,transcriber's ,notes3 ,underscores 80 before and after a word or phrase indicate italics in the original text4 ,carat symbol 80 designates a superscript4 ,a single underscore between two characters indicates a subscript4 ,equal signs 80 before and after a word or phrase indicate bold in the original text4 ,small capitals have been converted to ,,solid capitals4 ,illustrations have been moved so they do not break up paragraphs4 ,typographical errors have been silently corrected but other variations in spelling and punctuation remain unaltered4 ,,pyrometry ,,by ,,the ,,same ,,author ,,heat ,,for ,,engineers ,a ,,treatise ,,on ,,heat ,,with ,,special ,,regard ,,to ,,its ,,practical ,,applications ,third ,edition1 revised1 with #aaj illustrations1 xiv #dcj pp4 ,demy #hvo4 ,price #abs4 #f;d4 net4 ,,liquid ,,drops ,,and ,,globules ,,their ,,formation ,,and ,,movement ,,three ,,lectures ,,delivered ,,to ,,popular ,,audiences ,with #dc illustrations1 x #hd pp4 ,crown #hvo1 cloth4 ,price #cs4 net4 ,e4 ,f4 ,n4 ,,spon1 ,,ltd41 ,,haymarket1 ,,london1 ,s4,w4 #a ,,pyrometry ,a ,,practical ,,treatise ,,on ,,the ,,measurement ,,of ,,high ,,temperatures ,,by ,,chas4 ,r4 ,,darling ,,associate ,,of ,,the ,,royal ,,college ,,of ,,science1 ,,dublin2 ,,whitworth ,,exhibitioner2 ,,fellow ,,of ,,the ,,institute ,,of ,,chemistry2 ,,fellow ,,of ,,the ,,physical ,,society1 ,,etc4 ,,lecturer ,,in ,,physics ,,at ,,the ,,city ,,and ,,guilds ,,technical ,,college1 ,,finsbury1 ,e4,c4 ,,author ,,of 8,,heat ,,for ,,engineers0 ,,second ,,edition1 ,,revised ,,and ,,enlarged ,,sixty-,,nine ,,illustrations ,illustration ,london ,e4 ,f4 ,n4 ,,spon1 ,ltd41 #eg ,,haymarket1 ,s4,w4 #a ,new ,york ,,spon ,,chamberlain1 #abj ,,liberty ,,street #aibj ,contents ,,page ,,preface ,,to ,,second ,,edition ix ,,preface ,,to ,,first ,,edition xi ,,chap4 ,i4 ,,introduction #a ,,ii4 ,,standards ,,of ,,temperature #i ,absolute or ,thermodynamic ,scale--,constant ,volume ,gas ,thermometer--,fixed ,points for ,calibration--,national ,physical ,laboratory ,scale--,temperatures above the ,present ,limit of the ,gas ,thermometer4 ,,iii4 ,,thermo-,,electric ,,pyrometers #bj ,general ,principles--,metals used for ,thermal ,junctions-- ,changes in ,thermal ,junctions when constantly used--,electromotive ,force developed by ,typical ,junctions--,practical ,forms of ,thermocouples-- ,liquid ,element ,thermocouples--,indicators for ,thermo-electric ,pyrometers--,special ,features of ,indicators--,standardizingof ,indicators to read ,temperatures directly--,standardization by ,fixed ,points-- ,standardization by ,measurement of ,e4,m4,f4--,cold-,junction ,compensators--,constant ,temperature ,cold ,junctions-- ,special-,range ,indicators--,potentiometer,indicators-- ,recorders for ,thermo-electric ,pyrometers--,the ,thread ,recorder--,the ,siemens ,recorder--,foster's ,recorder-- ,paul's ,recorder--,the ,leeds-,northrup ,recorder-- ,control of ,furnace ,temperatures--,contact-,pen ,recorders-- ,installations of ,thermo-electric ,pyrometers--,management of ,thermo-electric ,pyrometers--,laboratory ,uses of ,temperatures ,thermo-electric ,pyrometers--,measurement of ,lower by the ,thermo-electric ,method--,measurement of ,surface of ,surface ,temperatures--,measurement of ,low ,temperatures-- ,temperature of ,steam1 ,exhaust ,gases--,measurement of ,differences of ,temperature--,advantages of the ,thermo-electric ,method of ,measuring ,temperatures4 ,,iv4 ,,resistance ,,pyrometers #aja ,general ,principles--,measurement of ,resistance by the ,differential ,galvanometer--,measurement of ,resistance by the ,wheatstone ,bridge--,relation between ,resistance of ,platinum and ,temperature--,changes in ,resistance of ,platinum when constantly ,heated--,terms used in ,resistance ,pyrometry--,practical ,forms of ,resistance ,pyrometers--,indicators--,siemens' ,indicator--,whipple's ,indicator--,the ,harris ,indicator-- ,the ,leeds-,northrup ,indicator--,siemens' ,differential ,indicator--,recorders for ,resistance ,pyrometers--,the ,leeds-,northrup ,recorder--,paul's ,recorder--,installation of ,resistance ,pyrometers--,management of ,resistance ,pyrometers--,special ,uses of ,resistance ,pyrometers4 ,v4 ,,radiation ,,pyrometers #acd ,general ,principles--,practical ,forms of ,radiation ,pyrometers--,fry's ,mirror ,pyrometer--,fry's ,spiral ,radiation ,pyrometer--,foster's ,fixed-,focus ,radiation ,pyrometer--,paul's ,radiation ,pyrometer--,indicators for ,radiation ,pyrometers--,calibration of ,indicators-- ,recorders--,management of ,radiation ,pyrometers--,special ,uses of ,radiation ,pyrometers4 ,,vi4 ,,optical ,,pyrometers #afg ,general ,principles--,wien's ,law--,practical ,forms of ,optical ,pyrometers--,fry's ,optical ,pyrometer--,le ,chatelier's ,optical ,pyrometer--,wanner's ,pyrometer-- ,cambridge ,optical ,pyrometer--,holborn-,kurlbaum ,pyrometer--,lovibond's ,pyrometer--,mesur and ,nouel's ,pyrometer--,colour-extinction ,pyrometers-- ,management of ,optical ,pyrometers--,special ,uses of ,optical ,pyrometers4 ,,vii4 ,,calorimetric ,,pyrometers #aie ,general ,principles--,practical ,forms--,siemens' ,calorimetric or 8,water0 ,pyrometer--,special ,uses4 ,,viii4 ,,fusion ,,pyrometers #bjd ,general ,principles--,seger ,pyramids or 8,cones0-- ,watkin's ,heat ,recorder--8,sentinel0 ,pyrometers-- ,stone's ,pyrometer--,fusible ,metals--,fusible ,pastes4 ,,ix4 ,,miscellaneous ,,appliances #baa ,expansion and ,contraction ,pyrometers--,wedgwood's ,pyrometer--,daniell's ,pyrometer--,northrup's ,molten ,tin ,pyrometer--,vapour-,pressure ,pyrometers--,water-,jet ,pyrometers--,pneumatic ,pyrometers--,conduction ,pyrometers--,gas ,pyrometers--,wiborgh's ,thermophones-- ,joly's ,meldometer--,brearley's ,curve ,tracer4 ,,index #bbb ,preface to the ,second ,edition ,since the publication of the first edition in #aiaa1 a great extension has been witnessed in the use of pyrometers in industrial processes and laboratory work1 to which development the author hopes his book has contributed in some measure4 ,during the stress occasioned by the war1 pyrometers have proved invaluable in many processes1 and ,british makers were fully able to meet the demands1 owing to the status attained in pre-war days4 ,the increasing use of pyrometric appliances renders necessary some book of reference which will provide the user with information to enable him to get the best results out of his instruments1 and it is hoped that the present treatise meets this need4 ,in preparing the second edition1 certain parts have been revised in conformity with modern practice1 and the later developments included4 ,the scope of the book remains as before4 ,the author desires to acknowledge the assistance he has received from the ,british makers of pyrometers1 all of whom have liberally provided him with information of a most useful kind1 of which he has availed himself in the production of the present edition4 ,,chas4 ,r4 ,,darling4 ,,woolwich1 #aibj4 ,preface to the ,first ,edition ,the present treatise has been founded on a course of ,cantor ,lectures on 8,industrial ,pyrometry10 delivered by the author before the ,royal ,society of ,arts in the autumn of #aiaj4 ,the practice of pyrometry in recent years has progressed at a greater rate than the literature bearing upon it2 and the author is not aware of the existence of any other book written in ,english which treats the subject from the standpoint of the actual daily use of the instruments4 ,in the succeeding pages the exact measurement of temperature1 as an end in itself1 is made subordinate to the practical utility of pyrometers in controlling various operations2 and consequently descriptions of appliances of interest only theoretically have either been omitted1 or at the most briefly described4 ,nevertheless1 the fundamental principles are in all cases fully explained1 as an understanding of these is essential to the intelligent use of the appliances dealt with in the book4 ,when necessary1 numerical examples are given to illustrate the applications of the principles2 and the reader who finds any difficulty in following the various explanations--which of necessity involve an understanding of many portions of the subject of heat--is referred to the author's treatise on 8,heat for ,engineers10 issued by the publishers of the present volume4 ,with regard to temperature scales1 the author has in the main employed ,centigrade degrees1 but has recognised that the ,fahrenheit degree is still largely used1 and has therefore frequently expressed temperatures in terms of both scales4 ,the number of those who find it an advantage in their calling to measure and control high temperatures is constantly increasing2 and the manufacture of pyrometric appliances now gives employment to considerable numbers4 ,the author trusts that the present treatise will prove of service to all thus concerned1 and also to those who pursue the fascinating study of high temperature measurement from the purely scientific standpoint4 ,in conclusion1 the author expresses his thanks to the various firms1 mentioned in the text1 who have loaned blocks for the purpose of illustration1 and who have furnished him with much valuable information4 ,,chas4 ,r4 ,,darling4 ,,woolwich1 #aibj4 ,,pyrometry ,,chapter ,i ,,introduction ,the term 8pyrometer0--formerly applied to instruments designed to measure the expansion of solids--is now used to describe any device for determining temperatures beyond the upper limit of a mercury thermometer4 ,this limit1 in the common form1 is the boiling point of mercury3 #ceg ,c4 or #fgb ,f4 ,by leaving the bore of the tube full of nitrogen or carbon dioxide prior to sealing1 the pressure exerted by the enclosed gas when the mercury expands prevents boiling2 and with a strong bulb of hard glass the readings may be extended to #eej ,c4 or #ajbj ,f4 ,above this temperature the hardest glass is distorted by the high internal pressure1 but1 by substituting silica for glass1 readings as high as #gjj ,c4 or #abij ,f4 may be secured4 ,whilst such thermometers are useful in laboratory processes they are too fragile for workshop use2 and if made of the length necessary in many cases in which the temperature of furnaces is sought1 the cost would be as great as that of more durable and convenient appliances4 ,no other instrument1 however1 is so simple to read as the thermometer2 and for this reason it is used whenever the conditions are favourable4 ,the latest proposal in this direction is due to ,northrup1 who has constructed a thermometer containing tin enclosed in a graphite envelope1 which is capable of reading up to #aejj ,c4 or higher4 ,this instrument is described on page #baf4 ,the origin and development of the science of pyrometry furnish a notable example of the value of the application of scientific principles to industry4 ,sir ,isaac ,newton was the first to attempt to measure the temperature of a fire by observing the time taken to cool by a bar of iron withdrawn from the fire2 but1 although ,newton's results were published in #agja1 it was not until #aghb that a practical instrument for measuring high temperatures was designed4 ,in that year ,josiah ,wedgwood1 the famous potter1 introduced an instrument based on the progressive contraction undergone by clay when baked at increasing temperatures1 which he used in controlling his furnaces1 finding it much more reliable than the eye of the most experienced workman4 ,this apparatus 7described on page #baa7 remained without a serious rival for forty years1 and its use has not yet been entirely abandoned4 ,the next step in advance was the introduction of the expansion pyrometer by ,john ,daniell in #ahbb4 ,the elongation of a platinum rod1 encased in plumbago1 was made to operate a magnifying device1 which moved a pointer over a scale divided so as to read temperatures directly4 ,although inaccurate as compared with modern instruments1 this pyrometer was the first to give a continuous reading1 and required no personal attention4 ,the expansion pyrometer--with different expanding substances--is still used to a limited extent4 ,the year #ahbb was also marked by ,seebeck's discovery of thermo-electricity4 ,the generation of a current of electricity by a heated junction of two metals1 increasing with the temperature1 appeared to afford a simple and satisfactory basis for a pyrometer1 and ,becquerel constructed an instrument on these lines in #ahbf4 ,pouillet and others also endeavoured to measure temperatures by the thermo-electric method1 but partly owing to the use of unsuitable junctions1 and partly to the lack of reliable galvanometers1 these workers failed to obtain concordant results4 ,the method was for all practical purposes abandoned until #ahhf1 when its revival in reliable form led to the enormous extension of the use of pyrometers witnessed during recent years4 ,in #ahbh ,prinsep initiated the use of gas pyrometers1 and enclosed the gas in a gold bulb4 ,later workers used porcelain bulbs1 on account of greater infusibility1 but modern research has shown that porcelain is quite unsuitable for accurate measurements1 being porous to certain gases at high temperatures1 even when glazed4 ,gas pyrometers are of little use industrially1 but are now used as standards for the calibration of other pyrometers1 the bulb being made of an alloy of platinum and rhodium4 ,calorimetric pyrometers1 based on ,regnault's 8method of mixtures10 were first made for industrial purposes by ,bystrm1 who patented an instrument of this type in #ahfb4 ,this method has been widely applied1 and a simplified form of 8water0 pyrometer1 made by ,siemens1 is at present in daily use for industrial purposes4 ,it is not capable1 however1 of giving results of the degree of accuracy demanded by many modern processes4 ,the resistance pyrometer was first described by ,sir ,w4 ,siemens in #ahga1 and was made by him for everyday use in furnaces4 ,many difficulties were encountered before this method was placed on a satisfactory footing1 but continuous investigation by the firm of ,siemens ,co41 and also the valuable researches of ,callendar and ,griffiths1 have resulted in the production of reliable resistance pyrometers1 which are extensively used at the present time4 ,in #ahgb ,sir ,william ,barrett made a discovery which indirectly led to the present development of the science of pyrometry4 ,barrett observed that iron and steel1 on cooling down from a white heat1 suddenly became hotter at a definite point1 owing to an internal molecular change2 and gave the name of 8recalescence0 to the phenomenon4 ,workers in steel subsequently discovered that this property was intimately connected with the hardening of the metal2 thus ,hadfield noticed that a sample of steel containing #aaf per cent4 of carbon1 when quenched just below the change-point was not hardened1 but when treated similarly at #ae ,c4 higher it became totally hard4 ,the demand for accurate pyrometers in the steel industry followed immediately on these discoveries1 for even the best-trained workman could not detect with the eye a difference in temperature so small1 and yet productive of such profound modification of the properties of the finished steel4 ,in this instance1 as in many others1 the instruments were forthcoming to meet the demand4 ,the researches of ,le ,chatelier1 published in #ahhf1 marked a great advance in the progress of pyrometry4 ,he discovered that a thermo-electric pyrometer1 satisfactory in all respects1 could be made by using a junction of pure platinum with a rhodioplatinum alloy1 containing #aj per cent4 of rhodium2 a d',arsonval moving-coil galvanometer being used as indicator4 ,this type of galvanometer1 which permits of an evenly-divided scale1 is now universally employed for this purpose1 and has made thermo-electric pyrometers not only practicable1 but more convenient for general purposes than any other type4 ,continuous progress has since been made in connection with this method1 which is now more extensively used than any other4 ,attempts to deduce temperature from the luminosity of the heated body were first made by ,ed4 ,becquerel in #ahfc1 but the method was not successfully developed until #ahib1 when ,le ,chatelier introduced his optical pyrometer4 ,this instrument1 being entirely external to the hot source1 enabled readings to be taken at temperatures far beyond the melting point of platinum1 which would obviously be the extreme limit of a pyrometer in which platinum was used4 ,the quantitative distribution of energy in the spectrum has since been worked out by ,wien and ,planck1 who have furnished formula based on thermodynamic reasoning1 by the use of which optical pyrometers may now be calibrated in terms of the thermodynamic scale of temperature4 ,other optical pyrometers1 referred to in the text1 have been devised by ,wanner1 ,holborn and ,kurlbaum1 ,fry1 and others2 and the highest attainable temperatures can now be measured satisfactorily by optical means4 ,the invention of the total-radiation pyrometer by ,fry in #aijb added another valuable instrument to those already available4 ,based on the fourth-power radiation law1 discovered by ,stefan and confirmed by the mathematical investigations of ,boltzmann1 this pyrometer is of great service in industrial operations at very high temperatures1 being entirely external1 and capable of giving permanent records4 ,modifications have been introduced by ,foster and others1 and the method is now widely applied4 ,recorders1 for obtaining permanent evidence of the temperature of a furnace at any time1 were first made for thermo-electric pyrometers by ,holden and ,roberts-,austen1 and for resistance pyrometers by ,callendar4 ,numerous forms are now in use1 and the value of the records obtained has been abundantly proved4 ,for scientific purposes1 all pyrometers are made to indicate ,centigrade degrees1 #ajj of which represent the temperature interval between the melting-point of ice and the boiling point of water at #gfj mm4 pressure1 the ice-point being marked #j and the steam-point #ajj4 ,in industrial life1 however1 the ,fahrenheit scale is often used in ,english-speaking countries1 the ice-point in this case being numbered #cb and the steam-point #bab2 the interval being #ahj4 ,a single degree on the ,centigrade scale is therefore #ah times as large as a ,fahrenheit degree1 but in finding the numbers on each scale which