ,nernst lamp has fallen somewhat into disuse for commercial purposes1 but it possesses certain characteristics that render it eminently suitable for the purpose under discussion4 ,the main principle of this type of lamp depends upon the discovery made by ,professor ,nernst in #ahih1 after whom the lamp is named1 that filaments of certain earthy bodies when raised to a red heat became conductive sufficiently well to pass a current which raised it to a white heat1 and furthermore that the glowing filament emitted a brighter light for a given amount of current than carbon filaments4 ,illustration3 ,,fig4 #eb;a4 ,nernst lamps are made in two sizes1 the larger #ii being intended for the same work as usually done by arc lamps1 and the smaller to replace incandescent lamps2 the smaller type being made to fit into the ordinary bayonet lampholders4 ,the principal parts of a ,nernst lamp consist of the filament1 the heater1 the automatic cut-out1 and the resistance1 and their arrangement in the smaller type of lamp is given in the diagram1 ,fig4 #eb;a4 ,the current enters at the positive terminal1 passes through the heater ,m1 and out through the negative terminal4 ,the filament ,b1 which consists of a short length of an infusible earth made of the oxides of several rare minerals1 of which zirconia is one1 is a non-conductor at first1 but becomes a conductor upon being raised to a high temperature by means of the heater ,m4 ,as soon as the filament becomes conductive the current then passes through the automatic cut-out ,h1 and the armature ,d is attracted1 thus breaking the heater circuit4 ,the current then flows from the positive terminal #ajj ,illustration through the cut-out ,h1 resistance ,j1 and filament ,b1 and from thence out of the lamp4 ,since the resistance of the filament decreases the hotter it gets1 it is necessary to insert a ballasting resistance in series with it which has the opposite property of increasing its resistance as it gets hotter1 to prevent the filament taking too much current and destroying itself4 ,such a resistance1 ,j1 consists of a filament of fine iron wire1 which1 to prevent oxidation from exposure to the air1 is enclosed in a glass bulb filled with hydrogen gas4 ,fig4 #eb;b shows the form of ballast resistance used in the small and large type of lamp respectively4 ,either direct or alternating current can be used with these lamps1 and with direct current the polarity must be strictly observed1 and that the positive wire is connected to the positive and the #aja negative wire to the negative terminal4 ,with the smaller type of lamp once it has been correctly placed in its holder it is essential that it should not be turned1 as a change in the direction of the current will rapidly destroy the filament4 ,illustration3 ,,fig4 #eb;c4 ,the arrangement of the larger type of ,nernst lamp can be readily seen from the drawing1 ,fig4 #eb;c4 ,care must be taken to see that the voltage required by the burner and resistance equals the voltage of the supply circuit1 and that only parts of the same amperage are used together on the same lamp4 ,no advantage is obtained by over-running a ,nernst lamp1 this only shortening its life without increasing the light4 ,under normal conditions the average life of the burner is about #gjj hours4 ,the efficiency of the ,nernst lamp is fairly high1 being only #a4de to #a4ge watts per c4p4 ,the light given is remarkably steady1 and the lamps are adaptable for all voltages from #ajj to #cjj4 ,in one of the large type of lamps for use on a #bce-volt #ajb circuit the burner takes #j4e ampere at #bae volts1 and the resistance #j4e ampere at #bj volts1 while one of the smaller lamps for use on the same circuit takes #j4be ampere at #bae volts and #j4be ampere at #bj volts for the burner and resistance respectively4 ,the burner and heater are very fragile1 and should never be handled except by the porcelain plate to which they are attached4 ,the lamps burn in air and emit a brilliant white light of high actinic power1 the intrinsic brilliancy 7c4p4_/square inch7 varying from #ajjj to #bejj1 as compared with #ajjj to #abjj for ordinary metal filament lamps1 and #cjj to #ejj for carbon filament lamps4 ,the chief advantage of the ,nernst lamp from a photographic point of view lies in the fact that it produces abundantly the blue and violet rays which have the greatest chemical effect upon a photographic plate or film4 ,these rays are known as chemical or actinic rays1 and are only slightly produced in some types of incandescent electric lamps4 ,carbon-filament lamps are very poor in this respect4 ,because a light is visually brilliant it must by no means be assumed that it is the best to use for purposes of photography1 and this is a point over which many photographers stumble when using artificial light4 ,many sources of light1 while excellent for illumination1 have very low actinic powers1 while others may have low illuminating but high #ajc actinic powers4 ,a lamp giving a light yellowish in colour has usually low actinic power1 while all those lamps giving a soft white light are generally found to be highly actinic4 ,in addition to the actinic value of the source of illumination1 the photographic film used must be very carefully chosen1 as the chemical inertia of the sensitised film plays an important part in the successful reproduction of the picture1 and also1 to a certain extent1 affects the speed of transmission4 ,the length of exposure1 the amount of light admitted to the film1 and the characteristics of the film itself1 are all factors which have a decided bearing upon the quality of the results obtained1 and the film found to be most suitable in one case will perhaps give very unsatisfactory results in another4 ,in photo-telegraphy the length of exposure is determined by the time taken by the transmitting stylus to trace over a conducting strip on the metal print1 and this time1 of course1 varies with the density of the image and also with the speed of transmission4 ,the film in ordinary photography is chosen with regard to the subject and the existing light conditions1 and the amount of light admitted to the film and the length of exposure are regulated accordingly4 ,no such latitude is1 however1 possible in photo-telegraphy4 ,with each set of apparatus #ajd the various factors1 such as the light value1 the amount of light admitted to the film1 and the length of exposure1 will be practically fixed quantities1 and the film that will give the most satisfactory results under these fixed conditions can only be found by the rough-and-ready method of trial and error4 ,the films in common use are manufactured in four qualities1 namely1 ordinary1 studio1 rapid1 and extra rapid4 ,these terms should really relate to the light sensitiveness of the film 7or1 as it is technically termed1 the speed71 but at the best they are a rough and very unsatisfactory guide1 for the reason that some unscrupulous makers1 purely for business purposes1 do not hesitate to label their films and plates as slow1 rapid1 etc41 without troubling to make any tests for correct classification4 ,the speed of photographic films and plates is generally indicated by a number1 and the system of standardisation adopted by the majority of makers in this country is that originated by ,messrs4 ,hurter ,driffield1 abbreviated ,h4 ,d4 ,in their system the speed of the film and the exposure varies in geometrical proportion1 a film marked ,h4 ,d4 #ej requiring double the exposure of one marked ,h4 ,d4 #ajj4 ,the highest number always denotes the highest speed1 and the exposure varies inversely with the speed4 ,besides the ,hurter ,driffield method of #aje obtaining the speed numbers of plates and films adopted by a large number of makers in this country1 there are also two standard ,english systems known as the ,w4,p4 ,no4 7,watkin's power number7 and ,wynne ,f4 ,no41 both of which are used to a fair extent4 ,the ,actinograph number or speed number of a plate in the ,h4 ,d4 system is found by dividing #cd by a number known as the ,inertia1 the ,inertia1 which is a measure of the insensitiveness of the plate1 being determined according to the directions laid down by ,hurter ,driffield--that is1 by using pyro-soda developer and the straight portion only of the density curve4 ,if1 for instance1 the ,inertia was found to be one-fifth1 then the speed number would be #cd _/ #a_/e #agj1 and the plate is ,h4 ,d4 #agj4 ,the ,w4,p4 ,no4 is found by dividing #ej by the ,inertia4 ,thus #ej _/ #a_/e #bej1 and the plate is ,w4,p4 #bej1 but for all practical purposes the ,w4,p4 ,no4 can be taken as one and a half times ,h4 ,d4 ,the ,wynne ,f4 numbers may be found by multiplying the square root of the ,watkins number by #f4d4 ,thus sqrt#bej #ae4ha1 and #ae4ha x #f4d ,w4,f4 #aja4 ,for those photographers who are in the habit of using an actinometer giving the plate speeds in ,h4 ,d4 numbers1 the following table1 taken from the ,photographer's ,daily ,companion1 is given1 #ajf which shows at a glance the relative speed numbers for the various systems4 ,the ,watkins and ,wynne numbers only hold good1 however1 when the inertia has been found by the ,h4 ,d4 method4 ,,table ,,of ,,comparative ,,speed ,,numbers ,,for ,,plates ,,and ,,films ---------------------------------------- -------------- ,h4 ,d4,w4,p4 ,no4,w4,f4 ,no4,h4 ,d4,w4,p4 ,no4,w4,f4 ,no4 ---------------------------------------- ---------- #aj #ae #bd #bbj #cbc #aad #bj #cj #bh #bdj #ceb #abj #dj #fj #di #bfj #chb #abd #hj #abj #fi #bhj #dab #abi #ajj #adg #gg #cjj #dda #acd #abj #agf #hd #cbj #dgj #ach #adj #bjf #ia #cdj #ejj #adb #afj #bce #ajc #chj #eeh #aej #bjj #bid #aji #djj #ehh #aed ---------------------------------------- -------------- ,although theoretically the higher the speed of the film the less the duration of exposure required1 there is a practical limit1 as besides the intensity and actinic value of the light admitted to the film a definite time is necessary for it to overcome the chemical inertia of the sensitised coating and produce a useful effect4 ,with every make of film it is possible to give so short an exposure that although light does fall upon the film it does no work at all--in other words1 we can say that for every film there is a minimum amount of light action1 and anything below this is of no use4 ,the exposure that enables the smallest amount of light action to take place is termed the limit of the smallest useful exposure4 #ajg ,there is also a maximum exposure in which the light affects practically all the silver in the film1 and any increased light action has no increased effect4 ,this is the limit of the greatest useful exposure4 ,in photo-telegraphy the duration of exposure1 as already pointed out1 is determined by certain conditions connected with the transmitting apparatus1 and with conditions similar to those mentioned on page #ge the length of exposure will vary roughly from #a-ejth to #a-aejth of a second4 ,the most suitable film to use for purposes of photo-telegraphy is one having a fairly slow speed in which the range of exposure required comes well within the limits of the film4 ,there is no advantage in using a film having a speed of1 say1 ,h4 ,d4 #cjj if good results can be obtained from one with a speed of1 say1 ,h4 ,d4 #bjj1 as the use of the higher speed increases the risk of overexposure4 ,with the high-speeded films the difficulties of development are also greatly increased1 there being more latitude in both exposure and development with the slower speeds1 and consequently a better chance of obtaining a good negative4 ,another point1 often puzzling to the beginner1 and which increases the difficulty of choosing a suitable make of film1 is that1 although one make of film marked ,h4 ,d4 #ajj will give good results1 another make1 also marked ,h4 ,d4 #ajj1 will give #ajh very poor results4 ,this is owing1 not to a poor quality film1 as many suppose1 but to the almost insurmountable difficulty of makers being able to employ exactly the same standard of light for testing purposes1 so that although various makes may all be standardised by the ,h4 ,d4 method1 films bearing the same speed numbers may vary in their actual speed by as much as #cj to #ej per cent4 99 99 99 99 99 #aji ,,appendix ,a ,,selenium ,,cells ,selenium is a non-metallic element1 and was first discovered by ,berzelius in #ahag1 in the deposit from sulphuric acid chambers1 which still continues the source from which it is obtained for commercial purposes1 although it is found to a small extent in native sulphur4 ,its at4 wt4 is #gi4b1 and its sp4 gr4 #d4h4 ,symbol1 ,se4 ,in its natural state selenium is practically a non-conductor of electricity1 its resistance being forty thousand million times greater than copper4 ,its practical value lies in the property which it possesses1 that when in a prepared condition it is capable of varying its electrical resistance according to the amount of light to which it is exposed1 the resistance decreasing as the light increases4 ,selenium is prepared by heating it to a temperature of #abj;deg ,c41 keeping it there for some hours1 and allowing it to cool slowly1 when it assumes a crystalline form and changes from a bluish grey to a dull slate colour4 ,a selenium cell in its simplest form consists merely of some prepared selenium placed between two or more metal electrodes1 the selenium acting as a high resistance conductor between them4 ,the form given by ,bell and ,tainter to the cells used in their experiments is given in ,figs4 #ec and #ec;a4 ,it consists of a number of rectangular brass plates ,p1 ,p'1 separated by very thin sheets of mica ,m1 the mica sheets being slightly narrower than the brass plates1 the whole being clamped together in the frame ,f by the two bolts ,b4 #aaj ,by means of a sand-bath the cell is raised to the desired temperature1 and selenium is rubbed over the surface1 which melts and fills the small spaces between the brass plates4 ,all the plates ,p are connected together to form one terminal1 and the plates ,p' to form the other4 ,by using very thin mica sheets1 and a large number of elements1 a very narrow transverse section of selenium1 together with a large active surface1 can be obtained4 ,the cell used for commercial purposes is usually constructed as follows4 ,a small rectangular piece of porcelain1 slate1 mica1 or other insulator1 is wound with many turns of fine platinum wire4 ,the wire is wound double1 as shown in ,fig4 #ed1 the spaces between the turns being filled with prepared selenium4 ,a thin glass cover is sometimes placed over the cell to protect the surface from injury4 ,illustration3 ,,fig4 #ec4 ,p1 ,p'1 plates2 ,m1 mica2 ,s1 selenium4 ,illustration3 ,,fig4 #ec;a4 ,a strong light falling upon a cell lowers its resistance1 and vice versa1 the resistance of a cell being at its highest when unexposed to light2 the light is apparently absorbed and made to do work by varying the electrical resistance of the selenium4 ,selenium cells vary very considerably as regards their quality as well as in their electrical resistance1 it being possible to obtain cells of the same size for any resistance between #aj and #a1jjj1jjj ohms1 and also1 a cell may remain in good working condition for several months1 while another will become useless in as many weeks4 ,the ability of a cell to respond to very rapid changes in the illumination to which it is exposed is determined largely upon its inertia1 it being taken as a general rule #aaa that the higher the resistance of a cell the less the inertia1 and vice versa1 and also1 that the higher the resistance the greater the ratio of sensitiveness4 ,inertia plays an important part in the working of a cell1 slightly opposing the drop in resistance when illuminated1 and opposing to a ,illustration much greater degree the return to normal for no-illumination4 ,the effects of inertia or lag1 as it is termed1 can readily be seen by reference to ,fig4 #ee4 ,it will be noticed that the current value rapidly increases when the cell is first illuminated1 but if after a short time t the light is cut off1 the current value1 instead of returning at once to normal for no-illumination1 only partially rises owing to the interference of the inertia1 and some time elapses before the cell returns to its normal condition2 the time varying from a few seconds to several minutes1 depending upon the characteristics of the cell and the amount of light to which it is exposed4 ,an actual curve is given in ,fig4 #ee;a4 ,the inertia or lag of a cell produces upon an intermittent current an effect similar to that produced by the capacity ,illustration of a line1 as was noted in ,chapter ,i41 preventing the incoming signals from being recorded separately1 and distinctly4 ,to obtain the best results in photo-telegraphy1 the resistance of a cell should only be decreased to an extent sufficient to pass the current required to operate the recording apparatus1 and the illumination should be regulated so that this condition of the cell takes place4 ,the comparative slowness of selenium in responding to #aab any great changes in the illumination offers a serious difficulty to its use in photo-telegraphy1 but various methods have been devised whereby the effects of inertia can be counteracted4 ,in the system of ,de' ,bernochi 7see ,chapter ,i47 the changes in the illumination are neither very rapid nor very great1 and the inertia effects would therefore be very slight2 but in any photo-telegraphic system in which a metal line print is used for transmitting1 where the source of illumination is constant and the resistance of the cell is required to drop to a definite value and return to normal instantly1 many times in succession1 the inertia effects are very pronounced4 ,the most successful method of counteracting the inertia is that adopted by ,professor ,korn of always keeping the cell sufficiently illuminated to overcome it1 so that any additional