designate a given temperature1 the difference in the zero position on the two scales must be taken into account4 ,when it is desired to translate readings on one scale into the corresponding numbers on the other1 the following formula may be used3-- 7,c4 reading7 7,f4 reading - #cb7 ------------------------ ---------------------------------- 4 #e #i ,thus by substituting in the above expression1 #ffj ,c4 will be found to correspond to #abbj ,f4 and #aecj ,f4 to #hcb ,c4 ,it is greatly to be regretted that all pyrometers are not made to indicate in ,centigrade degrees1 as confusion often arises through the use of the two scales4 ,an agreement on this point between instrument makers would overcome the difficulty at once1 as the ,centigrade scale is now so widely used that few purchasers would insist on ,fahrenheit markings4 ,it may be pointed out here that no single pyrometer is suited to every purpose1 and the choice of an instrument must be decided by the nature of the work in hand4 ,a pyrometer requiring skilled attention should not be entrusted to an untrained man2 and it may be taken for granted that to obtain the most useful results intelligent supervision is necessary4 ,in the ensuing pages the advantages and drawbacks of each type will be considered2 but in all cases it is desirable1 before making any large outlay on pyrometers1 to obtain a competent and impartial opinion as to the kind best suited to the processes to be controlled4 ,catalogue descriptions are not always trustworthy1 and instances are not wanting in which a large sum has been expended on instruments which1 owing to wrong choice1 have proved practically useless4 ,an instrument suited to laboratory measurements is often a failure in the workshop1 and all possibilities of this kind should be considered before deciding upon the type of pyrometer to be used4 ,,chapter ,,ii ,,standards ,,of ,,temperature ,the ,absolute or ,thermodynamic ,scale of ,temperatures4--,all practical instruments for measuring temperatures are based on some progressive physical change on the part of a substance or substances4 ,in a mercury thermometer1 the alteration in the volume of the liquid is used as a measure of hotness2 and similarly the change in volume or pressure on the part of a gas1 or the variation in resistance to electricity shown by a metal1 and many other physical changes1 may be employed for this purpose4 ,in connection with the measurement of high temperatures1 many different physical principles are relied upon in the various instruments in use1 and it is of the greatest importance that all should read alike under the same conditions4 ,this result would not be attained if each instrument were judged by its own performances4 ,in the case of a mercury thermometer1 for example1 we may indicate the amount of expansion between the temperatures of ice and steam at #gf centimetres pressure1 representing #ajj ,centigrade1 by a2 and then assume that an expansion of #b;a will signify a temperature of #bjj1 and so on in proportion4 ,similarly1 we may find the increase in resistance manifested by platinum between the same two fixed points1 and indicate it by r1 and then assume that an increase of #br will correspond to #bjj4 ,if now we compare the two instruments1 we find that they do not agree1 for on placing both in a space in which the platinum instrument registered #bjj1 the mercury thermometer would show #bjc4 ,a similar1 or even greater1 discrepancy would be observed if other physical changes were relied upon to furnish temperature scales on these lines1 and it is therefore highly desirable that a standard independent of any physical property of matter should be used4 ,such a standard is to be found in the thermodynamic scale of temperatures1 originally suggested by ,lord ,kelvin4 ,this scale is based upon the conversion of heat into work in a heat engine1 a process which is independent of the nature of the medium used4 ,a temperature scale founded on this conversion is therefore not connected with any physical property of matter1 and furnishes a standard of reference to which all practical appliances for measuring temperatures may be compared4#a ,when readings are expressed in terms of this scale1 it is customary to use the letter ,k in conjunction with the number3 thus #hej ,k would mean #hej degrees on the thermodynamic scale4 ,when existing instruments are compared with this standard1 it is found that a scale based on the assumption that the volume of a gas free to expand1 or the pressure of a confined gas1 increases directly as the temperature is in close agreement with the thermodynamic scale4 ,it may be proved that if the gas employed were 8perfect10 a scale in exact conformity with the standard described would be secured2 and gases which approach nearest in properties to a perfect gas1 such as hydrogen1 nitrogen1 and air1 may therefore be used to produce a practical standard1 the indications of which are nearly identical with the thermodynamic scale4 ,if any other physical change be chosen1 such as the expansion of a solid1 or the increase in resistance of a metal1 and a temperature scale be based on the supposition that the change in question varies directly as the temperature1 the results obtained would differ considerably from the absolute standard4 ,for this reason the practical standard of temperature now universally adopted is an instrument based on the properties of a suitable gas4 ,the ,constant ,volume ,gas ,thermometer4--,in applying the properties of a gas to practical temperature measurement1 we may devise some means of determining the increase in volume when the gas is allowed to expand1 or the increase in pressure of a confined gas may be observed4 ,the latter procedure is more convenient in practice1 and the instrument used for this purpose is known as the constant volume gas thermometer1 one form of which is shown in fig4 #a4 ,the gas is enclosed in a bulb ,b1 connected to a tube bent into a parallel branch1 into the bend of which is sealed a tap ,c1 furnished with a drying cup4 ,the extremity of the parallel branch is connected to a piece of flexible tubing ,t1 which communicates with a mercury cistern which may be moved over a scale1 the rod ,g serving as a guide4 ,in using this instrument the bulb ,b is immersed in ice1 and the tap ,c opened4 ,when the temperature has fallen to #j ,c41 the mercury is brought to the mark ,a by adjusting the cistern1 and the tap ,c then closed4 ,the bulb ,b is now placed in the space or medium of which the temperature is to be determined1 and expansion prevented by raising the cistern so as to keep the mercury at ,a4 ,when steady1 the height of the mercury in the cistern above the level of ,a is read off1 and furnishes a clue to the temperature of ,b4 ,if the coefficient of pressure of the gas used 7in this case1 air7 be known1 the temperature may be calculated from the equation ,p#a ,p#j7a bt71 where ,p#a is the pressure at t2 ,p#j the pressure at #j2 and b the coefficient of pressure2 that is1 the increase in unit pressure at #j for a rise in temperature of #a4 ,thus if ,p#j #gf cms42 b #jjjcfg2 height of mercury in cistern above ,a #eeh cms42 then ,p#a 7#gf #eeh7 #acah cms41 and by inserting these values in the above equation t is found to be #bjj4 ,in the instrument described1 ,p#j is equal to the height of the barometer1 since the tap ,c is open whilst the bulb is immersed in ice4 ,the coefficient of pressure may be determined by placing the bulb in steam at a known temperature1 and noting the increased pressure4 ,in the equation given1 ,p#a1 ,p#j1 and t are then known1 and the value of b may be calculated4 ,illustration3 ,,fig4 #a4--;,,constant ,,volume ,,air ,,thermometer4 ,in using this instrument for exact determinations of temperature1 allowance must be made for the expansion of the bulb1 which causes a lower pressure to be registered than would be noted if the bulb were non-expansive4 ,again1 the gas in the connecting tube is not at the same temperature as that in the bulb2 an error which may be practically eliminated by making the bulb large and the bore of the tube small4 ,the temperature of the mercury column must also be allowed for1 as the density varies with the temperature4 ,when the various corrections have been made1 readings of great accuracy may be secured4 ,when applied to the measurement of high temperatures1 the bulb must be made of a more infusible material than glass4 ,gold1 porcelain1 platinum1 and quartz have been used by different investigators1 but the most reliable material for temperatures exceeding #ijj ,c4 has been found to be an alloy of platinum with #bj per cent4 of rhodium4 ,the most suitable gas to use inside the bulb is nitrogen1 which is chemically inert towards the materials of the bulb1 and is not absorbed by the metals mechanically4 ,when measuring high temperatures with this instrument1 a considerable pressure1 amounting to #a atmosphere for every increase of #bgc degrees above the ice point1 is requisite to prevent expansion of the nitrogen2 and this pressure tends to distort the bulb and so to falsify the indications4 ,this trouble has been overcome by ,day1 who surrounded the bulb by a second larger bulb1 and forced air or nitrogen into the intervening space until the pressure on the exterior of the thermometer bulb was equal to that prevailing in the interior4 ,even then it was not found possible to secure higher readings than #aeej ,c41 as the bulb commenced to alter in shape owing to the softening of the material4 ,this temperature represents the highest yet measured on the gas scale2 but by using a more refractory material1 such as fused zirconia1 it may be found possible to extend this range to #bjjj ,c4 or more4 ,experiments in this direction are very desirable1 in order that high-reading pyrometers may be checked directly against the gas scale4 ,fixed ,points for ,calibration of ,pyrometers4--,it is evident that the gas thermometer is totally unsuited for use in workshops or laboratories when a rapid determination of a high temperature is required4 ,its function is to establish fixed points or temperature standards1 by means of which other instruments1 more convenient to use1 may be graduated so as to agree with each other and with the gas scale itself4 ,the temperature scales of all modern pyrometers are thus derived1 directly or indirectly1 from the gas thermometer4 ,in the table on next page1 a number of fixed points1 determined by various observers1 is given2 the error1 even at the highest temperatures1 probably not exceeding #b ,c4 ,in preparing the temperature scale of a pyrometer for practical use1 the instrument is subjected successively to a number of the temperatures indicated in the table1 and in this manner several fixed points are established on its scale4 ,the space between these points is then suitably subdivided to represent intermediate temperatures4 ,,table ,,of ,,fixed ,,points4 ,substance4 ,physical ,condition4 ,deg4 ,deg4 ,cent4 ,fahr4 ,water 7ice7 ,at ,melting ,point #j #cb ,water 0 ,boiling 0 #ajj #bab ,aniline 0 0 0 #ahd #cfc ,naphthalene 0 0 0 #bah #dbd ,tin 0 ,melting 0 #bcb #ddi ,lead 0 0 0 #cbg #fbj ,zinc 0 0 0 #dai #ghf ,sulphur 0 ,boiling 0 #dde #hcc ,antimony 0 ,melting 0 #fca #aafg ,aluminium 0 0 0 #feg #abad ,common ,salt 0 0 0 #hjj #adgb ,silver 7in air7 0 0 0 #iee #agea ,silver 7free from oxygen7 0 0 0 #ifb #agfc ,gold 0 0 0 #ajfd #aidg ,copper 7in air7 0 0 0 #ajfd #aidg ,copper 7,graphite covered7 0 0 0 #ajhd #aihc ,iron 7pure7 0 0 0 #aebj #bgfh ,palladium 0 0 0 #aedi #bhbj ,platinum 0 0 0 #agee #caij ,it is necessary to point out that the figures given in the table refer only to pure substances1 and that relatively small quantities of impurities may give rise to serious errors4 ,the methods by which the physical condition to which the temperatures refer may be realised in practice will be described in the succeeding chapter4 ,national ,physical ,laboratory ,scale4--,exact agreement with regard to fixed points has not yet been arrived at in different countries1 and an effort to co-ordinate the work of the ,national ,physical ,laboratory1 the ,united ,states ,bureau of ,standards1 and the ,reichsanstalt1 with a view to the formation of an international scale1 was interrupted by the war4 ,in #aiaf the ,national ,physical ,laboratory adopted a set of fixed points on the ,centigrade thermodynamic scale1 in conformity with which all ,british pyrometers have since been standardised4 ,it will be seen that the figures differ very slightly from those given in the previous table1 which represent the average results of separate determinations in different countries4 ,,national ,,physical ,,laboratory ,,scale 7#aiaf74 ,substance4 ,physical ,condition4 ,deg4 ,deg4 ,cent4 ,fahr4 ,water 7ice7 ,at ,melting ,point #j #cb ,water 0 ,boiling 0 7#gfj mm47 #ajj #bab ,naphthalene 0 0 0 0 #bagi #dbd ,benzophenone 0 0 0 0 #cjei #ehb ,zinc 0 ,melting 0 #daid #ghg ,antimony 0 0 0 #fcj #aaff ,common ,salt 0 0 0 #hja #adgd ,silver 7in reducing atmosphere7 0 0 0 #ifa #agfa ,gold 0 0 0 #ajfc #aide ,copper 7in reducing atmosphere7 0 0 0 #ajhc #aihb ,for higher temperatures the melting points of nickel 7#adeb ,c47 and palladium 7#aedi ,c47 are employed1 but the accuracy in these cases is not so certain as with the substances named in the table4 ,a useful point1 intermediate between copper and nickel1 has been established by ,e4 ,griffiths1 and is obtained by heating nickel with an excess of graphite1 when a well-defined eutectic is formed which freezes at #accj ,c41 or #bdbf ,f4 ,temperatures above the ,present ,limit of the ,gas ,thermometer4--,as it is not yet possible to compare an instrument directly with the gas thermometer above #aeej ,c41 all higher temperatures must be arrived at by a process of extrapolation4 ,by careful observation of a physical change at temperatures up to the limit of #aeej ,c41 the law governing such change may be discovered2 and assuming the law to hold indefinitely1 higher temperatures may be deduced by calculation4 ,an amount of uncertainty always attaches to this procedure1 and in the past some ludicrous figures have been given as the result of indefinite extrapolation4 ,wedgwood1 for example1 by assuming the uniform contraction of clay1 gave #abjja ,c41 or #bafcg ,f41 as the melting point of wrought iron1 whereas the correct figure is #aebj ,c41 according to the gas scale4 ,even in recent times1 the extrapolation of the law connecting the temperature of a thermal junction with the electromotive force developed1 obtained by comparison with the gas scale up to #aajj ,c41 led ,harker to the conclusion that the melting point of platinum was #agaj ,c41 a figure #de degrees lower than that now accepted4 ,the laws governing the radiation of energy at different temperatures1 however1 appear to be capable of mathematical proof from thermodynamic principles1 and temperatures derived from these laws are in reality expressed on the absolute or thermodynamic scale4 ,extrapolation of these laws1 when used to deduce temperatures by means of radiation pyrometers1 appears to be justified2 but it is still desirable to extend the gas scale as far as possible to check such instruments4 ,assuming the radiation laws to hold1 it is possible to determine the highest temperatures procurable1 such as that of the electric arc1 with a reasonable degree of certainty4 #a ,for a fuller account of the thermodynamic scale1 see the author's treatise ,heat for ,engineers1 pp4 #cia-b4 ,,chapter ,,iii ,,thermo-,,electric ,,pyrometers ,general ,principles4--,seebeck1 in #ahbb1 made the discovery that when a junction of two dissimilar metals is heated an electromotive force is set up at the junction1 which gives rise to a current of electricity when the heated junction forms part of a closed circuit4 ,becquerel1 in #ahbf1 attempted to apply this discovery to the measurement of high temperatures1 it having been observed that in general the ,e4,m4,f4 increased as the temperature of the junction was raised4 ,no concordant results were obtained1 and the same fate befell the investigations of others who subsequently attempted to produce pyrometers based on the ,seebeck effect4 ,these failures were due to several causes1 but chiefly to the non-existence of reliable galvanometers1 such as we now possess4 ,it was not until #ahhf that the problem was satisfactorily solved by ,le ,chatelier of ,paris4 ,although any heated junction of metals will give rise to an electromotive force1 it does not follow that any pair1 taken at random1 will be suited to the purposes of a pyrometer4 ,a junction of iron and copper1 for example1 gives rise to an ,e4,m4,f4 which increases with the temperature up to a certain point1 beyond which the ,e4,m4,f4 falls off although the temperature rises1 and finally reverses in direction--a phenomenon to which the name of 8thermo-electric inversion0 has been applied4 ,evidently1 it would be impossible to measure temperatures in this case from observations of the electromotive force produced1 and any couple chosen must be free from this deterrent property4 ,moreover1 the metals used must not undergo deterioration1 or alteration in thermo-electric properties1 when subjected for a prolonged period to the temperature it is desired to measure4 ,these and other considerations greatly restrict the choice of a suitable pair of metals1 which1 to give satisfaction1 should conform to the following conditions3-- #a4 ,the ,e4,m4,f4 developed by the junction should increase uniformly as the temperature rises4 #b4 ,the melting point of either component should be well above the highest temperature to be measured4 ,an exception to this rule occurs when the ,e4,m4,f4 of fused materials is employed4 #c4 ,the thermo-electric value of the couple should not be altered by prolonged heating4 #d4 ,the metals should be capable of being drawn into homogeneous wires1 so that a junction1 wherever formed1 may always give rise to the same ,e4,m4,f4 under given conditions4 ,it is a further advantage if the metals which fulfil the above conditions are cheap and durable4 ,the exacting character of these requirements delayed the production of a reliable thermo-electric pyrometer until #ahhf1 when ,le ,chatelier discovered that a junction formed of platinum as one metal1 and an alloy of #ij per cent4 of platinum and #aj per cent4 of rhodium as the other1 gave concordant results4 ,in measuring the ,e4,m4,f4 produced1 ,le ,chatelier took advantage of the moving-coil galvanometer introduced by d',arsonval1 which possessed the advantages of an evenly-divided scale and a dead-beat action4 ,this happy combination of a suitable junction with a simple and satisfactory indicator immediately established the reliability of the thermo-electric method of measuring temperatures4 ,as platinum melts at #agee ,c41 and the rhodium alloy at a still higher temperature1 a means was thus provided of controlling most of the industrial operations carried out in furnaces4 ,so far1 the effect of heating the junction has been considered without regard to the temperature of the remainder of the circuit1 and it is necessary1 before describing the construction of practical instruments1 to consider the laws governing the thermo-electric circuit1 the simplest form of which is represented in fig4 #b4 ,one of the wires is connected at both ends to separate pieces of the other wire1 the free ends of which are taken to the galvanometer ,two junctions1 ,a and ,b1 are thus formed1 which evidently act in opposition2 for if on heating ,a the direction of current be from ,a to ,b1 then on heating ,b the direction will be from ,b to ,a4 ,hence if ,a and ,b were equally heated no current would flow in the circuit1 the arrangement being equivalent to two cells of equal ,e4,m4,f4 in opposition4 ,thermal junctions are formed at each of the galvanometer terminals1 but the currents to which they give rise1 when the temperature changes1 are opposed and cancel each other4 ,the law which holds for this circuit may be expressed thus3-- 8,if in a thermo-electric circuit there be two junctions1 ,a and ,b1 the electromotive force developed is proportional to the difference in temperature between ,a and ,b40 ,illustration3 ,,fig4 #b4--,,two-,,junction ,,thermo-,,electric ,,circuit4 ,it is customary to refer to the two junctions as the 8hot0 and 8cold0 junctions2 but it is important to remember that fluctuations in the temperature of either will alter the reading on the galvanometer or indicator4 ,a second law1 which applies to all thermo-electric circuits1 is that 8the ,e4,m4,f4 developed is independent of the thickness of the wire40 ,this does not mean that the deflection of the galvanometer is the same whether thin or thick wires are used to form the junction4 ,the deflection depends upon the current flowing through the circuit1 and this1 according to ,ohm's law1 varies inversely as the total resistance of the circuit4 ,consequently1 the use of thin wires of a given kind will tend to give a less deflection than in the case of thick wires1 as the resistance of the former will be greater1 and unless the resistance of the galvanometer be great compared with that of the junction1 the difference in deflection will be conspicuous4 ,the ,e4,m4,f41 however1 is the same under given conditions1 whatever thickness of wire be used4 ,reference to fig4 #b will show that in order to realise this circuit in practice1 one of the wires forming the couple must be used in the form of leads to the galvanometer4 ,this can readily be done if the material of the wire is cheap2 but if platinum or other expensive metal be used1 and the galvanometer be some yards distant1 the question of cost necessitates a compromise1 and the circuit is then arranged as in fig4 #c4 ,the wires forming the hot junction are brought to brass terminals ,t ,t1 from which copper wires lead to the galvanometer ,g4 ,this arrangement results in three effective junctions1 viz4 the hot junction ,a to ,b2 the junction ,a to brass1 and the junction ,b to brass4 ,it will be seen that the two junctions of copper to brass are in opposition1 and cancel each other for equal heating2 and the same applies to the galvanometer connections4 ,a circuit thus composed of three separate junctions does not permit of a simple expression for the net ,e4,m4,f4 under varying temperature conditions1 and to avoid errors in readings care must be taken to prevent any notable change of temperature at the terminals ,t ,t in a practical instrument arranged as in the diagram4 ,illustration3 ,,fig4 #c4--,,three-,,junction ,,thermo-,,electric ,,circuit4 ,a point of practical utility in thermo-electric work is the fact that if a wire be interrupted by a length of other metal1 as indicated at ,c in fig4 #c1 no current will be set up in a circuit if both joints are equally heated1 as the electromotive forces generated at each junction are in opposition4 ,it is thus possible to interrupt a circuit by a plug-key or switch1 without introducing an error2 always provided that an even temperature prevails over the region containing the joints4 ,another useful fact is that if two wires be brought into contact1 they may be fastened over the joint by soldering or using a third metal1 without alteration of thermo-electric value1 except in rare cases4 ,thus a copper-constantan or iron-constantan junction may suitably be united by silver solder1 using borax as a flux1 thus avoiding the uncertainty of contact which must always occur when the wires are merely twisted together4 ,welding1 however1 is preferable to soldering4 ,metals used for ,thermal ,junctions4--,until recent years it was customary to employ a platinum-rhodioplatinum or platinum-iridioplatinum junction for all temperatures beyond the scope of the mercury thermometer4 ,the almost prohibitive price of these metals has caused investigations to be made with a view to discovering cheaper substitutes1 with successful results up to #ajjj ,c4 or #ahjj ,f41 thus comprehending the range of temperatures employed in many industrial processes4 ,above this temperature the platinum series of metals are still used for accurate working1 but it will be of great advantage if the range measurable by cheap or 8base0 metals can be further extended4 ,promise in this direction is afforded by the properties of fused metals when used in thermal junctions4 ,an investigation by the author has shown that in general the ,e4,m4,f4 developed by a junction does not undergo any sudden change when one or both metals melt1 but continues as if fusion had not occurred4 ,by making arrangements to maintain the continuity of the circuit after fusion1 it may be possible to read temperatures approximating to the boiling points of metals such as copper and tin1 both of which are over #bjjj ,c4 ,the base metals are not so durable as platinum and kindred metals1 but as the cost of replacement is negligible1 this drawback is of little importance4 ,moreover1 base-metal junctions usually develop a much higher ,e4,m4,f4 than the platinum metals1 which enables stronger and cheaper galvanometers to be used as indicators4 ,,thermal ,,junctions ,,used ,,in ,,pyrometers4 ,upper limit to which ,junction may be used4 ,couple4 ,deg4 ,deg4 ,cent4 ,fahr4 ,platinum and rhodioplatinum 7#aj per cent4 ,rh7 #adjj #beej #b ,rhodioplatinum alloys of different composition #afjj #bijj ,platinum and iridioplatinum 7#aj per cent4 ,ir7 #aajj #bjjj ,nickel and constantan #ijj #afej ,nickel and copper #hjj #adge ,nickel and carbon #ajjj #ahej ,nickel and iron #ajjj #ahej ,iron and constantan #ijj #afej ,copper and constantan #hjj #adge ,silver and constantan #hjj #adge #b ,nickel chrome alloys of different composition 7,hoskin's alloys7 #aajj #bjaj ,nickel-chrome alloy and nickel-aluminium alloy #aajj #bjaj #b ,iron-nickel alloys of different composition #ajjj #ahej ,the electromotive force developed by a junction of any given pair of metals when heated to a given temperature varies according to the origin of the metals4 ,it is not unusual1 for example1 for two samples of #aj per cent4 rhodioplatinum1 obtained from different sources1 to show a difference in this respect of #dj per cent4 when coupled with the same piece of platinum4 ,equal or greater divergences may be noted with other metals2 and hence the replacement of a junction can only be effected1 with accuracy1 by wires from the same lengths of which the junction formed a part4 ,as showing how platinum itself is not uniform1 it may be mentioned that almost any two pieces of platinum wire1 if not from the same length1 will cause a deflection on a sensitive galvanometer when made into a junction and heated4 ,it is therefore customary for makers to obtain considerable quantities of wire of a given kind1 homogeneous as far as possible1 in order that a number of identical instruments may be made1 and the junctions replaced1 when necessary1 without alteration of the scale of the indicator4 ,the alloy known as 8constantan10 which figures largely in the foregoing table1 is composed of nickel and copper1 and is practically identical with the alloy sold as 8,eureka0 or 8,advance40 ,it has a high specific resistance1 and a very small temperature coefficient1 and is much used for winding resistances4 ,couples formed of constantan and other metals furnish on heating an ,e4,m4,f4 several times greater than that yielded by couples of the platinum series1 and show an equally steady rise of ,e4,m4,f4 with temperature4 ,this alloy has proved of great service in connection with the thermo-electric method of measuring temperatures4 ,couples formed of nickel-chrome alloys1 known as 8,hoskin's alloys10 have been introduced into ,britain by the ,foster ,instrument ,company1 which may be used continuously to #aajj ,c41 and for occasional readings up to #acjj ,c4 ,another couple1 much used in ,america1 consists of an alloy of #ij per cent4 nickel and #aj per cent4 chromium1 and an alloy of #ih per cent4 nickel and #b per cent4 aluminium1 which may be used up to #aajj ,c4 ,other couples1 formed of alloys of nickel1 chromium1 iron1 aluminium1 etc41 have been introduced by different makers1 but have not proved so satisfactory as those mentioned above4 ,changes in ,thermal ,junctions when constantly used4--,no metal appears to be able to withstand a high temperature continuously without undergoing some physical alteration2 and for this reason the ,e4,m4,f4 developed by a given junction is liable to change after a period of constant use4 ,at temperatures above #aajj ,c41 platinum1 for example1 undergoes a notable change in a comparatively short period1 but below #ajjj ,c41 the change is very slight1 and if this range be not exceeded1 a platinum-rhodioplatinum or iridioplatinum junction may be used for years without serious error arising from this cause4 ,this liability to change is one of the factors which restricts the range of thermal junctions1 which should never be used continuously beyond the temperature at which the alteration commences to become large4 ,a second cause of discrepancy is the possible alteration in the composition of an alloy1 due to one of the constituents leaving in the form of vapour1 as is noted with iridioplatinum alloys1 from which the iridium volatilises in tangible quantities above #aajj ,c41 causing a fall of #aj per cent4 or more in the thermo-electric value of the junction of these alloys with platinum4 ,constantan appears to be very stable in its thermo-electric properties1 and the various junctions in which it plays a part show a high degree of stability if not overheated4 ,rhodioplatinum alloys are very stable1 and for temperatures exceeding #aajj ,c4 a junction of two of these alloys1 of different composition1 is more durable than one in which pure platinum is used4 ,an extended series of tests on base-metal junctions made in ,america by ,kowalke showed that continuous heating of couples as received from the makers altered the ,e4,m4,f4 considerably1 the change in some cases representing over #ajj ,c4 on the indicator4 ,a stable condition1 due to the relief of strains or other change1 was finally reached1 and the conclusion drawn that the materials should be thoroughly annealed before calibration4 ,it is desirable in all cases periodically to test the junctions at some standard temperature1 and if any conspicuous error be noted1 to replace the old junction by a new one4 ,in addition to the errors due to slow physical changes1 a junction may be altered considerably1 if imperfectly protected1 owing to the chemical action of furnace gases1 or of solids with which the junction may come into contact4 ,the vapours of metals such as lead or antimony are very injurious2 and platinum in particular is seriously affected by vapours containing phosphorus1 if in a reducing atmosphere4 ,so searching is the corrosive action of furnace gases that adequate protection of the junction is essential if errors and damage are to be avoided4 ,when a wire has once been corroded1 a junction made with it will not develop the same ,e4,m4,f4 as before4 ,electromotive ,force developed by ,typical ,junctions4--,the following table exhibits the ,e4,m4,f4 generated by several junctions for a range of #ajj ,c41 taken at the middle part of the working range in each case4 ,these figures are subject to considerable variation1 according to the origin of the metals4 ,e4,m4,f4 in millivolts for ,couple4 a rise of #ajj at middle of working range4 ,platinum-rhodioplatinum 7#aj per cent4 ,rh7 #aa ,platinum-iridioplatinum 7#aj per cent4 ,ir7 #ab ,nickel-constantan #bc ,copper-constantan #eh ,nickel-copper #fa ,iron-constantan #fg ,hoskin's alloys #gd ,it will be noted that the base-metal junctions give much higher values than the platinum series1 and hence can be used with a less sensitive1 and therefore cheaper1 indicator4 ,base-metal junctions are also1 in consequence of the greater ,e4,m4,f4 furnished1 capable of yielding more sensitive readings over a selected range of temperature4 ,illustration3 ,,fig4 #d4--,,practical ,,form ,,of ,,thermo-,,electric ,,pyrometer4 ,practical ,forms of ,thermocouples4--,when expensive junctions are employed1 wires of the minimum thickness consistent with strength and convenience of construction are used1 a diameter of ,no4 #be standard wire gauge being suitable4 ,a common arrangement is shown in fig4 #d1 in which ,j is the hot junction1 the wires from which are passed through thin fireclay tubes which serve as insulators 7or through twin-bore fireclay7 to the reels ,r ,r1 in the head of the pyrometer1 upon which a quantity of spare wire is wound to enable new junctions to be made when required4 ,two brass strips1 ,s1 are screwed down on to the wires at one end1 and are furnished with screw terminals at the other end1 from which wires are taken to the galvanometer or indicator4 ,a protecting-tube1 ,t1 surrounds the wires and hot junction4 ,the head1 ,h1 may be constructed of wood1 fibre1 or porcelain1 and should be an insulator for electricity and heat4 ,there are various modifications in use1 but the general method described is adopted by most makers4 ,in order to guard against errors arising from alterations in the temperature of the cold junctions in the end of the pyrometer1 some firms construct the head so as to leave a hollow space1 through which cold water is constantly circulated 7fig4 #e71 the arrangement being known as a 8water-cooled head40 ,in some forms the supply of spare wires is made to take the form of two spiral springs in a hollow head1 the upper ends of the springs being taken to terminals4 ,illustration3 ,,fig4 #e4--,,pyrometer ,,with ,,water-,,cooled ,,head4 ,the choice of a protecting-tube is a matter of considerable importance4 ,obviously1 such a tube should not soften at the highest temperature attained1 and when expensive metals are used to form the junction the sheath should not be permeable to gases or vapours4 ,it should also1 if possible1 be a good conductor of heat1 so that the junction may respond quickly to a change of temperature in its surroundings1 and should be mechanically strong4 ,it is difficult to secure all these properties in any single material1 and the choice of a sheath is decided by the conditions under which the couple is to be used4 ,the substances employed1 and their properties and special uses1 may be enumerated as follows3-- #a4 ,iron or ,mild ,steel4--,for temperatures not exceeding #aajj ,c4 iron or mild steel covers are cheap and efficient from the standpoint of conductivity1 although liable to deteriorate owing to oxidation4 ,the tendency to oxidise is greatly diminished by 8calorising0 the exterior by ,ruder's process1 in which the iron is heated in a mixture of metallic aluminium and oxide of aluminium1 a surface alloy being formed which resists oxidation4 ,a result nearly as good may be obtained by smearing the surface with fine aluminium powder1 and bringing to a white heat4 ,this treatment greatly prolongs the life of an iron sheath4 ,some makers employ an inner steel tube round the wires1 and an outer tube which comes into contact with the furnace gases1 corrosion of the latter being detected before the inner tube has given way and exposed the junction4 ,some makers do not consider it safe to expose heated platinum to an iron surface1 with only air intervening1 and hence use an inner cover of silica or porcelain1 which the outer iron or steel tube protects from mechanical damage4 ,for ordinary work seamless steam or hydraulic steel tubing1 with a welded end1 is satisfactory2 but for dipping into molten lead or other metals the tube should be bored from the solid4 ,the great advantage of an iron or steel sheath is its mechanical strength1 which protects the couple from damage in case of rough usage4 #b4 ,nichrom4--,certain alloys of nickel and chromium1 and especially that known as ,nichrom ,,ii1 may be kept at #aajj ,c4 without oxidising to any appreciable extent2 and hence sheaths of this material may be used up to the temperature named4 ,in addition to being more durable than iron1 nichrom possesses the same advantages of strength and good conductivity2 on the other hand1 it is more costly4 #c4 ,molybdenum4--,this metal1 which possesses a melting point of about #bejj ,c41 may be dipped in molten brass1 bronze1 copper1 etc41 without being attacked1 and has been used to form the tip of a protecting-tube designed to measure the temperature of molten alloys4 ,a junction covered only by a thin tube of molybdenum quickly attains the temperature of its surroundings4 #d4 ,graphite and ,graphite ,compositions4--,carbon has the highest melting point of all known substances1 and in the form of artificial or ,acheson graphite may be easily machined to any desired shape4 ,graphite sheaths are sometimes used for immersion