light acts very rapidly4 ,another method worked out and patented by ,professor ,korn1 and known as the compensating cell method1 gives a practically dead beat action1 the resistance returning to its normal condition as soon as the illumination ceases4 ,the arrangement is given in the diagram ,fig4 #ef4 ,illustration3 ,,fig4 #ee;a4 ,light from the transmitting or receiving apparatus1 as the case may be1 falls upon the selenium cell ,s#a1 which is #aac placed on one arm of a ,wheatstone bridge1 a second cell ,s#b being placed on the opposite arm4 ,the selenium cell ,s#a should have great sensitiveness and small inertia1 the compensating cell ,s#b having proportionally small sensitiveness and large inertia4 ,two batteries ,b1 ,b'1 of about #ajj volts1 are connected as shown1 ,b being provided with a compensating variable resistance ,w2 ,w' is also a regulating resistance4 ,when no light is falling upon the cell ,s#a1 light from ,l is prevented from reaching the second cell ,s#b by a small shutter which is fastened to the strings of the ,einthoven galvanometer 7described in ,chapter ,,iii471 and the piece of apparatus ,c--relay or galvanometer as the case may be--remains in a normal condition4 ,when1 however1 light falls upon the cell ,s#a1 the balance of the bridge is upset1 and light from ,l falls a fraction of a second later upon the second cell ,s#b1 and the current flowing through ,c completes the circuit4 ,needless to say it is necessary that the two cells be well matched1 as it is very easy to have over-compensation1 in which case the current is brought below zero4 ,illustration3 ,,fig4 #ef4 ,it is also stated that by enclosing the cells in exhausted glass tubes1 their inertia can be greatly reduced and their life considerably prolonged4 ,the sensitiveness of a cell is the ratio between its resistance in the dark and its resistance when illuminated4 ,the majority of cells have a ratio between #b3a and #c3a1 but ,professor ,korn has shown mathematically that by conforming to certain conditions regarding the construction the ratio of sensitiveness may be between #d3a and #e3a4 ,thus a cell of ,r #bej1jjj ohms can be reduced to #fj1jjj ohms from the light of a #af c4p4 lamp placed only a short distance away2 the resistance may be still #aad further decreased by continuing the illumination1 but this produces a permanent defect in the cells termed fatigue1 the cells becoming very sluggish in their action and their sensitiveness gradually becoming less1 the ratio between their resistance in the dark and their resistance when illuminated being reduced by as much as #cj per cent4 ,excessive illumination will also produce similar results4 ,the inertia of a cell is practically unaffected by the wavelength of the light used1 but the maximum sensitiveness of a cell is towards the yellow-orange portion of the spectrum4 ,in addition to light1 heat has also been found to vary the electrical resistance of selenium in a very remarkable manner4 ,at #hj;deg ,c4 selenium is a non-conductor1 but up to #baj;deg ,c4 the conductivity gradually increases1 after which it again diminishes4 99 99 99 99 99 #aae ,,appendix ,b ,,preparing ,,the ,,metal ,,prints ,electricians who desire to experiment in photo-telegraphy1 but who have no knowledge of photography1 may perhaps find the following detailed description of preparing the metal prints of some value4 ,the would-be experimenter may feel somewhat alarmed at the amount of work entailed1 but once the various operations are thoroughly grasped1 and with a little patience and practice1 no very great difficulty should be experienced4 ,the simpler photographic operations1 such as developing1 fixing1 etc41 cannot be described here1 and the beginner is advised to study a good text-book on the subject4 ,the method to be given of preparing the photographs is practically the only one available for wireless transmission1 and although the manner given of preparing is perhaps not strictly professional1 having been modified in order to meet the requirements of the ordinary amateur experimenter1 the results obtained will be found perfectly satisfactory4 ,as will have been gathered from ,chapter ,,ii41 the camera used for copying has to have a single line screen placed a certain distance in front of the photographic plate1 and the object of this screen is to break the image up into parallel bands1 each band varying in width according to the density of the photograph from which it has been prepared4 ,thus a white portion of the photograph would consist of very narrow lines wide apart1 while a dark portion would be made up of wide lines close together2 a black part would appear solid and show no lines at all4 ,it is1 of course1 obvious #aaf that the lines on the negative cannot be wider apart1 centre to centre1 than the lines of the screen4 ,a good screen distance has been found to be #a to #fd1 i4e4 the diameter of the stop is #a_/fdth of the camera extension1 and the distance of the screen lines from the photographic plate is #fd times the size of the screen opening4 ,the following table shows what this distance is for the screen most likely to be used4 ,the line screens used consist of glass plates upon which a number of lines are accurately ruled1 the width of the lines and the spaces between being equal2 the lines are filled in with an opaque substance4 ,these ruled screens are very expensive1 and are only made to order1#aj a screen half-plate size costing from #bas4 to #bgs4 #f;d4 ,an efficient substitute for a ruled screen can be made by taking a rather large sheet of ,bristol board and ruling lines across in pure black drawing ink1 the width of the lines and the spaces between being #a_/abth of an inch respectively4 ,a photograph must be taken of this card1 the reduction in size determining the number of lines to the inch4 ,a card #bj x #ae inches1 with #ab lines to the inch1 would1 if reduced to #e x #d inches1 make a screen having #dh lines to the inch4 ,preparing the board is rather a tedious operation1 but the line negative produced will be found to give results almost as good as those obtained from a purchased screen4 ,,diameter ,,of ,,stop ,,used #a_/fd,,th ,,of ,,camera ,,extension4 ---------------------------------------- ---------------------- ,screen ruling ,actual space ,distance of ,in #a_/cb;,in milli- lines per inch4 in inches4 screen ruling inches metres4 in inches4 ---------------------------------------- ---------------- #ce #a_/gj 4#ia #bh4h #ba4h #ej #a_/ajj 4#fd #bj4e #af4b #fe #a_/acj 4#di #ae4g #ab4d ---------------------------------------- ---------------------- ,as it is impossible for many to have the use of professional apparatus designed for this particular kind of work1 #aag the fixing of the screen into an ordinary camera must be left to the ingenuity of the worker4 ,a half-plate back focussing camera will be found suitable for general experimental work1 but if this is not available1 a large box camera can be pressed into service4 ,illustration3 ,,fig4 #eg4 ,the writer has never seen a half-plate box camera1 but one taking a #e x #d inch plate can be obtained second-hand very cheaply4 ,it is a comparatively simple matter to fix the line screen into a camera of this description1 the drawings ,figs4 #eg and #eh showing the method adopted by the writer4 ,the two clips ,d1 made from fairly stout brass about #a_/b inch wide1 are bent to the shape shown 7an enlarged section is given at ,c7 and soldered at the top and bottom of one of the metal sheaths provided for holding the plates4 ,the distance between the front of the photographic plate 7the film side7 and the back of the line screen 7also the film side71 indicated by the arrow at ,a1 is determined by the number of lines on the screen4 ,as will be seen from the table given1 the distance for a screen having #ej lines to the inch will be #da_/fdths of an inch4 ,illustration3 ,,fig4 #eh4 ,m1 sheath2 ,p1 photographic plate2 ,d1 clips2 ,s1 line screen4 ,in all probability there will be enough clearance between the top of the sheath and the top of the camera to allow for the thickness of the clip1 but if not1 a shallow groove a little wider than the clip should be carefully cut in the top of the camera1 so that it will slide in easily4 ,the screen should be placed between the clips1 the film side on the #aah inside1 i4e4 facing the photographic plate4 ,as with a box camera the extension is a fixture1 the size of stop to be used is a fixture also4 ,the extension of a camera 7this term really applies to a bellows camera7 is measured from the front of the photographic plate to the diaphragm1 and if this distance in our camera is #h inches1 then the diameter of the stop to give the best results would be #a_/fdth of this1 or #a_/hth inch4 ,although for all ordinary experimental work the lens fitted to the camera will be suitable1 the best type of lens for process work of all kinds is the ,anastigmat4 ,the picture or photograph from which it is desired