in molten metals1 but at #ajjj ,c4 and higher ,acheson graphite oxidises easily and becomes friable4 ,it is a good conductor of heat1 but is easily broken4 ,compositions of natural graphite and refractory earths1 such as ,morgan's 8,salamander10 are inferior to pure graphite in conductivity1 but are stronger and not readily oxidised1 and may be used to form sheaths for temperatures up to #adjj ,c4 or possibly higher1 when penetration of furnace gases to the junction is not of moment4 #e4 ,porcelain4--,this material1 in its best forms1 may be used up to #adjj ,c41 but must be efficiently glazed to prevent the ingress of furnace gases to the junction4 ,it is easily broken by a blow1 and when circumstances permit should be protected by an iron covering-sheath4 ,the variety known as 8,marquardt0 has been found very satisfactory for high-reading thermal couples4 ,porcelain is not a good conductor of heat1 and a junction encased in it does not respond quickly to external changes in temperature4 #f4 ,vitrified ,silica4--,this substance1 which may be worked in the oxy-hydrogen blowpipe1 is largely used as a protecting-tube4 ,it is not advisable1 however1 to use it for continuous work above #aajj ,c41 as beyond this temperature devitrification occurs1 and the tube becomes porous4 ,it is a fairly good conductor of heat1 and withstands rapid changes in temperature without cracking4 ,it is very brittle1 and for this reason is generally encased in iron4 #g4 ,alundum4--,this material is made from fused bauxite1 and has a melting point of #bjej ,c4 ,a special form of alundum1 used for protecting-tubes1 is non-porous up to #acjj ,c41 and forms a satisfactory covering4 ,alundum is a moderately good conductor of heat1 but is easily broken4 #h4 ,carborundum4--,this is an electric furnace product1 which may be heated above #bjjj ,c4 without damage4 ,for making into pyrometer tubes1 it is bonded with a suitable material1 and baked after shaping4 ,carborundum1 and the amorphous variety known as 8silfrax10 have proved useful for protecting junctions at temperatures as high as #afjj ,c4 ,the thermal conductivity is relatively good1 but the tubes are easily broken4 #i4 ,magnesia4--,tubes of this material1 which melts at a temperature considerably above #bjjj ,c41 have been used for special work4 ,magnesia is a poor conductor of heat1 and has little mechanical strength4 #aj4 ,zirconia4--,this is a very refractory material1 its melting point exceeding #bejj ,c4 ,it may be made into a vitreous variety1 which is non-porous and proof against sudden temperature changes4 ,at present1 only a moulded form of pyrometer tube1 made from zirconia powder1 is available1 the material worked in this manner being termed 8zirkite40 ,although zirconia is a bad conductor of heat1 its other qualities are such that it forms an excellent material for work at the highest temperatures possible for thermal junctions2 and when the vitreous variety is available1 may come into extended use4 ,illustration3 ,,fig4 #f4 ,,pyrometer ,,with ,,special ,,cold ,,junction ,,in ,,head4 ,it will be seen from the foregoing that the ideal protecting-tube has yet to be found1 and the user must choose the one which comes nearest to his requirements4 ,special consideration must be given in cases when chemical fumes are present1 and a sheath selected which is not attacked or penetrated by them4 ,returning to the junction1 it is advisable always to weld the wires1 and not to rely upon the contact resulting from twisting them together4 ,platinum and the platinum alloys may be welded readily by placing the junction in a coal-gas blowpipe fed with oxygen instead of air4 ,for work at lower temperatures the platinum metals may be soldered by means of a small quantity of gold1 in the flame of a ,bunsen burner4 ,when cheap metals are used for the junction the construction may be considerably modified1 and often with advantage4 ,in fig4 #f1 for example1 which represents a thermocouple made by ,a4 ,gallenkamp ,co41 the metals used are copper and constantan1 and the hot junction1 fastened by silver solder1 is supplemented by a cold junction of the same metals located in the head4 ,the copper wire from the hot junction passes directly to a copper terminal1 from whence a copper wire lead is carried to the galvanometer2 and the same procedure is carried out with the copper wire from the cold junction1 thus realising the circuit shown in fig4 #b4 ,the cold junction is kept in oil1 the temperature of which is registered by a short thermometer1 thus enabling 7as will be explained later7 the correct temperature of the hot junction to be deduced under any circumstances4 ,in this instrument twin-bore fireclay is used to insulate the wires1 and the protecting-tube is of iron--which suffices for the upper limit 7#hjj ,c47 to which the junction may be used4 ,iron and constantan could be used in this manner by employing iron leads to the galvanometer4 ,another type of instrument1 rendered practicable by the use of cheap metals1 and which may be termed the 8heavy type10 is constructed of thick pieces of the metals welded together instead of wires1 thus ensuring greater strength and longer life4 ,messrs ,crompton ,co4 were the first to introduce thermocouples of this type1 consisting of a heavy steel tube1 to one end of which a nickel rod is welded1 the other end being free1 and the length of the rod suitably insulated from the steel tube2 leads for the rod and tube being taken to the galvanometer4 ,fig4 #g shows a couple of this kind1 made by ,paul1 consisting of an iron tube down the middle of which a constantan rod is passed1 insulated from the tube by magnesia4 ,at the tapered end the two metals are welded together1 and at the free end a special cap1 fitted over the tube and rod1 the contact parts being insulated from one another1 serves to enable leads to be taken to the galvanometer4 ,similar thermocouples are made by the ,foster ,instrument ,company 7fig4 #h71 and are simple1 cheap1 and reliable up to #ijj ,c4 with an iron-constantan couple1 and to #aajj ,c4 with nichrom couples4 ,when worn out they may be replaced1 at a trifling cost1 by others made from the same batch of metal4 ,illustration3 ,,fig4 #g4--;,,heavy ,,type1 ,,cheap-,,metal ,,pyrometer4 ,the drawback to the use of carbon as one of the materials for a junction is the difficulty experienced in securing a good contact with the metal with which it is coupled4 ,in nickel-carbon junctions the contact is sometimes ensured by the aid of a spring1 which presses the two substances together4 ,such an arrangement is evidently not so reliable as one in which the materials are welded1 and a defective contact1 arising from any cause1 would lead to serious error4 ,a preferable plan is to screw both the nickel and carbon rods into a cross-piece of either element4 ,illustration3 ,,fig4 #h4--;,,foster',s ,,cheap-,,metal ,,pyrometer4 ,when applying a thermal junction to the measurement of surface temperatures1 such as steam-pipes or the exterior of furnaces1 the wires may be passed through a thin disc of metal1 about in4 in diameter1 and soldered at the back4 ,suitable materials are copper and constantan1 soldered to a thin copper disc with silver solder1 and brought to a cold junction in the head of the instrument as shown in fig4 #f4 ,the terminal piece of the insulation may be made of hard wood1 with the holes countersunk so as to cover the solder and enable the wood to touch the disc1 which1 when pressed on the hot surface1 will then rapidly acquire the temperature4 ,the author has found1 by trials under varying circumstances1 that this method of measuring surface temperatures gives reliable and concordant results4 ,for very high surface temperatures a platinum disc1 with one of the usual platinum metal couples soldered to the disc with pure silver1 and a piece of twin-bore fireclay brought to the back of the disc1 will be found to suffice for most cases arising in practice4 ,a small blowpipe flame is best for soldering the wires to the disc1 borax being used as flux in the first case2 but no flux is necessary in soldering the platinum metals with pure silver4 ,in deciding upon the length of a thermocouple it must be remembered that the temperature recorded is that prevailing in the region of the hot junction4 ,when the temperature of a furnace is uniform it is sufficient to allow the end of the thermocouple to protrude about #ab inches into the interior1 but when following the change of temperature undergone by objects in a furnace the end must be located near the objects4 ,if the distance from the exterior of the furnace to the objects exceed #b feet1 the thermocouple should be inserted through the roof so as to hang vertically1 as if placed through the side it would droop by its own weight at high temperatures4 ,the distance between the exterior of the furnace and the cold junctions should be at least #ae inches in all cases in which the heating of the cold junction is not automatically compensated4 ,after inserting the couple the opening through the furnace wall should be closed by means of suitable luting-clay4 ,in certain instances1 such as flues1 it is necessary to use a long instrument in a horizontal position4 ,a rail may then be placed across the flue1 at a suitable place1 to serve as a support and so to prevent drooping4 ,liquid ,element ,thermocouples4--,an investigation by the author and ,a4 ,w4 ,grace has shown that the continuity of the ,e4,m4,f4 produced by a rising temperature is not interrupted by fusion1 except in the cases of bismuth and antimony1 which both show an abrupt change in thermo-electric properties at the melting point4 ,it would therefore appear feasible to measure temperatures by constructing a thermocouple so as to retain the circuit after fusion1 the advantage gained being that the range is restricted by the boiling point of the metals instead of the melting point and higher readings are rendered possible4 ,the boiling points of some of the common metals are appended3-- ,metal4 ,boiling ,point4 ,deg4 ,c4 ,deg4 ,f4 ,aluminium #ahjj #cbgj ,silver #aiee #ceej ,tin #bbgj #dabj ,copper #bcaj #daij ,nickel #bccj #dbbe ,iron #bdej #dddj ,from inspection of these figures1 it will be seen that if a suitable couple could be obtained1 common metals might be used to measure temperatures equalling or even exceeding the limit of the range covered by wire junctions of metals of the platinum series4 ,instead of using two metals1 graphite might form one member of the couple1 provided that no objection to its use existed on other grounds4 ,illustration3 ,,fig4 #i4--,,liquid-,,element ,,thermocouple4 ,the form of thermocouple designed by the author to permit of the use of molten elements is shown in fig4 #i4 ,a rod of refractory material1 ,r1 is perforated longitudinally by two holes1 down which are passed rods of the thermo-elements1 ,a and ,b4 ,the lower ends of ,a and ,b are inserted in a graphite block ,g1 which is jointed on its upper face to ,r2 the whole being surrounded by the refractory cover ,c4 ,on either or both of the elements melting1 the circuit is maintained through ,g1 which serves also to prevent the mixing of ,a and ,b when molten1 whilst not affecting the ,e4,m4,f4 developed4 ,in order to allow for the expansion of the metals on melting1 ,a and ,b are made to fit loosely in ,r4 ,when inserted in a furnace to a depth represented by ,,ef1 only the portion of the metals adjacent to the closed end will melt1 the outer parts remaining solid4 ,at present it has not been found possible to procure the refractory parts in a form suited to commercial use1 but when this obstacle is overcome this type of thermocouple should prove of service for measuring temperatures beyond the scope of ordinary base-metal junctions4 ,indicators for ,thermo-electric ,pyrometers4--,as the electromotive force developed by a single junction when heated is small1 a sensitive galvanometer is required to indicate the minute current flowing through the circuit4 ,delicate millivoltmeters1 of the moving-coil type1 are universally employed1 as they possess the advantage of an evenly-divided scale combined with the requisite degree of sensitiveness4 ,the original d',arsonval galvanometer1 consisting of a coil suspended by a metallic strip between the poles of a horse-shoe magnet1 was used by ,le ,chatelier1 who1 by its aid1 was enabled to lay the foundations of this branch of pyrometry4 ,three forms of this instrument are now in use1 viz4 7a7 the suspended coil 8mirror0 type2 7b7 the suspended coil 8pointer0 type2 and 7c7 the pivoted type4 ,examples of each will now be described4 ,illustration3 ,,fig4 #aj4--;,,holden-,d',,arsonval ,,mirror ,,galvanometer4 ,fig4 #aj represents a mirror galvanometer working on the d',arsonval principle1 designed by ,gen4 ,holden1 ,f4,r4,s4 ,the horse-shoe magnet is laminated1 and an iron core1 supported by a pillar1 is placed between the poles4 ,the coil1 which moves in the space between the core and the poles of the magnet1 is suspended by a thin1 flat strip of phosphor-bronze1 which carries a small circular mirror4 ,a similar phosphor-bronze strip is fastened to the lower part of the coil1 and is continued to an adjusting-screw in the base4 ,the ends of the suspension strips communicate with the terminals of the galvanometer1 and a current entering at one terminal passes through the metallic suspensions and the coil to the other4 ,the effect of passing a current through the coil1 which is located in a powerful magnetic field is to produce an axial movement tending to twist the suspension strips1 which movement is greatly magnified by a spot of light reflected from the mirror on to a distant scale4 ,when the current ceases1 the untwisting of the strip restores the coil to its former position4 ,galvanometers of this type are remarkably 8dead-beat0 in action1 that is1 the movement and restoration of the coil are accomplished without vibration4 ,a semi-transparent scale1 placed at #a metre distance1 and #ej centimetres long1 is suitable for use with this galvanometer4 ,when used in workshops1 it is necessary to protect a mirror galvanometer from the vibrations produced by machinery1 which would cause the spot of light to become unsteady4 ,the best method of effecting this is shown in fig4 #aa1 which represents the mode of suspension devised by ,w4 ,j4 ,lambert for use in the ,royal ,gun ,factory1 ,woolwich ,arsenal4 ,the usual supports of the galvanometer are abolished1 and the instrument suspended from the ring of a brass tripod1 so as to keep three springs partly in compression4 ,when suspended in this manner1 a mirror galvanometer is quite suited to commercial use2 in the quiet of the laboratory the ordinary supports may be employed4 ,the advantage gained by using the mirror type is that a much longer scale is possible than with instruments furnished with a pointer1 and hence greater accuracy in determining temperature readings may be secured4 ,illustration3 ,,fig4 #aa4--,,lambert',s ,,anti-,,vibration ,,stand ,,for ,,galvanometers4 ,illustration3 ,,fig4 #ab4--,,siemens' ,,thermo-,,electric ,,indicator4 ,in suspended coil instruments furnished with a pointer1 the construction differs only in detail from the foregoing4 ,in place of the mirror1 a light pointer is attached to the suspension so as to rest on the coil and a scale is furnished over which the pointer moves4 ,fig4 #ab is an example of this type1 made by ,messrs ,siemens1 the suspension being contained in the tube which rises from the body of the instrument4 ,the maximum length of scale moved over by the extremity of the pointer is about #f inches1 as a longer and therefore heavier pointer would reduce the sensitiveness below the point requisite for thermo-electric work4 ,in the double-pivoted type1 the suspension is eliminated1 and pivots are fastened to each end of the moving coil which rest in bearings4 ,the turning of the coil is made to compress a hair spring1 made of phosphor-bronze2 and when the current ceases the unwinding of this spring restores the coil to its former position4 ,the coil carries a pointer which moves over a scale4 ,these instruments are not so sensitive as those in which the coil is suspended1 but can be made sufficiently sensitive to work with any kind of junction in practical use4 ,the pivoted form is cheaper and stronger than the suspended type1 and is used whenever sufficiently sensitive4 ,the 8,uni-pivot0 galvanometer1 made by ,r4 ,w4 ,paul1 is shown in figs4 #ac and #ag4 ,the coil1 which carries the pointer1 is circular1 and moves round a spherical core of iron placed between the poles of the magnet4 ,a hole is drilled in the iron core1 and the coil rests on a single bearing at the bottom of this hole4 ,a phosphor-bronze control-spring serves to restore the coil to the zero position4 ,the lessened friction due to the use of a single pivot enables this instrument to be made very sensitive when needed1 so that a relatively small rise in the temperature of a junction may cause the pointer to traverse the whole length of the scale4 ,illustration3 ,,fig4 #ac4--,,principle ,,of ,,uni-,,pivot ,,galvanometer4 ,special ,features of ,indicators4--,all moving-coil instruments1 whether suspended or pivoted1 are liable to alteration of the zero point owing to what is termed 8creep40 ,the suspension strip1 when first fixed in position1 generally possesses a certain amount of initial torsion1 which comes into operation gradually and causes a slight movement of the coil4 ,similarly1 in a pivoted instrument1 the strength or shape of the control-spring undergoes a gradual alteration at first1 causing the pointer to move away from the zero position4 ,for this reason adjusting arrangements are fitted by means of which the spot of light or pointer may be brought back to the zero4 ,this creeping ceases after a time--often requiring twelve months--and if not subjected to any strain1 error from this cause does not recur to any notable extent4 ,with a mirror galvanometer it is better to move the scale1 or turn the galvanometer round on its axis to restore the correct zero1 rather than to twist the coil back2 but with a fixed scale and pointer the only remedy is to turn the coil bodily round4 ,in a single-pivot indicator constantly used in the author's laboratory1 the creep amounted to a movement of the end of the pointer through an angle of #b degrees