to make a print should be fastened out perfectly flat upon a board with drawing pins1 and if a copying stand is not available it must be placed upright in some convenient position4 ,the diagram ,fig4 #ei gives the disposition of the apparatus required for copying4 ,a simple and inexpensive copying stand is shown in ,fig4 #fj4 ,the blackboard ,a should be about #cj inches square1 and must be fastened perfectly upright upon the base-board ,b4 ,the stand ,c should be made so that it slides without any side play between the guides ,d1 and should be of such a height that the lens of the camera comes exactly opposite the #aai ,illustration ,illustration centre of the board ,a4 ,the camera1 if of the box type1 can be secured to the stand by means of a screw and wingnut1 the screw being passed from the inside as shown4 ,the beginner is advised to photograph only very bold and simple subjects1 such as black and white drawings or enlargements4 ,it is not safe to trust to the view-finders as to whether the whole of the picture is included on the plate1 a piece of ground glass the same size as the plate sheaths1 and used as a focussing screen1 being much more reliable4 ,it is a good plan to focus the camera for a number of different-sized pictures1 marking the board ,a1 and the #abj guides ,d1 so that adjustment is afterwards a very simple matter4 ,the make of plate used is also a great factor in getting a good negative1 and ,wratten ,process ,plates will be found excellent4 ,as already mentioned1 such subjects as the exposure and the development of the plate cannot be dealt with here1 these subjects having been exhaustively treated in several text-books on photography4 ,with an arc lamp the exposure is about twice as long as in daylight1 but the exposure varies with the amount of light admitted to the plate1 character of the source of light1 and the sensitiveness of the plate used1 etc4 ,the writer has used acetylene gas lamps for this purpose with great success4 ,the beginner is advised to use artificial light1 as this can be kept perfectly even4 ,with daylight1 however1 the light is constantly fluctuating1 and this renders the use of an actinometer a necessity for correct exposure4 ,after development1 if the plate is required for immediate use1 it can be quickly dried by soaking for a few minutes in methylated spirit4 ,having obtained a good negative1 the next operation is to prepare what is known as a metal print4 ,for this we shall require some stout tin-foil or lead-foil1 about #ab or #ae square feet to the pound1 and this should be cut into pieces of such a size that it allows a lap of #c_/af inch when wrapped round the drum of the transmitting machine4 ,obtain some good fish-glue and add a saturated solution of bichromate of potash in the proportion of #d parts of potash to #dj or #ej parts of glue4 ,pour a little of this glue into a shallow dish1 lay a sheet of foil upon a flat board1 and with a fairly stiff brush 7a flat hog's-hair as wide as possible7 proceed to coat the sheet of foil with a thin but perfectly even coating of glue4 ,the thickness of the coating can only be found by trial1 for if the coating is too thick a longer time will be required for printing2 but it must not be thin enough to show interference colours4 ,after the coating has been laid on1 a soft brush1 such as photographers use for dusting dry #aba plates1 should be passed up and down1 and across and across1 with light1 even strokes to remove any unevenness4 ,a glue solution used by professional photo-engravers is as follows3 ,fish-glue #ab oz4 ,bichromate of ,ammonia #c_/d oz4 ,water #ah to #bd oz4 ,ammonia 4#hhj #cj minims4 ,the bichromate should be dissolved in the water1 and1 when added to the glue1 stir very thoroughly in order that complete mixing may take place4 ,the coating may be done in a good light1 not bright sunlight1 but it must be dried in the dark1 because1 although insensitive while in a moist condition1 it becomes sensitive immediately on desiccation4 ,if allowed to dry in the light the whole coating will become insoluble1 and for this reason the brushes used should be washed out as soon as they are finished with4 ,the sheets will take about #ae minutes to dry in a perfectly dry room1 but it is not advisable to prepare many sheets at once1 as they will not keep for more than two or three days4 ,the prepared negative must now be placed in an ordinary printing frame1 and a print taken off upon one of the metal sheets in the same way as a print is taken off upon ordinary sensitised paper4 ,in daylight the exposure varies from #e to #bj minutes1 but in artificial light various trials will have to be made in order to get the best results1 the exposure varying with the amount of bichromate in the coating2 the proportion of the bichromate to the glue should remain about #f per cent4 ,light from a #be ampere arc lamp for #b to #e minutes1 at a distance of #ah inches1 will generally suffice to print the impression on the metal sheets4 ,the printing finished1 the metal print should be laid upon a sheet of glass and held under a running stream of water4 ,the washing is complete as soon as the unexposed parts of the glue coating have been entirely washed away leaving the bare metal1 and this will take anything from #c to #g #abb minutes1 depending upon the thickness of the film4 ,as soon as it is dry the print is ready for use4 ,as already mentioned1 the negative from which the metal print is made requires that the lines be perfectly sharp and opaque1 and the spaces between perfectly transparent4 ,ordinary dry plates are too rapid1 a rather slow plate being required4 ,wratten ,process ,plates give excellent results1 and the following is a good developer to use with them3 ,glycin #ae grammes #a oz4 ,sulphite of ,soda #dj 11 #b-a_/b 11 ,carbonate of ,potash #hj 11 #e 11 ,water #ajjj c4c4 #fj 11 ,this developer should be used for #f minutes at a temperature of #ej;deg ,f41 #c-a_/b minutes at #fe;deg1 and #a-c_/d minutes at #hj;deg4 ,it is best only used once4 ,if an intensifier is required1 the following formula will be found to give satisfactory results3 ,bichloride of ,mercury #a oz4 #fj grammes4 ,hot ,water #af 11 #ajjj c4c4 ,allow to cool1 completely pour off from any crystals1 and add3 ,hydrochloric ,acid #cj minims #d c4c4 ,allow negative to bleach thoroughly1 wash well in water1 and blacken in #aj per cent ammonia 4#hhj1 or #e per cent sodium sulphide4 ,in preparing the negatives and metal prints the following points should be observed3 ,a good negative should have the lines perfectly sharp and opaque2 there should be no fluff between the lines even when they are close together4 ,a properly exposed and developed negative should not require any reducing or intensifying4 ,if the lamps used for illuminating the copying board are placed #b feet away1 and the exposure required is #e minutes1 the exposure1 if the lamps are placed #d feet away1 will be #abc #bj minutes1 as the amount of light which falls upon an object decreases as the inverse square of the distance4 ,get the coating on the foil as thin as possible1 and err on the side of over-exposure1 for if the coating is thick and has been under-exposed1 excessive washing will dissolve the whole coating2 for1 unless insolubilisation has taken place right up to the metal base1 the under parts will remain in a more or less soluble condition4 ,on no account must the unexposed sheets be placed near a fire1 otherwise they will be spoilt1 the whole coating becoming insoluble2 heat acting in the same manner as light4 ,in washing1 keep the print moving so that the stream of water does not fall continually in one place4 ,it is best to hold the print so that the water runs off in the direction of the lines4 ,to dry the prints after washing they can be laid out flat in a moderately warm oven1 or before a stove1 the heat of course not being sufficient to cause the coating to peel4 ,to render the glue image more distinct the print should be immersed for a few seconds in an aniline dye solution1 the glue taking up the colour readily4 ,these dyes are soluble in either water or alcohol4 ,a dye known as magenta is very good4 ,the process of coating the metal sheets must be performed as quickly as possible 7about #aj seconds71 as owing to the peculiar nature of the bichromated glue it soon sets1 and once this has taken place it is impossible to smooth down any unevenness4 ,see that the negative and metal sheet make good contact while printing4 ,if the glue solution does not adhere to the surface of the foil in a perfectly even film1 but assumes a streaky appearance1 a little liquid ammonia1 or a weak solution of nitric acid1 rubbed over the surface of the foil1 which is afterwards gently scoured with precipitated chalk on a tuft of cotton #abd wool1 will remove the grease which is the cause of the difficulty4 ,a photograph of a picture prepared from a line negative is given in ,fig4 #fa4 ,for a great many experiments1 and in order to save time1 trouble1 and expense1 sketches drawn upon stout lead-foil in an insulating ink will answer the purpose admirably1 but if any exact work is to be done a single line print is of course absolutely necessary4 ,the insulating ink can be prepared by dissolving shellac in methylated spirit1 or ordinary gum can be used4 ,a very fine brush should be used in place of a pen1 as the gum will not flow freely from an ordinary nib unless greater pressure than the foil will safely stand be applied4 ,a sketch prepared in this manner is shown in ,fig4 #fb4 ,a little aniline dye should be added to the gum to render it more visible1 or a mixture of gum and liquid indian ink will be found suitable4 ,illustration3 ,,fig4 #fc4 ,with the copying arrangement already described it is only possible to employ it for reducing1 it being necessary to employ a bellows camera with a back focussing attachment for purposes of enlarging1 and this constitutes the chief drawback to the use of a fixed focus camera4 ,by replacing the box camera with a focussing camera of the same size1 we shall have a piece of apparatus capable of reducing or enlarging1 only in this case the camera should be a fixture and the board1 ,a1 arranged to slide backwards and forwards instead4 ,illustration3 ,,fig4 #fa4 ,portions of photographs 7full size7 of single line screen1 and single line print4 ,screen #dj lines to the inch4 ,illustration3 ,,fig4 #fb4 #abe ,an extra improvement would be to rule the surface of the copying board1 ,a1 in a manner similar to that shown in the diagram1 ,fig4 #fc4 ,the rulings should be marked off from the centre of the board1 and should enclose parallelograms of the various plate sizes ranging from #c-a_/d x #d-a_/d inches up to the full size of the board4 ,by fastening the picture or photograph to be copied in the space on the board corresponding in size1 we can ensure that it is in the correct position for the whole to be included on the photographic plate1 providing1 of course1 that the centre of lens and board coincide4 ,with regard to the lens required1 the practice adhered to by most photographers is to use a lens having a focal length equal to the diagonal of the plate used4 ,thus for a #a_/d-plate camera a #e-;inch lens should be used1 and for a #a_/b-plate an #h-;inch lens1 and so on4 ,for a #e x #d inch camera a #f-;inch lens will be required4 ,the following is a simple rule for finding the conjugate foci of a lens1 and is useful in obtaining the distance from the lens to the photographic plate and the picture to be copied4 ,let us suppose that we wish to make a #a-a_/b times enlarged line negative from a #d-a_/d x #c-a_/d inch print4 ,add #a to the number of times it is required to enlarge and multiply the result by the focal length of the lens in inches4 ,in the present case this will be #a-a_/b #a #b-a_/b2 and if a #f-;inch lens is used1 #b-a_/b x #f #ae inches will be the distance of the lens from the plate4 ,divide this number by the number of times it is desired to enlarge1 and the distance of the lens from the picture to be copied is obtained2 in this instance #ae _/ #a-a_/b #aj inches4 ,the same rule can be followed when it is required to reduce any given number of times1 only in this case the greater number will represent the distance between the lens and the picture to be copied1 and the lesser number the distance between the lens and the plate4 ,in reducing1 a #a_/d-plate lens will be found to fully cover a #e x #d inch plate1 providing the reduction is not greater than three to one4 99 99 99 99 99 #abf ,,appendix ,c ,,lenses ,in this small volume it is not desirable1 neither is it intended1 to give an exhaustive treatment on the subject of lenses and their action1 but as optics plays an important part in the transmission of photographs1 both by wireless and over ordinary conductors1 the following notes relating to a few necessary principles have been included as likely to prove of interest4 ,light always travels in straight lines when in a medium of uniform density1 such as water1 air1 glass1 etc41 but on passing from one medium to another1 such as from air to water1 or air to glass1 the direction of the light rays is changed1 or1 to use the correct term1 refracted4 ,this refraction of the rays of light only takes place when the incident rays are passed obliquely2 if the incident rays are perpendicular to the surface separating the two media they are not refracted1 but continue their course in a straight line4 ,all liquid and solid bodies that are sufficiently transparent to allow light rays to pass through them possess the power of bending or refracting the rays1 the degree of refraction1 as already explained1 depending upon the nature of the body4 ,the law relating to refraction will perhaps be better understood by means of the following diagram4 ,in ,fig4 #fd let the line ,,ab represent the surface of a vessel of water4 ,the line ,,cd1 which is perpendicular to the surface of the #abg water1 is termed the normal1 and a ray of light passed in this direction will continue in a straight line to the point ,e4 ,if1 however1 the ray is passed in an oblique direction1 such as ,,nd1 it will be seen that the ray is bent or refracted in the direction ,,dm2 if the ray of light is passed in any other oblique direction1 such as ,,jd1 the refracted ray will be in the direction ,,dk4 ,the angle ,,ndc is called the angle of incidence and ,,mde the angle of refraction4 ,if we measure accurately the line ,,nc1 we shall find that it is #a-a_/c1 or more exactly #a4ccf1 times greater than the line ,,em4 ,if we repeat this measurement with the lines ,,jh and ,,pk we shall find that the line ,,jh also bears the proportion of #a4ccf to the line ,,pk4 ,the line ,,nc is called the sine of the angle of incidence ,,ndc1 and ,,em the sine of the angle of refraction ,,mde4 ,illustration3 ,,fig4 #fd4 ,therefore in water the sine of the angle of incidence is to the sine of the angle of refraction as #a4ccf is to #a1 and this is true whatever the position of the incident ray with respect to the surface of the water4 ,from this we say that the sines of the angles of incidence and refraction have a constant proportion or ratio to one another4 ,the number #a4ccf is termed the refractive index1 or coefficient1 or the refractive power of water4 ,the refractive power varies1 however1 with other fluids and solids1 and a complete table will be found in any good work on optics4 ,glass is the substance most commonly used for refracting the rays of light in optical work1 the glass being worked up into different forms according to the purpose for which it #abh is intended4 ,solids formed in this way are termed lenses4 ,a lens can be defined as a transparent medium which1 owing to the curvature of its surfaces1 is capable of converging or diverging the rays of light passed through it4 ,according to its curvature it is either spherical1 cylindrical1 elliptical1 or parabolic4 ,the lenses used in optics are always exclusively spherical1 the glass used in their construction being either crown glass1 which is free from lead1 or flint glass1 which contains lead and is more refractive than crown glass4 ,the refractive power of crown glass is from #a4ecd to #a4ebe1 and of flint glass from #a4fbe to #a4eij4 ,spherical surfaces in combination with each other or with plane surfaces give rise to six different forms of lenses1 sections of which are given in ,fig4 #fe4 ,illustration3 ,,fig4 #fe4 ,all lenses can be divided into two classes1 convex or converging1 or concave or diverging4 ,in the figure1 b1 c1 g are converging lenses1 being thicker at the middle than at the borders1 and d1 e1 f1 which are thinner at the middle1 being diverging lenses4 ,the lenses e and g are also termed meniscus lenses1 and a represents a prism4 ,the line ,,xy is the axis or normal of these lenses to which their plane surfaces are perpendicular4 ,let us first of all notice the action of a ray of light when passed through a prism4 ,the prism1 ,fig4 #ff1 is represented by the triangle ,,bbb1 and the incident ray by the line ,,ta4 #abi ,where it enters the prism at ,a its direction is changed and it is bent or refracted towards the base of the prism1 or towards the normal1 this being always the case when light passes from a rare medium to a dense one1 and where the light leaves the opposite face of the prism at ,d it is again refracted1 but away from the normal in an opposite direction to the incident ray1 since it is passing from a dense to a rare medium4 ,the line ,,dp is called the emergent or refracted ray4 ,if the eye is placed at ,t1 and a bright object at ,p1 the object is seen not at ,p1 but at the point ,h1 since the eye cannot follow the course taken by the refracted rays4 ,in other words1 objects viewed through a prism always appear deflected towards its summit4 ,illustration3 ,,fig4 #ff4 ,in considering the action of a lens we can regard any lens as being built up of a number of prisms with