in the first few months1 since when1 after the lapse of several years1 no further alteration has occurred4 ,it is advisable to test the zero point of an indicator from time to time by breaking the circuit1 and if an error be discovered the pointer should be re-set1 or an allowance made in taking a reading4 ,the resistance of an indicator should be so high that the readings should not be perceptibly altered by any fluctuations in the resistance of the circuit which may arise in practice4 ,if leads of considerable length were used to connect the pyrometer with the indicator1 and were subject to fairly large alterations of temperature1 the consequent changes in the resistance of such leads would be noticeable on a low-resistance indicator2 and similarly1 if a pyrometer were inserted at different depths in a furnace at separate times1 thus heating up varying lengths of the junction wires1 a discrepancy would arise for the same reason4 ,the resistance of an indicator1 however1 cannot be raised beyond a certain point without reducing the sensitiveness below the required limit4 ,a mirror galvanometer of the type described may have a resistance--partly in the coil and partly in an added series resistance--of #ajjj ohms or more1 and still be sufficiently sensitive2 and in the latest types of instruments provided with pointers the resistance may be made as high as #ajjj ohms1 although it is more usually #djj to #ejj ohms4 ,many indicators are in use1 however1 in which the resistance is #ajj ohms or less4 ,as1 from ,ohm's law1 the current varies inversely as the total resistance in the circuit1 any alteration in resistance should be small relatively to the total to render the error negligible4 ,this point is made clear in the following example3-- ,example4--,a thermocouple and leads have a resistance of #e ohms and are subject to alterations amounting to #a ohm4 ,to find the errors resulting when indicators of resistances #hjj1 #djj1 and #ej ohms respectively are used4 ,from ,ohm's law1 ,c ,e_/,r1 the variation in ,c1 with ,e constant1 will be #a in #hje1 #a in #dje1 and #a in #ee respectively4 ,as the indications are proportional to the current1 the alterations caused will be approximately per cent41 per cent41 and #b per cent4 ,the first two may be ignored2 the last may be quite serious and lead to the failure of an operation4 ,it will be seen from the foregoing that low-resistance indicators should only be used for fixed thermocouples and short leads not subject to temperature changes1 or1 in other words1 in a circuit of fixed resistance4 ,the resistance of an indicator1 when unknown1 may be found by the following method1 suggested by the author3--,a resistance box is joined at one end to one terminal of the indicator4 ,to the other terminal a fairly stout iron wire1 #ah inches long1 is connected1 and a similar length of constantan wire is coupled to the other end of the resistance box4 ,the free ends of the wires are twisted into a junction which is dipped into boiling water4 ,the deflection obtained with no resistance in the box 7,d#a7 is noted1 and resistances 7,r7 are then unplugged until the deflection 7,d#b7 is approximately one-half of ,d#a4 ,the resistance 7,g7 of the indicator1 ignoring that of the wires1 is then given by the formula ,d#b,r ,g 7,d#a - ,d#b7 as may readily be proved from ,ohm's law1 ,e being constant4 ,this method is extremely simple and reasonably accurate4 ,reliable indicators are now procurable from many instrument-makers at a comparatively small cost1 progress in this direction having been most marked in recent years1 particularly in the case of pivoted instruments4 ,the most convenient form for workshop use is made with an edgewise scale 7fig4 #ad7 and may be placed in a suitable position fixed to a bracket4 ,the flat-scale pattern is preferable for use on a laboratory table1 or for a portable pyrometer4 ,the sector pattern is also good for workshop use1 the dial being visible from a distance4 ,illustration3 ,,fig4 #ad4--;,,indicator ,,with ,,edgewise ,,scale4 ,standardizing of ,indicators to read ,temperatures directly4--,the temperature scale of an indicator1 for use with a given thermal couple1 is always marked by the maker in the case of instruments furnished with a pointer1 and1 generally speaking1 is correct within reasonable limits4 ,it is customary and necessary to send with the instrument a statement of the cold-junction temperature for which the markings are correct2 say #bj ,c4 or #fj ,f4 ,the user should then endeavour to maintain the cold junction at this specified temperature when taking a reading1 or otherwise a considerable error may be introduced4 ,it is highly desirable1 however1 that the user should be able to perform the standardizing himself1 if only for checking purposes2 and when using a mirror galvanometer as indicator it is necessary to standardize on the spot at which the instrument is fixed4 ,ability to prepare a temperature scale is further useful1 inasmuch as any good millivoltmeter1 of range #j to #bj millivolts1 may be used for thermo-electric work of all kinds1 and may be calibrated for different junctions1 a suitable series resistance being added to enable ,e4,m4,f4's higher than #bj millivolts to be measured4 ,such an instrument may thus be made extremely useful1 both in the workshop and laboratory4 ,standardization may be effected either by subjecting the hot junction to several known temperatures1 and noticing the deflections corresponding thereto2 or by measuring the electromotive force developed by the junction1 and calculating the corresponding temperature from a formula which is known to hold for the range comprehended by the instrument4 ,the former method is simpler2 and if carefully conducted is quite accurate4 ,the latter method possesses the advantage that readings in millivolts may be translated directly into temperatures when the constants of a given thermal couple are known4 ,it is now usual to mark indicators with a double scale1 one reading millivolts and the other temperatures4 ,standardization by ,fixed ,points4--,taking any millivoltmeter which1 with a maximum of #bj millivolts at the terminals1 will give a full scale deflection1 the first step is to arrange that the pointer 7or spot of light7 shall just remain on the scale at the highest temperature to be attained by the junction4 ,this may be done by placing the hot junction in boiling water and noting the deflection obtained1 either in millivolts or equal arbitrary divisions1 and also the temperature of the cold junction4 ,the deflection observed is due to a difference of temperature 7#ajj-t7 deg4 ,c1 where t is the temperature of the cold junction4 ,if the highest temperature to be measured is #aj times 7#ajj-t71 the deflection should be rather less than #a_/aj of the scale1 and similarly for any other required temperature limit4 ,if the observed deflection exceed this proportion1 a series resistance should be added until the correct value is obtained4 ,this resistance is then permanently installed in the circuit for use with the junction under trial4 ,before proceeding further it is necessary to consider whether the pyrometer is to possess a single cold junction of ascertainable temperature 7as in fig4 #f71 or whether it will be arranged with two cold junctions in the head1 as in fig4 #d4 ,in the former case it is simpler to prepare a 8difference0 scale2 that is1 one which reads differences of temperature between the hot and cold junctions1 from which the temperature of the hot end may be obtained by adding to the difference that of the cold junction4 ,in the latter case the cold end should be kept by artificial means at the temperature likely to be attained in practice--say #be ,c4--a water-bath being suitable for this purpose4 ,it is advisable to remove the shield of the pyrometer when standardizing1 so as to expose the hot junction1 as closer readings can then be taken4 ,a number of materials--preferably cheap--of known boiling points or melting points are then selected from a table of fixed points 7page #af7 so as to give about six points1 distributed fairly evenly over the scale4 ,as an example1 if it were desired to prepare a temperature scale from #j to #ajjj ,c41 the following might be chosen3-- ,substance and ,condition4 ,temperature4 ,water at boiling point #ajj ,c4 #bab ,f4 ,tin at melting point #bcb #ddi ,zinc at melting point #dai #ghf ,antimony at melting point #fca #aafg ,common salt at melting point #hjj #adgb ,copper at melting point 7covered with graphite7 #ajhd #aihc ,the hot junction is allowed to attain these temperatures successively1 and the corresponding deflection in each case is noted4 ,it is then possible to divide up the whole of the scale to read temperatures directly4 ,the first reading is taken by placing the junction in a vessel of boiling water1 and for a locality near sea level it is not necessary in ordinary work to take account of fluctuations in the boiling point due to alterations of atmospheric pressure4 ,to ensure that the other readings are taken when the substances are exactly at the melting point1 the procedure is as follows3 about #b-c lb4 of the substance are melted in a salamander crucible1 and a small fireclay tube1 closed at one end1 is inserted in the molten mass4 ,the hot junction is placed in the fireclay tube1 and the intervening space filled with asbestos fibre4 ,great care must be taken not to let the junction touch the fused substance4 ,the crucible is now allowed to cool1 and a reading of the deflection taken every half-minute4 ,when the substance is exactly at its solidifying point--identical in general with the melting point--the deflection remains stationary for several consecutive readings1 owing to the liberation of latent heat of fusion in sufficient quantity to balance the loss by radiation4 ,this stationary reading is noted for each substance1 and represents the deflection given when the hot junction is at the temperature corresponding to the melting point1 and the cold junction or junctions at the temperature existing when the observation is made4 ,for melting the materials1 a ,davies furnace with a large ,teclu or ,meker burner is convenient up to #hej ,c42 but to melt the copper a blast lamp is requisite4 ,the molten mass may be allowed to cool in the furnace4 ,from these observations a calibration curve may be drawn either for differences between hot and cold junctions1 or for a steady temperature of the cold junctions4 ,two sets of data are appended to illustrate the procedure4 ,temperature ,pyrometer #a4 ,iron-constantan4 ,pyrometer #b4 of ,hot 7,series resistance in ,platinum- ,junction4 galvanometer circuit47 iridioplatinum4 ,deflection4 ,cold ,difference4,deflection4 ,cold ,junction4 ,junction4 #ajj ,c4 #hi #ae ,c4 #he ,c4 #ee #bcb #bah #ag #bae #aef ,constant #dai #djf #ai #djj #bid at #fca #fch #ai #fab #dee #be ,c4 #hjj #hcj #bj #ghj #eij #ajhd 444 444 444 #hbj ,illustration3 ,,fig4 #ae4--;,,calibration ,,curves ,,for ,,two ,,thermo-,,electric ,,pyrometers4 ,fig4 #ae1 ,a1 is a calibration curve for thermocouple #a1 connecting deflections with corresponding differences between the temperatures of the hot and cold junctions4 ,in order to read from this curve the temperature of the hot end1 the reading corresponding to the observed deflection is added to the existing temperature of the cold junction4 ,thus if a deflection of #ef divisions were obtained with the cold junction at #be1 the temperature of the hot junction would be 7#edj #be7 #efe ,c4 ,the advantage of this method of calibration is that it is unnecessary to take precautions to keep the cold junction at a steady temperature2 and when a single cold junction is used1 as in fig4 #f1 this plan should always be followed4 ,it will be noted that this curve passes through zero1 as no deflection represents no difference of temperature4 ,fig4 #ae1 ,b1 represents the calibration curve for pyrometer #b1 and is such that direct readings may be obtained corresponding to any given deflection1 for a cold junction temperature of #be4 ,this curve1 therefore1 cuts the axis of zero deflection at #be1 as no deflection corresponds to the condition when both hot and cold junctions are at #be4 ,this method of calibration may be used with advantage for couples of the type shown in fig4 #d1 where two cold junctions exist in the head1 and the simple rule of adding the cold junction temperature does not apply4 ,many suggestions have been made for correcting for alterations in the temperature of the cold end of such a couple1 but none are accurate1 and it is necessary to keep this part at the temperature of standardization to secure correct readings4 ,in both of the above calibrations the galvanometer used possessed a scale divided into #ajj equal arbitrary divisions4 ,in making permanent temperature scales from these curves to attach to the existing galvanometer scale1 intervals of #ajj may be taken and marked opposite to the corresponding divisions on the existing scale4 ,each #ajj may then be equally subdivided into as many parts as the length of scale permits1 and numbered at suitable intervals4 ,if the junction used yield a calibration curve departing greatly from a straight line1 every #ej interval should be taken1 or1 if necessary1 every #be4 ,in the examples given both curves are nearly straight lines in the working region1 viz4 #djj to #hjj for the iron-constantan junction1 and #ejj to #aajj for the platinum-iridioplatinum4 ,one precaution necessary in standardizing an indicator by this method is to ensure that the metals used are pure1 as impurities lower the melting points4 ,if ordered as 8pure0 from any dealer of repute1 the metals will generally be found satisfactory4 ,the common salt used should be the ordinary salt sold in blocks1 and not a prepared table salt4 ,a second precaution1 when observing melting points1 is to guard against a possible error due to the substance becoming 8surfused0 or 8overcooled02 in which case the temperature falls below the ordinary freezing point before solidification commences4 ,when freezing occurs1 however1 the temperature rises to and remains at the true melting point1 and an increase of deflection following a gradual fall always indicates overcooling4 ,the higher deflection then attained is the true freezing point4 ,antimony frequently overcools to #fjj before freezing1 but on setting rises to the correct figure--#fca4 ,all metals and salts are liable to overcooling occasionally4 ,standardization by ,measurement of ,e4,m4,f4--,it has been found1 as the result of experiments1 that the relation between the ,e4,m4,f4 developed by a junction and its temperature--under constant conditions of the cold junction--may be expressed approximately by a formula as under3-- log ,e ,a log t ,b 7,holman's formula71 where ,e electromotive force in microvolts1 t temperature in ,centigrade degrees1 and ,a and ,b are constants depending upon the junction4 ,with certain junctions this formula may be applied over the working part of the scale with an error not exceeding #b ,c41 but with others the discrepancy is greater4 ,in order to determine the constants ,a and ,b1 it is necessary to measure the ,e4,m4,f4 at two known temperatures1 which should be chosen as far apart as possible in the working region4 ,when these constants are known1 a measurement of ,e enables the temperature t to be found by calculation4 ,example4--,le ,chatelier found that a junction at the temperature of melted aluminium 7#feg ,c47 gave #fbjj microvolts2 at the melting point of copper in air 7#ajfb ,c47 the figure was #ajehj4 ,applying in the above formula log #fbjj ,a log #feg ,b and log #ajehj ,a log #ajfb ,b1 the value of ,a is #abaif and of ,b #jcjb1 as may be found by taking logarithms and solving for ,a and ,b4 ,the values of the constants ,a and ,b vary for different junctions1 and also for different melts of what are reputed to be the same materials4 ,when once determined for a quantity of homogeneous wires1 to which the formula applies with sufficient accuracy1 it is evident that an indicator with a millivolt scale may be made to read temperatures directly without any necessity for further experiment1 although it is always advisable to take one check reading at a fixed point in the working range4 ,illustration3 ,,fig4 #af4--,,potentiometer ,,method ,,of ,,measuring ,e4,m4,f4 ,in order to determine the ,e4,m4,f4 of a junction at different temperatures1 the potentiometer method is used1 in which the ,e4,m4,f4 of the test-couple is balanced against the known ,e4,m4,f4 furnished by a constant cell4 ,the circuit is shown in fig4 #af1 in which ,b is an accumulator which sends a current through the resistances ,r#a1 ,r#b1 and the calibrated wire ,,de4 ,the cold ends of the couple are attached at ,p so as to be in opposition to ,b1 and in this branch of the circuit are included a sensitive galvanometer ,g and a portion of the wire ,,de4 ,a standard cadmium cell1 ,s1 is connected between ,r#a and ,r#b at one end1 and may be put in circuit with the galvanometer through the switch ,a4 ,in commencing1 ,s is connected to the galvanometer and ,r#a adjusted until no deflection is obtained on ,g4 ,the switch ,a is now moved over to the circuit of the couple1 and the terminal ,f moved along the wire until zero deflection is again obtained4 ,the ,e4,m4,f4 of the couple is determined from the relation ,e of junction ,resistance of ,,df 4 ,e of standard cell ,r#b ,by exposing the hot end of the junction to successive standard temperatures1 and maintaining the cold ends at a known constant temperature1 the necessary data for inclusion in a formula may be obtained4 ,in fixing a permanent temperature scale1 calculated from the formula1 to a millivoltmeter1 it must be remembered that the values given by the experiment are absolute1 and independent of the resistance of the circuit composed of the thermo-element and galvanometer4 ,on the other hand1 a millivoltmeter is marked to read difference of potential at its terminals2 and if in series with a junction and leads of notable resistance1 its indications will not be the ,e4,m4,f4 of the junction4 ,an example will make this point clear4 ,example4--,a millivoltmeter has a resistance of #ajj ohms1 and is marked to read ,p4,d4 at its terminals4 ,a thermocouple and leads connected to the millivoltmeter have a resistance of #e ohms4 ,to find the