curved faces in contact4 ,such a lens is shown in ,fig4 #fg1 the light rays being refracted towards the base of the prisms or towards the normal1 as already explained2 while the top half of the lens will refract all the light downwards1 the bottom half will act as a series of inverted prisms and refract all the light upwards4 ,illustration3 ,,fig4 #fg4 ,illustration3 ,,fig4 #fh4 ,if a beam of parallel light--such as light from the sun--be passed through a double convex lens ,l1 ,fig4 #fh1 we shall find that the rays have been refracted from their parallel course and brought together at a point ,f4 ,this point ,f is #acj termed the principal focus of the lens1 and its distance from the lens is known as the focal length of that lens4 ,in a double and equally convex lens of glass the focal length is equal to the radius of the spherical surfaces of the lens4 ,if the lens is a plano-convex the focal length is twice the radius of its spherical surfaces4 ,if the lens is unequally convex the focal length is found by the following rule3 multiply the two radii of its surfaces and divide twice that product by the sum of the two radii1 and the quotient will #aca be the focal length required4 ,conversely1 by placing a source of light at the point ,f the rays will be projected in a parallel beam the same diameter as the lens4 ,if1 however1 instead of being parallel1 the rays proceed from a point farther from the lens than the principal focus1 as at ,a1 ,fig4 #fi1 they are termed divergent rays1 but they also will be brought to a focus at the other side of the lens at the point a4 ,if the source of light ,a is moved nearer to the principal focus of the lens to a point ,a#a the rays will come to a focus at the point a#a1 and similarly when the light is at ,a#b the rays will come to a focus at the point a#b4 ,it can be found by direct experiment that the distance fa increases in the same proportion as ,,af diminishes1 and diminishes in the same proportion as ,,af increases4 ,the relationship which exists between pairs of points in this manner is termed the conjugate foci of a lens1 and though every lens has only one principal focus1 yet its conjugate foci are innumerable4 ,illustration3 ,,fig4 #fi4 ,the formation of an image of some distant object in its principal focus is one of the most useful properties of a convex lens1 and it is this property that forms the basis of several well-known optical instruments1 including the camera1 telescope1 microscope1 etc4 ,if we take an oblong wooden box1 ,,aa1 and substitute a sheet of ground glass1 ,c1 for one end1 and drill a small pinhole1 ,h1 in the centre of the other end opposite the #acb glass plate1 we shall find that a tolerably good image of any object placed in front of the box will be formed upon the glass plate4 ,the light rays from all points of the object1 ,,bd1 ,fig4 #gj1 will pass straight through the hole ,h1 and illuminate the ground glass screen at points immediately opposite them1 forming a faint inverted image of the object ,,bd4 ,the purpose of the hole ,h is to prevent the rays from any one point of the object from falling upon any other point on the glass screen than the point immediately opposite to it1 therefore the smaller we make ,h1 the more distinct will be the image obtained4 ,reducing the size of ,h in order to produce a more distinct image has the effect of causing the image to become very faint1 as the smaller the hole in ,h1 the smaller the number of rays that can pass through from any point of the object4 ,by enlarging the hole ,h gradually1 the image will become more and more indistinct until such a size is reached that it disappears altogether4 ,illustration3 ,,fig4 #gj4 ,if in this enlarged hole we place a double convex lens1 ,,ll1 ,fig4 #ga1 whose focal length suits the length of the box1 the image produced will be brighter and more distinct than that formed by the aperture1 ,h1 since the rays which proceed from any point of the object will be brought by the lens to a focus on the glass screen1 forming a bright #acc distinct image of the point from which they come4 ,the image owes its increased distinctness to the fact that the rays from any one point of the object cannot interfere with the rays from any other point1 and its increased brightness to the great number of rays that are collected by the lens from each point of the object and focussed in the corresponding point of the image4 ,it will be evident from a study of ,fig4 #ga that the image formed by a convex lens must necessarily be inverted1 since it is impossible for the rays from the end1 ,m1 of the object to be carried by refraction to the upper end of the image at n4 ,the relative positions of the object and image when placed at different distances from the lens are exactly the same as the conjugate foci of light rays as shown in ,fig4 #fi4 ,illustration3 ,,fig4 #ga4 ,the length of the image formed by a convex lens is to the length of the object as the distance of the image is to the distance of the object from the lens4 ,for example1 if a lens having a focal length of #ab inches is placed at a distance of #ajjj feet from some object1 then the size of the image will be to that of the object as #ab inches to #ajjj feet1 or #ajjj times smaller than the object2 and if the length of the object is #ejj inches1 then the length of the image will be the #a_/ajjjth part of #ejj inches1 or #a_/b inch4 #acd ,the image formed by the convex lens in ,fig4 #ga is known as a real image1 but in addition convex lenses possess the property of forming what are termed virtual images4 ,the distinction can be expressed by saying1 real images are those formed by the refracted rays themselves1 and virtual images those formed by their prolongations4 ,while a real image formed by a convex lens is always inverted and smaller than the object1 the virtual image is always erect and larger than the object4 ,the power possessed by convex lenses of forming virtual images is made use of in that useful but common piece of apparatus known as a reading or magnifying glass1 by which objects placed within its focus are made larger or magnified when viewed through it2 but in order to properly understand how objects seem to be brought nearer and apparently increased in size1 we must first of all understand what is meant by the expression1 the apparent magnitude of objects4 ,illustration3 ,,fig4 #gb4 ,the apparent magnitude of an object depends upon the angle which it subtends to the eye of the observer4 ,the image at ,a1 ,fig4 #gb1 presents a smaller angle to the eye than the angle presented by the object when moved to ,b1 and the image therefore appears smaller4 ,when the object is moved to either ,b or ,c1 it is viewed under a much #ace greater angle1 causing the image to appear much larger4 ,if we take a watch or other small circular object and place it at ,a1 which we will suppose is a distance of #ej yards1 we shall find that it will be only visible as a circular object1 and its apparent magnitude or the angle under which it is viewed is then stated to be very small4 ,if the object is now moved to the point ,b1 which is only #e feet from the eye1 its apparent magnitude will be found to have increased to such an extent that we can distinguish not only its shape1 but also some of the marking4 ,when moved to within a few inches from the eye as at ,c1 we see it under an angle so great that all the detail can be distinctly seen4 ,by having brought the object nearer the eye1 thus rendering all its parts clearly visible1 we have actually magnified it1 or made it appear larger1 although its actual size remains exactly the same4 ,when the distance between the object and the observer is known1 the apparent magnitude of the object varies inversely as the distance from the observer4 ,let us suppose that we wish to produce an image of a tree situated at a distance of #ejjj feet4 ,at this distance the light rays from the tree will be nearly parallel1 so that if a lens having a focal length of #e feet is fastened in any convenient manner in the wall of a darkened room the image will be formed #e feet behind the lens at its principal focus4 ,if a screen of white cardboard be placed at this point we shall find that a small but inverted image of the tree will be focussed upon it4 ,as the distance of the object is #ejjj feet1 and as the size of the received image is in proportion to this distance divided by the focal length of the lens1 the image will be as #ejjj _/ #e1 or #ajjj times smaller than the object4 ,if now the eye is placed six inches behind the screen and the screen removed1 so that we can view the small image distinctly in the air1 we shall see it with an apparent magnitude as much greater than if the same small image were equally far off with the tree1 as #f inches is to #ejjj #acf feet1 that is #aj1jjj times4 ,thus we see that although the image produced on the screen is #ajjj times less than the tree from one cause1 yet on account of it being brought