relation between the true ,e4,m4,f4 of the junction and the readings of the indicator4 ,if ,e the ,e4,m4,f4 developed by the junction1 and ,v1 the reading of the millivoltmeter1 ,p4,d4 at its terminals1 then the current in the circuit ,e_/#aje ,v_/#ajj2 and ,v 7#ajj_/aje7;,e4 ,that is1 the readings are lower by #e per cent4 than the true ,e4,m4,f4 of the junction4 ,in the same way a low resistance voltmeter1 if applied to a cell of high resistance1 shows a lower reading than the ,e4,m4,f4 of the cell4 ,this example indicates how a table connecting true ,e4,m4,f4's with reading in millivolts may be calculated when the resistances concerned are known4 ,it is presumed1 in preparing a scale in this manner1 that the resistance of the couple will not be subject to such alterations as to affect the reading4 ,the advantages of this method of calibration are manifest when a number of junctions are being made from a given batch of wires1 as it is only necessary to divide the scale of the indicator so as to represent millivolts--a simple operation--and then to attach a temperature scale4 ,this procedure is much more expeditious than standardizing each indicator at several fixed points when a number are concerned1 but for a single junction the fixed point method is easier4 ,the potentiometer method of measuring ,e4,m4,f4 may also be used to determine temperatures in place of an indicator1 and is of great service in cases where very accurate readings are specially required1 being far more delicate in detecting small differences of temperature than an indicator4 ,special potentiometers for thermo-electric work are made by the ,cambridge and ,paul ,instrument ,company1 ,siemens1 and others1 and are useful in conducting accurate research1 but are too elaborate for workshop or ordinary laboratory practice4 ,cold ,junction ,compensators4--,the necessity for paying attention to the cold junction has led to various attempts to compensate automatically for changes of temperature at this part of the pyrometer4 ,a thermometer located near the cold junction1 as in fig4 #f1 is all that is needed to correct a two-junction circuit2 but when a three-junction circuit is used a correct reading is not secured by adding the excess temperature of the thermometer over the calibration temperature to the reading on the indicator4 ,in ,bristol's arrangement a mercury thermometer1 with a large bulb and wide stem1 is stationed at the cold junction1 and participates in any temperature change4 ,in the stem is placed a loop of thin platinum wire1 which forms part of the pyrometer circuit4 ,when the mercury is heated it expands up the stem and short-circuits a portion of the loop1 thereby diminishing the resistance of the pyrometer circuit1 and tending to increase the deflection on the indicator4 ,simultaneously the cold junction will be heated1 tending to diminish the current1 and so to cause a less deflection4 ,by adjustment these two tendencies may be counterbalanced1 so that the reading is unaffected1 but such adjustment will only apply to a given ,e4,m4,f41 and therefore to one temperature of the hot junction4 ,hence this method fails in general application4 ,peake's compensated leads are intended to remedy cold-junction errors by transferring this junction1 in effect1 to the galvanometer4 ,they are used for pyrometers in which the platinum metals are employed1 and consist of wires of two different alloys of copper and nickel1 which connect the cold end to the indicator4 ,these alloys are such that the electromotive forces set up at the junctions in the head--,pt and ,cu-,ni #a1 and ,pt-,ir with ,cu-,ni #b--;are equal and opposite at all working temperatures1 and hence changes at the cold junctions do not affect the reading4 ,at the indicator1 however1 temperature changes would cause an alteration in deflection2 but as the indicator is generally placed well away from the furnace1 and is not liable to notable heating or cooling1 the possible errors are greatly reduced by the use of these leads4 ,they are obviously of no value for use with base-metal pyrometers1 as the wires used in such may be prolonged to the indicator1 with an identical result4 ,illustration3 ,,fig4 #ag4--;,,darling',s ,,compensator1 ,,fitted ,,to ,,galvanometer4 ,an automatic compensator for use with base-metal pyrometers has been devised by the author1 and is illustrated in figs4 #ag and #ah4 ,a spiral made of a compound strip of two metals is attached to the needle of the indicator1 and coils or uncoils when cooled or heated1 thereby moving the pointer over the scale4 ,the length of the spiral is such that an alteration of a given number of degrees in its temperature moves the pointer by the same number of degrees on the scale--or1 in other words1 the temperature scale of the pyrometer is identical with that of the spiral4 ,the metals forming the junction are continued1 in the form of wires1 to the interior of the galvanometer1 where a cold junction is formed1 which will always possess the same temperature as the spiral4 ,the scale is constructed to represent differences of temperature between the hot and cold junctions1 and before coupling up the pyrometer the pointer indicates the temperature of the spiral2 that is1 of the cold junction4 ,on connecting the thermocouple the pointer is moved by the coil of the indicator through an amount represented by the difference in temperature between the two junctions1 and therefore finally indicates the temperature of the hot junction4 ,illustration3 ,,fig4 #ah4--;,,indicator ,,fitted ,,with ,,darling',s ,,compensator4 ,example4--,if the cold junction were at #bj1 the pointer1 before connecting the couple1 would indicate #bj on the scale4 ,if the hot junction were #ehj hotter than the cold1 then on completing the circuit the pointer would move #ehj additional degrees along the scale1 so that the figure indicated would be 7#bj #ehj7 #fjj1 the temperature of the hot junction4 ,if now the indicator were heated by #aj1 the spiral would tend to augment the deflection by #aj1 but simultaneously the deflection due to the junctions would fall off by #aj1 and the reading would still be #fjj4 ,this method of compensation renders the readings independent of the cold junction1 and1 in addition to its use for high temperatures1 enables ordinary and low temperatures to be read simply and correctly1 as will be shown later4 ,the spiral is located in the tower rising from the top of the indicator in fig4 #ah4 ,in ,paul's method of compensation the thermocouple and indicator are placed across a ,wheatstone bridge1 two arms of which contain resistances of copper1 whilst the resistances in the other two arms are of manganin4 ,any change in temperature at the cold junction is shared by these four resistances1 and1 whilst affecting the resistance of the copper parts1 no change is caused in the manganin parts1 as this alloy has a negligible temperature coefficient4 ,if1 therefore1 the bridge were initially balanced at #bj ,c41 and the temperature rose to #cj1 the increased resistance of the copper would destroy the balance1 and permit of a small current passing through the indicator4 ,a fall to #aj1 by diminishing the resistance of the copper1 would cause an equal current to pass through the indicator in the opposite direction4 ,the amount of this current is arranged so as to add the rise in temperature of the cold junction to the reading of the indicator in the one case1 and to subtract the fall in the other1 thus retaining true readings for the cold-junction temperature at which the couple was standardized4 ,constant ,temperature ,cold ,junctions4--,if the cold junction can be kept at a steady temperature1 compensators are unnecessary1 but no good practical means of achieving this end has yet been devised4 ,water-cooled heads have already been referred to2 but in many situations the connecting-pipes entailed would be objectionable1 and hence this arrangement is not greatly used4 ,an alternative method1 suggested by ,prof4 ,a4 ,zeleny1 is to bury the cold junction in the ground4 ,recent experiments1 conducted at ,cambridge by ,r4 ,s4 ,whipple1 showed that a junction buried #aj feet deep did not vary in temperature by more than #b ,c4 over a period of three years4 ,this has led to the adoption of buried junctions in special cases2 but it is probable that much greater variations would be experienced in the ground beneath large furnaces1 in which case the advantages of this procedure would be lost4 ,a common workshop method is to locate the cold junction in a thermos flask filled with oil1 when a temperature constant to #b ,c4 may be secured1 although the changes in the temperature of the surrounding atmosphere may be as great as #aej ,c4 ,for special work1 ice may be used in the thermos flask1 thus securing absolute constancy2 but this procedure is not feasible in ordinary works practice4 ,special-,range ,indicators4--,when the working range of a pyrometer is from #fjj ,c4 upwards1 it is evident that the part of the scale occupied by the first #fjj is useless1 and that it would be an advantage if the whole scale could be utilised for the special working range1 so as to secure more exact readings4 ,this may be accomplished by a 8set-up0 against the movement of the pointer caused by the thermocouple1 so as to prevent any motion over the scale until an assigned temperature is reached4 ,for example1 a junction developing #ab millivolts at #ajjj ,c4 may be coupled to an indicator in which the full-scale deflection of the pointer is produced by #f millivolts4 ,if an ,e4,m4,f4 of #f millivolts be opposed to the junction1 no deflection will occur until the temperature at which the couple develops #f millivolts is reached--when the opposing ,e4,m4,f4 will be overcome4 ,this temperature may be #ejj ,c41 so that the whole scale may be divided up between #ejj and #ajjj4 ,the length of the indicator scale is thus effectively doubled2 and by using different values for the set-up1 it is evident that any desired range may be obtained within the limits of sensitivity of the indicator4 ,the method of procuring the opposing ,e4,m4,f4 varies with different makers4 ,the ,cambridge and ,paul ,instrument ,company employ a dry cell and a series resistance1 connected so as to oppose the thermocouple2 and by adjusting the resistance any desired set-up may be obtained1 the value of which1 in degrees1 may be read off by connecting the cell and resistance to the indicator1 the couple having been switched out of the circuit4 ,thus1 to adjust for a range of #ejj-ajjj on an indicator giving full-scale deflection for #ejj1 the resistance is regulated so that the cell alone causes the pointer to move to the end of the scale4 ,the method adopted by ,paul consists of suitable resistances inserted in a ,wheatstone bridge1 which may be thrown off the balance1 and thus cause an opposing ,e4,m4,f4 of the correct amount at the terminals of the indicator4 ,a mechanical set-up has been introduced by the ,cambridge and ,paul ,instrument ,company1 the indicator in this case having a suspended coil4 ,by turning a milled-head a twist may be given to the suspending strip1 and by the turning of a second head the pointer may be brought back to zero1 retaining the initial twist1 which is opposed to that produced by the current due to the couple4 ,thus1 if the imposed twist were such as to move the pointer to the #djj mark on the scale1 the temperature indicated by the junction would be the observed reading plus #djj4 ,by this method it is possible to obtain any desired range within the limits of the indicator4 ,the danger of producing errors due to 8creeping0 is said to be negligible4 ,illustration3 ,,fig4 #ai4--;,,circuit ,,of ,,northrup',s 8,,pyrovolter40 ,potentiometer ,indicators4--,the advantage of measuring ,e4,m4,f4 by the potentiometer method is that the result is independent of the resistance of the circuit under test1 whereas an indicator is affected by changes in the resistance of the circuit in which it is inserted4 ,when long leads are used to connect a couple to its indicator1 notable errors may be caused by the varying resistance of the leads1 due to changing temperature2 and1 in addition1 the resistance of the couple-wires varies according to temperature and depth of insertion in the furnace4 ,attempts have therefore been made to produce indicators based on the potentiometer principle1 suitable for workshop use1 and one form1 known as ,northrup's 8,pyrovolter10 is arranged as shown in fig4 #ai1 ,a4 ,a cell ,d sends a current through a rheostat ,r1 a copper coil ,c1 and a manganin coil ,s4 ,the copper coil has the same resistance as the copper winding of the indicator ,g4 ,the couple is connected1 with ,g in circuit1 across the manganin coil ,s1 the resistance of this material being unaffected by temperature4 ,by adjusting ,r until no deflection is shown on ,g1 the drop of volts across ,s is made equal to the ,e4,m4,f4 of the couple4 ,to measure this drop1 a key is pressed1 altering the circuit as shown in ,b1 the indicator being now in series with ,s and the couple detached4 ,the value of the current passing through ,s is unchanged1 as the indicator coil has the same resistance as the copper coil ,c1 which it now replaces4 ,the deflection on ,g indicates the value of this current1 and1 as the drop of volts across ,s is proportional to the current1 ,g may be marked off to read ,e4,m4,f4 and the corresponding temperature of the junction4 ,the advantages claimed are that the indicator may be used with any type of junction1 and is unaffected by temperature changes in the circuit4 ,a similar instrument is made by the ,brown ,company of ,philadelphia4 ,up to the present potentiometer indicators have not been adopted to any extent in ,britain1 and the adjustments necessary to obtain a reading must be accounted a distinct drawback from a workshop standpoint4 ,recorders for ,thermo-electric ,pyrometers4--,it is frequently of importance to know not only the existing temperature of a furnace1 but also the fluctuations to which it is subject4 ,continuous observation of a pyrometer would involve too much labour1 and it is therefore evident that an automatic recorder would possess many advantages in such cases4 ,a continuous record shows whether the attendant has maintained the temperature between the prescribed limits1 and furnishes a permanent history of a given operation1 which often serves as a guide to future procedure4 ,the first successful recorder1 suggested by ,sir ,w4 ,roberts-,austen and designed by ,gen4 ,holden1 ,f4,r4,s41 was used in conjunction with a mirror galvanometer4 ,in its original form1 the spot of light from the mirror was made to fall on a sensitized plate1 to which a gradual vertical motion was conveyed by connecting the dark slide to a water-float by means of a chain and pulley4 ,the float was placed in a tank of water1 which was gradually emptied through a tap1 causing the float to sink and the plate to rise4 ,if the deflection of the spot of light remained steady1 a vertical straight line was traced on the plate1 fluctuations producing a sinuous line4 ,trials at known temperatures enabled a standard plate to be obtained1 divided into degrees1 which could be superposed on a trial plate1 and the temperatures thus determined4 ,much valuable work was accomplished with this recorder by ,roberts-,austen for the ,alloys ,research ,committee of the ,institution of ,mechanical ,engineers4 ,illustration3 ,,fig4 #bj4--,,roberts-,,austen ,,recorder4 ,in its modern form 7fig4 #bj7 the photographic plate is replaced by a sheet of sensitized paper wound round a drum which rotates at a known rate--say1 once in #ab hours--by means of internal clockwork1 shown to the left of the figure4 ,the galvanometer is placed at the opposite end1 and the mirror is illuminated by means of an electric lamp placed externally1 the rays from which are reflected from a prism in the interior on to the mirror4 ,the ray of light leaving the mirror is broken into two portions1 one of which passes through a narrow slit on to the sensitized paper1 whilst the other portion is reflected on to a ground-glass scale on the lid1 divided so as to read temperatures4 ,in this manner the arrangement serves not only as a recorder1 but also indicates the existing temperature without necessitating the examination of the sensitized paper4 ,the whole arrangement is made impervious to light1 so that it may be used in daylight4 ,a dark room is necessary for fixing the records4 ,when desired1 records of two or more pyrometers may be taken on the same sheet1 a clockwork device being used to switch each instrument in turn on to the galvanometer for a given period1 an external dial indicating which pyrometer is for the time being in circuit4 ,whilst it is a drawback to the use of this recorder that the record is not visible1 the use of a mirror galvanometer confers a high degree of sensitiveness to the instrument1 not possessed by the recorders to be described subsequently4 ,illustration3 ,,fig4 #ba4--,,principle ,,of ,,thread ,,recorder4 ,the ,thread ,recorder4--,in this instrument an intermittent record is secured in ink1 possessing the advantages of visibility during the period over which readings are taken1 and of permanence without subsequent treatment of the chart4 ,the principle is shown in fig4 #ba1 where ,a is a boom terminating in a ,v-shaped piece of ivory1 and attached to the galvanometer suspension ,b4 ,by means of a cam ,e1 rotated by clockwork1 a bar ,d is made to descend at stated intervals1 pressing the end of ,a on to an inked thread ,g1 and causing the thread to touch a paper wound round the drum ,c4 ,this drum rotates on its axis once in #be hours by the action of internal clockwork4 ,the continued rotation of the cam ,e alternately raises and depresses the boom ,a1 leaving it free for a sufficient time to enable it to attain the position it would occupy if the mechanism were absent4 ,the thread ,g is passed over pulleys1 and is wound round through an ink-well1 so that the portion opposite ,a is always moist4 ,with the bar ,d descending every two minutes1 the successive dots form a nearly continuous line4 ,the paper on ,c is divided horizontally into temperatures1 and vertically into time units1 so that the temperature existing at any given time may readily be ascertained4 ,the front of the bar ,d1 or a separate strip parallel to it1 is divided so as to enable temperatures to be read without reference to the chart4 ,the actual instrument is shown in fig4 #bb4 ,when several simultaneous records are required1 the drum ,c is extended1 and other galvanometers introduced1 to which the separate pyrometers are connected4 ,several records can be taken on one chart by introducing a clockwork mechanism to couple each pyrometer in turn to the one galvanometer4 ,illustration3 ,,fig4 #bb4--,,thread ,,recorder4 ,illustration3 ,,fig4 #bc4--,,siemens' ,,recorder4 ,the ,siemens ,recorder4--,in this instrument 7fig4 #bc7 the boom from the galvanometer terminates in a knife-edge1 and moves over a thin horizontal rail1 the top of which is rounded4 ,between the rail and the boom are placed an inking ribbon and a paper chart1 which is moved forward by clockwork4 ,a chopper-bar1 also actuated by clockwork1 descends at about half-minute intervals1 and depresses the end of the galvanometer boom1 thus producing a small dot on the chart4 ,the paper is #ab cms4 wide and #dj yards long2 it is divided into time and temperature units1 and moves forward at the rate of #b cms4 per hour4 ,levelling screws are fixed to the base of the recorder4 ,illustration3 ,,fig4 #bd4--;,,foster',s ,,recorder4 ,foster's ,recorder4--,foster's recorder 7fig4 #bd7 is designed for use with base-metal couples of the nickel-chromium type1 known as ,hoskin's alloys1 which yield an ,e4,m4,f4 about five times as large as a platinum-rhodioplatinum couple4 ,the force available in this case enables the coil of the galvanometer to be pivoted in a horizontal position1 the pointer being vertical1 and yet to be sufficiently sensitive4 ,the chart is mounted on a vertical plate which rotates on its axis1 the time ordinates taking the form of concentric circles1 which are cut at an angle by the temperature ordinates4 ,at the terminus of the pointer is placed a small capillary tube1 fitted with an inked wick1 which1 when pressed upon the chart1 makes a mark4 ,the presser-bar is curved to the same radius as the pointer1 and carries a pad wetted with ink1 so that at each depression the supply of ink to the wick is replenished by an amount equal to that imparted to the chart4 ,this recorder is sometimes fitted with special contacts1 so that when the correct temperature exists an electric lamp with a white bulb remains lighted2 whereas when too low or too high a green or red lamp is lit up1 and an alarm thus given4 ,such an addition involves the use of a relay circuit1 but is advisable in cases where expensive articles might suffer if overheated4 ,it can be modified to permit of several simultaneous records being taken1 and possesses the advantage that the whole chart is visible at any time4 ,on the other hand1 the circular coordinates may be accounted a drawback by some1 as not being quite so familiar to read as charts in which the lines are straight4 ,robust construction is a feature of this recorder4 ,illustration3 ,,fig4 #be4 ,,paul',s ,,recorder4 ,paul's ,recorder4--,in the recorders previously described1 the motive power is furnished by clockwork4 ,r4 ,w4 ,paul has introduced an instrument in which all the moving parts are actuated by a motor driven with power from the mains4 ,this recorder is shown in fig4 #be4 ,the motor is furnished with a special type of governor to ensure constant speed1 and is connected by suitable gearing to the mechanisms moving the chart1 presser-bar1 and inking ribbon1 provision being made to vary the speeds of these movements by changing the gear4 ,the galvanometer is of uni-pivot pattern1 and the pointer is pressed at intervals on to a typewriter ribbon which lies above the chart4 ,immediately beneath the ribbon is placed a thin metal rod over which the paper passes1 and the result of the contact is to produce a small dot4 ,as in the thread recorder1 the chart is divided into rectilinear coordinates1 the ribbon in this case serving the same purpose as the thread in the former instrument4 ,the lower part of the recorder is prolonged so as to display a considerable length of the chart1 which is in the form of a roll1 and is drawn forward by the mechanism4 ,when two records are taken simultaneously the ribbon consists of two strips1 one moistened with black ink1 and the other with red2 and it is arranged that each strip in turn is over the thin rod on to which the pointer is pressed1 so that the records appear in separate colours4 ,this recorder can also be arranged for multiple records1 or fitted with a scale-control4 ,with a view to workshop use1 all the covers are fitted with faced metal joints1 which are much better for keeping out dust than wooden ones4 ,a further useful feature is that the various units in the recorder--galvanometer1 motor1 feed and record mechanism1 and reducing gear--are all separate and interchangeable4 ,by introducing a suitably divided chart this recorder will also serve for a radiation pyrometer1 or1 as will be shown later1 for a resistance pyrometer4 ,illustration3 ,,fig4 #bf4--,,leeds-,,northrup ,,recorder4 ,the ,leeds-,northrup ,recorder4--,the ,leeds and ,northrup ,company1 of ,philadelphia1 manufacture a recorder which is largely used in the ,united ,states4 ,as in ,paul's recorder1 all the mechanism is motor driven2 but the other arrangements are entirely distinct4 ,instead of measuring the deflection of the pointer1 a zero deflection method is used4 ,the pyrometer forms part of a potentiometer circuit1 and the function of the mechanism is to oppose an ,e4,m4,f4 equal to that of the pyrometer1 from which the temperature is known4 ,this has the advantage that the measurement is independent of the resistance of the leads1 and is capable of great accuracy4 ,the manner in which the adjustment of the opposing ,e4,m4,f4 is controlled may be understood from fig4 #bf1 in which the galvanometer coil is shown at the top of the figure4 ,the shaft from the motor carries four cams1 ,b1 ,c1 ,d1 ,d1 and at each revolution the cam ,b raises the bar 7#e7 so as to press it against an arm attached to the galvanometer coil4 ,at the same moment the cam ,c pushes against the bar 7#c71 and thereby releases a clutch 7#b7 from the disc beneath4 ,as shown1 the boom from the coil is to the right of the central position1 and is gripped between a bar 7#e7 and the lever 7#d7 when the former rises1 causing an angular movement of the clutch-arm 7#b74 ,as the rotation continues the cam ,c leaves the bar 7#c71 which then springs back and engages the clutch on the disc4 ,the cam ,d then descends and presses on the projection of the clutch-arm to the left1 causing the disc to rotate4 ,the movement of the disc is conveyed to an arm which moves over the slide wire of the potentiometer2 and this movement continues until the galvanometer boom is in the central or zero position1 when neither of the levers #d1 #d is gripped1 and consequently the disc is not fed in either direction4 ,if the boom swing to the left1 the movement of the disc will evidently be in the converse direction to that described4 ,in this recorder considerable power is available to drive printing or other mechanisms4 ,the arm moving over the potentiometer wire carries a pen which marks the moving chart1 or1 when several records are taken simultaneously1 a stamping machine is used which impresses the number of the pyrometer on the chart4 ,the same galvanometer mechanism serves also for use with resistance pyrometers1 as will be explained later4 ,control of ,furnace ,temperatures4--,many attempts have been made to secure the automatic regulation of furnace temperatures by means of mechanisms controlled by an indicator or recorder4 ,in the arrangement employed by the ,brown ,company of ,philadelphia1 movable stops are provided1 which may be brought to any part of the scale1 the mark between the stops representing the temperature it is desired to maintain4 ,the indicator 7or recorder7 is provided with a presser-bar which descends periodically2 and if the temperature be too low the depressed pointer completes a circuit through the inner stop1 whilst if too high the circuit is through the outer stop4 ,both circuits contain a relay which brings a mechanism into operation1 the result being to increase the supply of electricity or gas if the temperature be too low1 or to diminish the supply when too high4 ,when correct1 the depression of the pointer fails to complete either circuit1 and in this manner control between small limits may be ensured4 ,in the case of large furnaces the relay circuits are employed to light lamps of different colours1 the adjustment then being made by the man in charge of the furnace4 ,arrangements of this kind effect a considerable saving in fuel by preventing unnecessary heating1 and are particularly valuable in cases where overheating would be deleterious to the articles in the furnace4 ,the future will probably witness considerable developments along these lines4 ,contact-,pen ,recorders4--,the force with which the pointer of an indicator is urged over the scale is relatively small1 particularly in the case of pyrometers in which the platinum series of metals are used1 as these furnish only a low ,e4,m4,f4 ,if1 therefore1 the pointer terminate in a pen which is in continuous contact with the record-paper1 the friction thus occasioned interferes considerably with the free movement of the pointer4 ,when cheap-metal pyrometers are used1 which yield a much higher ,e4,m4,f41 the use of the pointer as contact-pen becomes more feasible1 and if uniform friction at all parts of the paper can be ensured1 records may be taken in this manner2 and a recorder so constructed is simpler and cheaper than those of the intermittent type4 ,contact-pen recorders are used in ,america to some extent1 being made by ,bristol1 ,brown1 and others2 but so far ,british makers have not developed the manufacture of these instruments4 ,at present1 contact-pen recorders must be considered less accurate and reliable than those in which the contact is intermittent4 ,installations of ,thermo-electric ,pyrometers4--,when a number of furnaces in the same establishment are to be controlled1 considerable economy may be effected by making one indicator serve for all the couples1 which in this case must necessarily be made up of wires identical in thermo-electric value4 ,such an arrangement is shown in fig4 #bg1 in which ,h#a and ,h#b represent two couples1 one wire from each being connected to one of the terminals of the galvanometer ,g4 ,the other terminal of the galvanometer is connected to the arm ,d of a switch1 and the remaining thermocouple leads are connected to the points #a and #b respectively on the circumference4 ,as shown1 ,h#a is connected to the galvanometer1 and by turning the arm ,d to the point #b the other couple would then be connected4 ,any number of junctions may thus be arranged with a single indicator4 ,when this arrangement is adopted in a workshop1 it is advisable to construct a small wooden building at a spot convenient for most of the furnaces1 in which the indicator and switchboard are kept1 and which could also contain a recorder if necessary2 a spot as free as possible from vibration being preferable4 ,separate indicators are only necessary when a furnace is used for special work4 ,illustration3 ,,fig4 #bg4--;,,connections ,,for ,,an ,,installation ,,of ,,pyrometers4 ,in some instances a second indicator is kept in the shop office1 to which all the pyrometers are wired1 and which serves as a standard4 ,the scale of the office indicator is checked daily at one point2 and by connecting a given couple first with the shop indicator1 and immediately afterwards with the office standard1 any errors can be detected4 ,it is also possible to ascertain the temperature of any given furnace in the office at any time1 and so to control the whole4 ,in fixing up such an arrangement it is necessary that each couple and its leads1 up to the indicator1 should possess the same resistance1 or should not differ by an amount sufficient to affect the readings4 ,the general experience of a properly managed installation is that the cost is saved in a few months in fuel alone2 and1 in addition1 the work is carried out to much better advantage owing to complete control from the office4 ,management of ,thermo-electric ,pyrometers4--,generally speaking1 thermo-electric pyrometers give little trouble in practice1 but the management should always be placed in skilled hands4 ,it is advisable to test each instrument periodically at a fixed point near the working temperature1 by the method explained on page #eg2 and if two or three pounds of material be used1 the protecting shield need not be removed4 ,a useful material for checking pyrometers near the critical range of steel is an alloy of #fj per cent4 of copper and #dj per cent4 of tin1 which gives a well-defined freezing point at #gch ,c41 and which may be used indefinitely in a reducing atmosphere ,any serious error is easily detected by observing that the indications differ widely from those generally obtained under the same working conditions4 ,if an error of #bj ,c4 or more is noted1 it is advisable to form a new junction1 as the discrepancy will probably become greater1 being due to a change at the hot junction4 ,a small error1 of the nature of #e or #aj ,c41 may be due to 8creep0 in the indicator1 which may be adjusted accordingly1 or a numerical correction may be made when taking a reading4 ,an iron protecting sheath may be saved from rapid oxidation by black-leading once per week1 which greatly prolongs its useful life1 but should be replaced immediately it becomes dangerously thin in any part4 ,coating with aluminium powder also greatly prolongs the life of an iron sheath4 ,when used in lead baths1 the immersed part1 if of iron or steel1 should be bored from the solid1 and left thick at the portion opposite the surface of the lead1 where most corrosion occurs4 ,a graphite tube1 or one made of a composition containing graphite1 is often useful in cases where iron is readily corroded1 and can be used to much higher temperatures4 ,when a number of instruments are in use1 it is advisable to keep a standard pyrometer for checking purposes1 preferably one which has been certified by the ,national ,physical ,laboratory4 ,in conducting a test1 the couples1 with protecting-tubes removed1 may be placed in the tube of an electric furnace of the type shown in fig4 #bi1 in close proximity with the standard junction4 ,on raising the temperature gradually1 the readings of each working instrument may be compared with the standard1 and the necessary corrections discovered4 ,care must be taken to prevent contact with the furnace tube1 and this may be accomplished by passing the wires through an asbestos stopper fitted into the end of the tube4 ,when recorders are used the attendant should make himself thoroughly conversant with the details of the mechanism1 so as to be able to remedy any minor ailments1 which are1 as a rule1 easily cured4 ,on no account should an unskilled workman be trusted with recorders2 it is better and safer to keep these in the office1 where they will not be likely to be damaged or tampered with4 ,all records should be kept for future reference1 properly dated1 and labelled according to the operations represented4 ,laboratory ,uses of ,thermo-electric ,pyrometers4--,numerous operations carried out in muffle furnaces at prescribed temperatures require no special precautions beyond those previously given4 ,in determining the melting points of metals or alloys1 however1 a porcelain or silica sheath is inadvisable1 as they are easily corroded4 ,an iron sheath is proof against some metals1 but not against others1 and it is always safer to fix a thin fireclay sleeve1 closed at the end1 over the part immersed4 ,a sheath of graphite or graphite composition may be used for temperatures above #aajj ,c42 and occasionally a sheath bored from a thick arc-lamp carbon1 coupled to an iron tube beyond the heated part1 will be found useful at high temperatures4 ,alundum is useful up to #afjj ,c1 and for temperatures of this order the higher refractories such as silfrax and zirkite may also be used to advantage4 ,illustration3 ,,fig4 #bh4--;,,differential ,,method ,,for ,,determining ,,critical ,,points ,,of ,,steel4 ,the determination of the 8critical0 points of steel call for special mention4 ,in cooling down a mass of steel the fall of temperature is arrested at one or more points1 observations of which are frequently of service in deciding the subsequent treatment of the steel4 ,a method commonly employed is known as the 8differential method10 and is indicated in fig4 #bh4 ,the sample of steel1 ,a1 is placed side by side with a piece of nickel1 ,b1 of equal dimensions1 in the tube of an electric furnace4 ,a naked junction1 ,c1 is placed in a hole drilled in ,a1 and is connected to the galvanometer ,g1 which is calibrated to read temperatures4 ,a two-junction circuit1 formed of a junction ,d placed in the hole in ,a1 and another junction ,e located in the hole in ,b1 are connected to a delicate galvanometer ,h4 ,the furnace is heated until the galvanometer ,g indicates #ijj ,c41 when the arrangement is allowed to cool4 ,as ,a and ,b1 under normal circumstances1 cool at an equal rate1 the junctions ,d and ,e will be at the same temperature1 and no deflection will be observed on ,h4 ,when1 owing to recalescence1 the cooling of ,a is arrested1 ,b1 not being thus affected1 will continue to cool1 thus producing a difference between the temperatures of ,d and ,e1 and consequently a deflection on ,h4 ,the temperature of ,a at the time this occurs is read off on ,g4 ,illustration3 ,,fig4 #bi4--;,,electric ,,tube-,,furnace4 ,the furnace illustrated in fig4 #bi is suited to this determination4 ,it consists of a silica tube #a foot long1 wound with a special resistance wire and efficiently lagged1 and may be heated in safety to #ajjj ,c4 for long periods1 and to #abjj for a short time4 ,it may be placed across the electric mains directly1 and reaches #ijj ,c4 in less than half an hour4 ,it consumes #fjj to #gjj watts at the highest temperatures1 and the cost of re-winding is small4 ,this furnace is useful as a general laboratory appliance1 and may be kept at a given steady temperature by the use of an external resistance4 ,the wires in this experiment should be platinum and