near to the eye it is #aj1jjj times greater in apparent magnitude2 therefore its apparent magnitude is increased as #aj1jjj _/ #ajjj1 or #aj times4 ,this means that by means of the lens it has actually been magnified #aj times4 ,this magnifying power of a lens is always equal to the focal length divided by the distance at which we see small objects most distinctly1 viz4 #f inches1 and in the present instance is #fj _/ #f1 or #aj times4 ,when the image is received upon a screen the apparatus is called a camera obscura1 but when the eye is used and sees the inverted image in the air1 then the apparatus is termed a telescope4 ,the image formed by a convex lens can be regarded as a new object1 and if a second lens is placed behind it a second image will be formed in the same manner as if the first image were a real object4 ,a succession of images can thus be formed by convex lenses1 the last image being always treated as a fresh object1 and being always an inverted image of the one before4 ,from this it will be evident that additional magnifying power can be given to our telescope with one lens by bringing the image nearer the eye1 and this is accomplished by placing a short focus lens between the image and the eye4 ,by using a lens having a focal length of #a inch1 and such a lens will magnify #f times1 the total magnifying power of the two lenses will be #aj x #f #fj times1 or #aj times by the first lens and #f times by the second4 ,such an instrument is known as a compound or astronomical telescope1 and the first lens is called the object glass and the second lens the magnifying glass1 or eye-piece4 ,we are now in a position to understand how virtual images are formed1 and the formation of a virtual image by means of a convex lens will be readily followed from a #acg study of ,fig4 #gc4 ,let ,l represent a double convex lens1 with an object1 ,,ab1 placed between it and the point ,f1 which is the principal focus of the lens4 ,the rays from the object ,,ab are refracted on passing through the lens1 and again refracted on leaving the lens1 so that an image of the object is formed at the eye1 ,n4 ,as it is impossible for the eye to follow the bent rays from the object1 a virtual image is formed and is seen at ,a#a;,b#a1 and is really a continuation of the emergent rays4 ,the magnifying power of such a lens may be found by dividing #f inches by the focal length of the lens1 #f inches being the distance at which we see small objects most distinctly4 ,a lens having a focal length of #a_/d inch would magnify #bd times1 and one with a focal length of #a_/ajjth of an inch #fjj times1 and so on4 ,the magnifying power is greater as the lens is more convex and the object near to the principal focus4 ,when a single lens is applied in this manner it is termed a single microscope1 but when more than one lens is employed in order to increase the magnifying power1 as in the telescope1 then the apparatus is termed a compound microscope4 ,illustration3 ,,fig4 #gc4 ,unlike a convex lens1 which can form both real and virtual images1 a concave lens can only produce a virtual image2 and while the convex lens forms an image larger #ach than the object1 the concave lens forms an image smaller than the object4 ,let ,l1 ,fig4 #gd1 represent a double concave lens1 and ,,ab the object4 ,the rays from ,,ab on passing through the lens are refracted1 and they diverge in the direction ,,rrrr1 as if they proceeded from the point ,f1 which is the principal focus of the lens1 and the prolongations of these divergent rays produce a virtual image1 erect and smaller than the object1 at ,a#a;,b#a4 ,the principal focal distance of concave lenses is found by exactly the same rule as that given for convex lenses4 ,illustration3 ,,fig4 #gd4 ,up to the present we have assumed that all the rays of light passed through a convex lens were brought to a focus at a point common to all the rays1 but this is really only the case with a lens whose aperture does not exceed #ab;deg4 ,by aperture is meant the angle obtained by joining the edges of a lens with the principal focus4 ,with lenses having a larger aperture the amount of refraction is greater at the edges than at the centre1 and consequently the rays that pass through the edges of the lens are brought to a focus nearer the lens than the rays that pass through the centre4 ,since this defect arises from the spherical form of the lens it is termed spherical aberration1 and in lenses that #aci are used for photographic purposes the aberration has to be very carefully corrected4 ,the distortion of an image formed by a convex lens is shown by the diagram1 ,fig4 #ge4 ,if we receive the image upon a sheet of white cardboard placed at ,a1 we shall find that while the outside edges will be clear and distinct1 the inside will be blurred1 the reverse being the case when the cardboard is moved to the point ,b4 ,illustration3 ,,fig4 #ge4 ,illustration3 ,,fig4 #gf4 ,illustration3 ,,fig4 #gg4 ,aberration is to a great extent minimised by giving to the lens a meniscus instead of a biconvex form1 but as it is desirable to reduce the aberration to below once the #adj thickness of the lens1 and as this cannot be done by a single lens1 we must have recourse to two lenses put together4 ,the thickness of a lens is the difference between its thickness at the middle and at the circumference4 ,in a double convex lens with equal convexities the aberration is #a-fg_/ajjths of its thickness4 ,in a plano-convex lens with the plane side turned towards parallel rays the aberration is #d-a_/b times its thickness1 but with the convex side turned towards parallel rays the aberration is only #a-ag_/ajjths of its thickness4 ,by making use of two plano-convex lenses placed together as at ,fig4 #gf1 the aberration will be one-fourth of that of a single lens1 but the focal length of the lens1 ,l#a1 must be half as much again as that of ,l4 ,if their focal lengths are equal the aberration will only be a little more than half reduced4 ,spherical aberration1 however1 may be entirely destroyed by combining a meniscus and double convex lens1 as shown in ,fig4 #gg1 the convex side being turned to the eye when used as a lens1 and to parallel rays when used as a burning glass or condenser4 99 99 99 99 99 #ada ,,index ,aberration1 #aci spherical1 #ach1 #adj ,accuracy of working1 #gj1 #gb ,acetylene gas lamps1 #abj ,actinic power1 #ajb ,actinograph1 #aje ,actinometer1 #abj ,alternating current1 #hb1 #ajj ,ammonia1 #abc ,angle of stylus1 #bd1 #gh ,aniline dye1 #abc ,arcing1 #bg1 #hb ,arc lamps1 #ae1 #abj1 #aba ,atmospherics1 #fa1 #he ,ballasting resistance1 #ajj ,belin1 #dg ,bernochi1 #g1 #aab system of1 #g1 #cd ,berzelius1 #aji ,bichromate of potash1 #abj ,blondel's oscillograph1 #dg ,camera obscura1 #acf extension1 #aaf1 #aah choice of1 #aag ,capacity of condenser1 #bd1 #gh electrostatic1 #c1 #e of cable1 #c of ,london-,paris telephone line1 #c ,carbon bisulphide1 #ec ,charbonelle1 #dh receiver of1 #dh ,chemical solution1 #ef ,circuit breaker1 #gf ,clutch1 details of1 #hh1 #hi1 #ia spring1 #ga ,coating the metal sheets1 #abj ,coherer1 #aa1 #dj ,collecting rings1 #ia ,commercial value of photo-telegraphy1 #a ,compensating selenium cell1 #aab ,contact breaker1 #cg ,copying arrangements1 #aah1 #abe ,cross screen1 #ba ,de' ,arsonval galvanometer1 #dg1 #gc ,decoherer1 #da ,design of machines1 #ba ,detectors1 #hc ,developing solutions1 #aje1 #abb ,diaphragm1 movement of1 #dh1 #eb1 #hd1 #hg ,dipping rods1 #ha1 #hc ,distance of transmission1 #cc ,duration of wave-trains1 #bb1 #be ,early experiments1 #b ,einthoven galvanometer1 #cb1 #dd1 #de1 #ed1 #aac ,electric clock1 #ic ,electrolytic receiver1 #d1 #cg1 #ed1 #fa1 #fd ,enlarging arrangements1 #abd1 #abe ,experimental machine1 #bj ,extraneous light1 #dg ,fastening electrolytic paper1 #eh ,fatigue of selenium cell1 #fd1 #aad ,fish glue1 #abj ,flexible couplings1 #gg ,frequency meter1 #fe ,friction brake1 #hh #adb ,high speed telegraphy1 #gj ,hughes governor1 #fe ,hughes printing telegraph1 #fc ,hurter and ,driffield1 #ajd ,hydrogen1 #ajj ,incidence1 angle of1 #abg ,inertia1 #fd1 #fe1 #aaa effects in photo-telegraphy1 #aaj method of counteracting1 #ajc1 #aab1 #aac effect of wave-length of light on1 #aad ,intensifying solution1 #abb ,isochroniser1 #hi1 #ia details of1 #ia1 #ib1 #ie ,isochronism1 #fd1 #fi1 #gj1 #ga ,kathode rays1 #ec ,knudsen1 #b apparatus of1 #i ,korn1 #cj1 #cc1 #de1 #fe1 #gb apparatus of1 #ca ,lamps1 coloured1 #id ,lenses1 #he1 #abe1 #abh principal focus of1 #acj conjugate foci of1 #aca action of1 #abi convex1 #abh1 #aca1 #acf concave1 #abh1 #ach focal length of1 #acj1 #ach aperture1 #ach meniscus1 #aci ,light1 diffusion of1 #hf extraneous1 #hg ,limit of error in synchronising1 #fd ,line balancer1 #c ,line screens1 #i1 #ae1 #af1 #aaf making1 #aaf ,magnifying power1 #acf1 #acg ,marconi valve1 #dd1 #ed coherer1 #dj ,mechanical inertia1 #cc ,mercury break1 #ha churning of1 #hb containers1 #hb ,mercury jet interrupter1 #bi ,metal prints1 #ae1 #ah1 #cb1 #ei1 #fd1 #ie1 #abj1 #abd drying the1 #aba1 #abc exposure of1 #aba size of1 #bb1 #bd1 #ge1 #gg pressing the1 #bb ,microscope1 #aca1 #acg ,military uses1 #ce ,mirror galvanometer1 #i1 #db1 #gc ,mirror1 #dg1 #ea ,morse code1 #ce ,motor speed1 #hi1 #ie driving1 #ia1 #ic1 #ie clockwork1 #fc electric1 #fc ,nernst lamps1 #dc1 #he1 #ih heater of1 #ii filament of1 #ii principle of1 #ih resistance of1 #ajj efficiency of1 #aja1 #ajb overrunning1 #aja ,nicol prism1 #ec ,paper for electrolytic receiver1 #ef ,parabolic reflector1 #h ,period of galvanometer1 #dc1 #dd1 #df ,photographic ,daily ,companion1 #aje ,photographic films1 #dj1 #dc1 #de1 #ec1 #ed1 #fb1 #he1 #hf1 #ih process1 #cg chemical inertia1 #ajc exposure of1 #ajc1 #ajg speed of1 #ajd1 #aje plates1 orthochromatic1 #ei plates1 #abj ,points to be observed in preparing metal prints1 #abc ,poulsen ,company1 #cb1 #dg arc1 #ca ,preparing selenium1 #aji photographs for transmitting1 #ae1 #aae sketches on metal foil1 #abd ,prism1 #abh action of1 #abi ,process plates1 #abb ,professor ,nernst1 #ih #adc ,radio-photography1 requirements of1 #gd ,refraction1 angle of1 #abg ,refractive power1 #abg ,relay1 #be1 #ci1 #di1 #ec1 #ee1 #fj1 #ge differential1 #gi polarised1 #ig working speed of1 #bf1 #ge ,reproducing for newspapers1 #fj ,resistance of selenium1 #aji of selenium cells1 #aaj regulating1 #aac ,retardation of current1 #f ,retouching1 #fb ,rotary spark-gap1 #bh ,selenium1 #ii cells1 #h1 #cd1 #ee1 #fj1 #fd1 #aji1 #aaj machines1 #de ,self-induction1 #bd1 #gh ,sensitiveness of selenium cells1 #aac ratio of1 #aac ,silvered quartz threads1 #dd1 #df ,spark-gap1 #bg ,speed regulator1 #fh adjustments of1 #fi ,spring clutch1 #ga ,starting position of machines1 #ih ,string galvanometer1 #cb ,stylus1 #ag1 #ah1 #eg1 #fa1 #gh1 #ie1 #ajc sparking at1 #bd ,stylus1 angle of1 #bd1 #gh defects of1 #eg ,submarine cable1 #d ,synchronism1 #aa1 #bj1 #cf1 #fd1 #fi1 #ga ,telephograph1 #gd advantages of1 #gf method of working1 #if ,telephone receiver1 #hc1 #he diaphragm1 #dh improved1 #ea ,telephone relay1 #dh1 #ej1 #eb1 #hc1 #he1 #ig ,telescope1 #aca1 #acf ,thermodetector1 #cb ,tow1 #hh ,transmission1 distance of1 #ce1 #gb speed of1 #be1 #ce1 #ge ,vibration1 natural period of1 #ci ,watkins1 #aje power number1 #aje ,waves1 damped1 #cj undamped1 #cj1 #ca ,wheatstone bridge1 #aac ,wireless apparatus1 #ac ,wireless ,world1 #ca ,wynne1 #aje ,zirconia1 #ii ,,the ,,end ,printed by ,r4 ,r4 ,,clark1 ,,limited1 ,edinburgh4 99 99 99 99 99 ,,publications ,,of ,,the ,,wireless ,,press1 ,,ltd4 #ab ,,and #ac ,,henrietta ,,street1 ,,strand1 ,,london1 ,w4,c4#b4 ,,the ,,year ,,book ,,of ,,wireless ,,telegraphy ,,and ,,telephony4 ,with ,map of the ,world1 showing ,wireless ,stations2 ,british1 ,colonial and foreign ,wireless ,laws and ,regulations4 ,price #aj,s4 #f;,d4 net4 7,,post ,,free1 #aa,s4 ,,inland2 #aa,s4 #d;,d4 ,,abroad47 ,,the ,,wireless ,,telegraphists' ,,pocket ,,book ,,of ,,notes1 ,,formulae ,,and ,,calculations4 ,by ,dr4 ,j4 ,a4 ,,fleming1 ,m4,a41 ,d4,sc41 ,f4,r4,s41 ,m4,inst4,e4,e41 etc4 ,a valuable compendium for ,wireless ,engineers and ,operators4 ,price #i,s4 net4 7,,postage #e;,d47 ,,the ,,handbook ,,of ,,technical ,,instruction ,,for ,,wireless ,,telegraphists4 ,by ,j4 ,c4 ,,hawkhead and ,h4 ,m4 ,,dowsett1 ,m4,i4,e4,e4 ,provides a complete theoretical course for the ,postmaster-,general's certificate of proficiency4 #caj pages4 #bdj ,diagrams and ,illustrations4 ,price #g,s4 net4 7,,postage #f;,d47 ,,manual ,,de ,,instruccion ,,tecnica ,,para ,,operadores ,,de ,,telegrafia ,,sin ,,hilos4 ,por ,j4 ,c4 ,,hawkhead y ,h4 ,m4 ,,dowsett1 ,m4,i4,e4,e4 ,precio3 ,espana1 #aj pesetas2 ,franqueo1 #a peseta extra4 ,america ,latina1 #b4be1 oro1 neto2 ,franqueo1 #be cents extra4 7,great ,britain1 #i,s42 ,,postage #f;,d47 ,,the ,,elementary ,,principles ,,of ,,wireless ,,telegraphy4 ,by ,r4 ,d4 ,,bangay4 ,in two ,parts4 ,price #c,s4 each4 7,,postage #d;,d47 ,or in one ,volume1 price #g,s4 net4 7,,postage #f;,d47 ,used by ,h4,m4 ,government for instructional purposes4 ,,principios ,,element ,,ales ,,de ,,telegrafia ,,sin ,,hilos4 ,por ,r4 ,d4 ,,bangay4 7,partes #a;a y #b;a en un ,volumen47 ,,precio3 ,espana1 #aj pesetas2 ,franqueo1 #a peseta extra4 ,america ,latina1 #b4be1 oro1 neto2 ,franqueo1 #be cents extra4 7,great ,britain1 #i,s42 ,,postage #f;,d47 ,,principes ,,elementaires ,,de ,,telegraphie ,,sans ,,fil4 ,par ,r4 ,d4 ,,bangay4 7,great ,britain1 #i,s42 ,,postage #f;,d47 ,,magnetism ,,and ,,electricity ,,for ,,home ,,study4 ,by ,h4 ,e4 ,,penrose4 ,crown #hvo4 ,over #ejj pages4 ,price #e,s4 net1 7,,postage #f;,d47 ,contains fifty complete lessons4 ,,the ,,calculation ,,and ,,measurement ,,of ,,inductance ,,and ,,capacity4 ,by ,w4 ,h4 ,,nottage1 ,b4,sc4 ,invaluable to all engaged in ,telegraph ,engineering4 ,indispensable to the ,wireless ,engineer1 ,student and ,experimenter4 ,price #c,s4 #f;,d4 net4 7,,postage #e;,d47 ,a ,,short ,,course ,,in ,,elementary ,,mathematics ,,and ,,their ,,application ,,to ,,wireless ,,telegraphy4 ,by ,s4 ,j4 ,,willis4 ,to ,students in ,wireless ,telegraphy1 as well as those engaged in the practical application of this ,science1 this book should prove of real value4 ,price #c,s4 #f;,d4 net4 7,,postage #f;,d47 ,,the ,,marconi ,,official ,,gramophone ,,records4 ,for self-tuition in receiving ,morse ,signals4 ,price #d,s4 each1 double-sided4 7,,postage #i;,d47 ,set of ,six ,records1 #bd,s4 post ,,free4 ,,the ,,maintenance ,,of ,,wireless ,,telegraph ,,apparatus4 ,by ,p4 ,w4 ,,harris4 ,an up-to-date ,manual1 full of practical hints and explanations4 ,diagrams of all ship installations1 from #a_/d kw4 to #e kw4 ,price #b,s4 #f;,d4 net4 7,,postage #d;,d47 ,,dictionary ,,of ,,technical ,,terms ,,used ,,in ,,wireless ,,telegraphy4 ,by ,,harold ,,ward4 ,vest ,pocket ,edition4 #bnd ,edition1 revised and enlarged4 ,contains over #aejj definitions4 ,price #b,s4 #f;,d4 net4 7,,postage #b;,d47 ,,armature ,,model ,,for #a-a_/b ,,kw4 ,,rotary ,,converter4 ,shows every ,winding of the ,converter ,armature from start to finish4 ,price #a,s4 net4 7,,postage #c;,d47 ,,morse ,,made ,,easy4 ,by ,a4 ,l4 ,,rye4 ,linen backed1 for rapidly learning the ,morse ,code4 ,price #c;,d4 net1 or post free #c-a_/b;,d4 ,,morse ,,code ,,card4 ,contains 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,,production4 ,book ,,iv4 ,,the #a-a_/b ,,kw4 ,,ship ,,set4 ,book ,v4 ,,the ,,oscillation ,,valve4 ,,the ,,oscillation ,,valve3 ,,the ,,elementary ,,principles ,,of ,,its ,,application ,,to ,,wireless ,,telegraphy4 ,by ,r4 ,d4 ,,bangay4 #bae pages4 ,price #e,s4 7,,postage #c;,d47 ,,the ,,thermionic ,,valve ,,and ,,its ,,developments ,,in ,,radio-,,telegraphy ,,and ,,telephony4 ,by ,dr4 ,j4 ,a4 ,,fleming1 ,m4,a41 ,d4,sc41 ,f4,r4,s41 ,m4,inst4,e4,e41 etc4 #bgi pages4 ,price #ae,s4 7,,postage #f;,d47 ,,alternating ,,current ,,work3 ,,an ,,outline ,,for ,,students ,,of ,,wireless ,,telegraphy4 ,by ,a4 ,,shore4 #afc pages4 ,price #c,s4 #f;,d4 7,,postage #d;,d47 ,,telephony ,,without ,,wires4 ,by ,,philip ,r4 ,,coursey1 ,b4,sc41 ,a4,m4,i4,e4,e41 ,f4,p4,s4,l4 #dad pages4 ,price #ae,s4 7,,postage #f;,d47 ,,the ,,wireless ,,world4 ,a ,monthly ,magazine devoted to ,wireless ,telegraphy and ,telephony4 ,price #i;,d4 7,,postage #c;,d47 ,annual ,subscription1 #aa,s4 post free4 ,,the ,,radio ,,review4 ,a ,monthly ,record of ,scientific ,progress in ,radio-telegraphy and ,telephony4 ,price #b,s4 #f;,d4 7,,postage #c;,d47 ,annual ,subscription1 #cj,s4 post free4 ,,conquest4 ,a ,popular ,illustrated ,monthly ,magazine dealing with ,science1 ,industry and ,invention4 ,price #a,s4 7,,postage #c;,d47 ,annual ,subscription1 #ae,s4 post free4 ,,continuous ,,wave ,,wireless ,,telegraphy4 ,part ,i4 ,by ,dr4 ,w4 ,h4 ,,eccles1 ,d4,sc41 ,a4,r4,c4,s41 ,m4,i4,e4,e4 ,in the ,press4 99 99 99 99 99 ,,complete ,,catalogue ,,post ,,free4 99 99 99 99 99 ,notes #a ,these measurements only apply to a single line4 ,where a double line is employed the capacity is halved4 #b ,see ,appendix ,a4 #c ,see ,appendix ,b4 #d ,in wireless telegraphy arcing is principally caused by the continuation of the supply current in the spark-gap after the capacity has been charged to a potential sufficient to break down the insulation of the gap4 #e ,see ,chapter ,v4 #f ,nernst lamps are the best to use1 as they produce abundantly the blue and violet rays which have the greatest chemical effect upon a photographic film4 ,carbon filament lamps are very poor in this respect4 #g ,a description of the apparatus required will be found in ,ganot's ,physics4 #h ,great care must be exercised in using this solution1 as it is exceedingly poisonous4 #i ,two clocks would isochronise if their hands travelled at precisely the same rate round the dials1 but would not synchronise unless they both registered the same time as well4 #aj ,line screens can be obtained from ,messrs4 ,penrose1 #aji ,farringdon ,street1 ,london2 or ,messrs4 ,fallowfield1 #adf ,charing ,cross ,road1 ,london4