iridioplatinum or rhodioplatinum1 or a good pair of base metals1 and the junctions in ,a should be separated from each other and from the specimen by asbestos2 the same precaution being taken to prevent the junction ,e from touching ,b4 ,a thin layer of mica should be used beneath ,a and ,b1 to avoid contact with the furnace tube1 which1 when hot1 allows of leakage of current from the heating coil4 ,both ,a and ,b may be #a in4 long1 in4 diameter1 with a hole in4 diameter drilled to a depth of inch4 ,an alternative method is to insert a junction in a hole in the specimen1 and to take direct readings as the temperature slowly rises or falls1 when an arrest in the movement of the pointer of the indicator shows that the change-point has been reached4 ,special sets are made for this purpose4 ,measurement of ,lower ,temperatures by the ,thermo-electric ,method4--,many cases arise in practice in which a thermal junction and a sensitive galvanometer are preferable to a mercury thermometer2 and below -#ci ,c41 at which temperature mercury freezes1 a thermal junction is frequently better to employ than an alcohol or pentane thermometer4 ,a number of practical examples of the use of thermal junctions for ordinary and low temperatures will now be considered4 ,measurement of ,surface ,temperatures4--,a mercury thermometer1 when laid on a hot surface1 only touches along a line1 and does not show the true surface temperature4 ,the construction of a thermal junction suitable for this purpose is described on page #da1 and for steam-pipe surfaces1 hot plates1 and the exterior of furnaces1 a specially calibrated millivoltmeter1 giving a full-scale deflection with #bj millivolts1 may be used4 ,in making the temperature scale1 boiling water 7#ajj;,c471 boiling aniline 7#ahd ,c471 and melting tin 7#bcb ,c47 are convenient standards4 ,if the surface temperature be less than #ajj ,c4 a mirror galvanometer should be used1 and the junction standardized in paraffin wax 7freezing point usually about #ej ,c41 but should previously be determined with an accurate thermometer71 absolute alcohol at boiling point 7#gi ,c47 and boiling water4 ,the author has found that this method yields excellent results in the case of steam-pipes1 the exterior of rotary cement kilns1 and hot surfaces generally4 ,measurement of ,low ,temperatures4--,junctions of iron and constantan1 ,hoskin's alloys1 copper and ,german silver1 or copper and constantan1 are suited to these measurements4 ,in a laboratory the cold junction may be kept in ice in a ,dewar vessel1 the mechanically protected form known as the 8,thermos0 flask being very useful for this purpose4 ,with a good mirror galvanometer precise readings may be secured1 #a_/aj of a degree ,c4 being easily detected4 ,calibration between -#dj and #dj;,c4 may be effected by comparison with a standard mercury thermometer1 a water-bath being used above #j1 and alcohol surrounded by a freezing mixture of ice and calcium chloride crystals below zero4 ,for very low temperatures 7-#bjj;,c4 or less7 the junction maybe calibrated in solid carbon dioxide 7-#gh;,c47 and liquid air 7-#ahd;,c474 ,dewar has found that copper and ,german silver form a reliable junction for very low temperatures1 and the author has successfully used a couple of ,hoskin's alloys for special work down to -#bjj;,c41 a pivoted indicator being employed4 ,no couples tested show a linear relation between ,e4,m4,f4 and temperature at these low ranges4 ,owing to the magnitude of the error caused by changes in the cold junction1 the thermo-electric method is not suited to the measurement of atmospheric temperatures1 or for explosive magazines or cold stores4 ,in such cases instruments of the resistance type1 to be described later1 are used4 ,temperature of ,steam1 ,exhaust ,gases1 etc4--,for measuring the temperature of ordinary or superheated steam1 the exhaust gases from internal combustion engines1 etc41 iron-constantan junctions1 with suitable indicators1 are satisfactory4 ,when placed in a pipe the junction should be as nearly as possible in the centre1 so as to avoid the cooling effect of the walls4 ,several junctions1 situated in different parts of the pipe1 may be used with a single indicator and suitable switchboard4 ,the above remarks also apply to the hot-blast for blast furnaces1 and similar instances where the temperature does not exceed #ijj ,c4 ,measurement of ,differences of ,temperature4--,cases frequently arise in practice in which the difference in temperature between two points is required1 and if this difference be subject to rapid changes1 a mercury thermometer1 from its large mass1 would not respond with sufficient rapidity to indicate these changes4 ,in such cases a circuit is made after the manner of fig4 #b1 one junction being located at each point2 thin wires of iron and constantan being used4 ,for small differences--#a ,c4 or less--a mirror galvanometer should be used4 ,calibration may be performed by placing one junction in hot water and the other in cold1 the water temperatures being read with an accurate thermometer4 ,advantages of the ,thermo-electric ,method of ,measuring ,temperatures4--,compared with other methods1 the thermo-electric possesses the following points of superiority3--7#a7 ,simplicity1 no special experiment being necessary to obtain a reading2 7#b7 cheapness of outfit2 7#c7 adaptability to a variety of purposes2 7#d7 ease of repair in case of damage2 7#e7 robustness1 not being liable to get out of order under workshop conditions2 and 7#f7 suitability to the purpose of a centrally controlled installation4 ,the drawbacks are3--7#a7 ,liability to error owing to fluctuations in the cold junction 7which may be avoided with care72 and 7#b7 lack of sensitiveness at very high temperatures compared with the resistance method--a point seldom of great practical importance1 as the limit of accuracy is usually within the amount by which an ordinary furnace fluctuates in temperature under working conditions4 ,,chapter ,,iv ,,resistance ,,pyrometers ,general ,principles4--,when a pure metal is heated1 its resistance to electricity increases progressively with the temperature4 ,certain alloys1 on the other hand1 show a practically constant resistance at all temperatures1 examples of such alloys being constantan1 manganin1 and platinoid4 ,all the elementary metals1 however1 exhibit a tangible rise in resistance when the temperature is augmented2 and ,sir ,w4 ,siemens1 in #ahga1 proposed to apply this principle to the measurement of high temperatures by determining the resistance1 and deducing the corresponding temperature from a table prepared under known conditions4 ,the choice of a metal is in this case more greatly restricted than in the selection of materials for a thermal junction4 ,a certain amount of external corrosion does not alter the ,e4,m4,f4 of a junction2 but an alteration in size produces a marked difference in the resistance of a wire1 which varies directly as the length and inversely as the area of cross-section4 ,to the necessity for the absence of any internal physical change affecting the resistance is therefore added the further condition of permanence of external dimensions4 ,for temperatures above a red heat the only feasible metals to use are platinum or the more expensive metals of the platinum series--and hence platinum is universally employed for this purpose4 ,the original ,siemens pyrometer consisted of #a metre of platinum wire1 #a millimetre in diameter1 wrapped round a porcelain rod1 and protected from furnace gases by an iron sheath ,an elaborate method of measuring the resistance1 involving the electrolysis of acidulated water1 was adopted for workshop use1 but was too involved to become popular4 ,later1 ,siemens employed the differential galvanometer method1 and finally the ,wheatstone bridge1 to measure the resistance4 ,both methods are still in use in connection with resistance pyrometers1 and the principle of each will now be explained4 ,measurement of ,resistance by the ,differential ,galvanometer4--,a differential galvanometer is one which possesses two windings1 arranged so that a current passing through the one tends to turn the pointer in one direction1 and through the other to cause a movement in the opposite direction4 ,if the currents in each winding simultaneously be equal1 the pointer remains at rest under the action of two equal and opposite forces4 ,the experimental attainment of the condition of rest serves as a means of measuring resistance1 the circuit being arranged as in fig4 #cj4 ,current from a battery ,b passes through a divided circuit1 one branch containing the adjustable resistance ,r and one coil of the galvanometer ,g2 and the other the unknown resistance ,p and the opposite coil4 ,the resistance ,r is adjusted until on tapping the key ,k no deflection on the galvanometer is noted1 when the current in each branch of the circuit will be the same4 ,the resistances of each coil of the galvanometer being equal1 it follows from ,ohm's law that ,p is equal to ,r when no deflection is obtained4 ,illustration3 ,,fig4 #cj4--;,,differential ,,galvanometer ,,method ,,of ,,measuring ,,resistance4 ,the accuracy of this method depends upon the sensitiveness of the galvanometer1 and also upon the extent to which the two coils may be regarded as truly differential1 as the measurement evidently assumes complete equality in resistance and effect on the moving part4 ,with modern galvanometers of this pattern1 it is possible to secure readings of sufficient accuracy for the purposes of pyrometry4 ,the method1 however1 is less sensitive than the ,wheatstone bridge1 now to be described4 ,illustration3 ,,fig4 #ca4--,,principle ,,of ,,wheatstone ,,bridge4 ,measurement of ,resistance by the ,wheatstone ,bridge4--,the principle of this method is shown in fig4 #ca1 where a and b are two fixed resistances of known value2 d is an adjustable resistance2 x the resistance to be measured2 ,b a battery2 and ,g a sensitive galvanometer4 ,if1 in this circuit1 d be adjusted until no deflection is shown on the galvanometer1 then a_/b x_/d2 or x 7a d7_/b4 ,hence1 if a b1 then x will be equal to d4 ,it is not difficult to construct a portable apparatus1 suitable for workshop use1 by means of which the value of x may be determined to #jja ohm2 and in the laboratory1 with a very delicate galvanometer1 #jjja ohm may readily be detected4 ,the ,wheatstone bridge method is the best for the accurate measurement of resistance2 but in resistance pyrometers it is sometimes advisable to sacrifice extreme accuracy in order to gain advantages in other directions1 as will be shown subsequently4 ,relation between the ,resistance of ,platinum and ,temperature4--,as platinum is the only feasible metal to use in the construction of resistance pyrometers1 it is essential that the effect of temperature on the resistance of this metal should be known4 ,difficulties were experienced1 in the early days of resistance pyrometers1 from the fact that different samples of platinum wire1 of varying degrees of purity1 gave widely differing results in this connection2 and no certainty was attained until #ahhf1 when ,professor ,callendar thoroughly investigated the subject1 and evolved a formula from which the temperature of a given kind of platinum could be deduced with great accuracy from the resistance4 ,in order to understand this formula and its application1 it will be necessary to consider the underlying principles upon which it is founded4 ,if the resistance of a platinum wire be measured at a number of standard gas-scale temperatures1 and the results depicted graphically by plotting resistances against corresponding temperatures1 the curve obtained is part of a parabola1 exhibiting a decrease in the rate at which the resistance increases at the higher temperatures4 ,a second platinum wire1 of different origin and purity1 and of the same initial resistance as the foregoing1 would furnish a curve which1 although parabolic1 would not overlap that obtained with the first wire4 ,the advance made by ,callendar was to deduce a formula from which the temperature of any kind of platinum wire could be deduced from its resistance1 after three measurements at known gas-scale temperatures had been determined4 ,the calibration of a resistance pyrometer was thereby reduced to three exact observations1 instead of a large number distributed over the scale2 and1 moreover1 the formula in question was found to give results of great accuracy over a wide range of temperature for any kind of platinum wire4 ,before dealing with ,callendar's formula1 the term 8degrees on the platinum scale0 will be explained4 ,such degrees are obtained by assuming that the increase of resistance of platinum is uniform at all temperatures2 that is1 that the temperature-resistance curve is a straight line1 and not a parabola4 ,for example1 a piece of platinum wire of #bf ohms resistance at #j ,c4 will show an increase to #cf ohms at #ajj ,c4--an addition of #a ohm for #ajj4 ,we now assume that a further augmentation of #a ohm1 bringing the total to #df ohms1 will represent an increase of #ajj1 or a temperature of #bjj4 ,similarly1 a total resistance of #ef ohms would indicate #cjj1 and #abf ohms #ajjj4 ,the temperature scale obtained by this process of extrapolation is called the 8platinum scale10 and differs considerably from the true or gas scale1 the difference becoming greater as the temperature rises4 ,this is indicated in fig4 #cb1 in which ,a represents the true parabolic relation between resistance and temperature1 and ,b the assumed straight-line relation4 ,reading from curve ,a1 the temperature corresponding to #h ohms resistance is #fjj ,c42 but from ,b the same resistance is seen to represent only #ede ,c41 which is the 8temperature on the platinum scale0 to which this resistance refers4 ,an inspection of fig4 #cb shows that at all temperatures1 except between #j and #ajj1 the platinum-scale readings for given resistances are less than those indicated on the gas scale4 ,callendar's formula is expressed in terms of the difference between the gas-scale and platinum-scale readings1 and takes the form t - p 7t_/#ajj7b - 7t_/#ajj71 where t temperature on the gas scale1 p temperature on the platinum scale4 a constant1 depending upon the purity of the wire4 ,illustration3 ,,fig4 #cb4--;,,connection ,,between ,,resistance ,,of ,,platinum ,,and ,,temperature3 ,a1 ,,on ,,gas ,,scale2 ,b1 ,,on ,,platinum ,,scale4 ,in order to determine the value of 1 it is necessary to measure the resistance of the wire at #j1 #ajj1 and a third temperature1 which should be considerably above #ajj4 ,the readings at #j and #ajj are requisite to establish the platinum scale of temperatures2 the third reading is required to calculate the value of 1 as p and t are equal at #j and #ajj1 these points forming the basis of both scales4 ,an example is appended to make this matter clear4 ,example4--,a platinum wire has a resistance in ice of #bf ohms2 in steam1 #cf ohms2 in boiling sulphur1 #fhae ohms4 ,to find the value of 1 the boiling point of sulphur being #ddde on the gas scale4 ,since an increase of 7#cf - #bf7 #a ohm is produced by #ajj1 the increase observed in boiling sulphur1 7#fhae - #bf7 #dbae ohms1 will represent a temperature1 on the platinum scale1 of 7#dbae #ajj7_/a #dbae p4 ,applying ,callendar's formula1 7#ddde - #dbae7 7#ddde_/ajj7b - 7#ddde_/ajj7 the value of is found to be #ae4 ,callendar1 in his experiments1 employed the boiling point of sulphur for the third point1 and determined this temperature on the gas scale with great accuracy1 ,the necessity for extreme precision in applying this formula is made clear by noting the effects on the value of resulting from small differences in the figures chosen in the above example4 ,if1 for instance1 the boiling point of sulphur on the gas scale were taken at #b lower1 or #ddbe1 the value of would work out to #acg2 and the error at #abjj ,c4 thus caused would amount to #ag4 ,the same discrepancy would be observed if the resistance in boiling sulphur were taken as #fhce ohms1 an error of #jjb ohm2 and a still greater error would result if the difference in resistance at #j and #ajj were measured as #jii ohm instead of #a ohm4 ,from an extensive experience of the difficulties attendant on correctly determining the value of 1 the author has found that no reliable result can be obtained unless measuring instruments of the highest precision are used1 and elaborate precautions taken to ensure the exact correction for alterations in the boiling points of water and sulphur occasioned by changes in atmospheric pressure4 ,unless the necessary facilities are at hand1 an operator would be well advised to standardize a resistance pyrometer by taking several fixed points and drawing a calibration curve1 after the manner recommended for a thermo-electric pyrometer4 ,if a resistance pyrometer be calibrated so as to read in platinum-scale degrees1 and the value of be known for the wire1 the correct gas-scale temperatures may be calculated from ,callendar's formula4 ,the table on next page gives the results of a number of calculations made in this manner4 ,changes in ,resistance of ,platinum when constantly ,heated4--,the resistance of platinum undergoes a gradual change when the wire is kept continuously above a red heat2 and if the temperature exceed #ajjj ,c4 the change becomes very marked after a time1 leading to serious errors in temperature indications when used in a pyrometer4 ,the alteration under notice is due1 as shown by ,sir ,william ,crookes1 to the fact that platinum is distinctly volatile above #ajjj ,c41 and hence the diameter of the wire diminishes4 ,this variation constitutes a serious drawback to the use of resistance pyrometers for temperatures exceeding #ajjj ,c4 ,,comparison ,,of ,,gas ,,and ,,platinum ,,scales4 #ae4 ,platinum ,thermometer,air ,thermometer ,reading ,difference ,reading 7,pt474 t 7deg4 ,c474 7t ,pt474 -#ajj -#iga #bi #j #j #j #ej #dif -#jjd #ajj #ajj #j #bjj #bjca #ca #cjj #cjih #ih #djj #dbjb #bjb #ejj #ecdi #cdi #fjj #fedd #edd #gjj #ggid #gid #hjj #iajg #aajg #ijj #ajdid #adid #ajjj #aaigj #aigj #aajj #aceej #beej #abjj #aebfg #cbfg #acjj #agafj #dafj ,terms used in ,resistance ,pyrometry4--,following on the researches of ,callendar and others1 certain terms relating to resistance pyrometers have come into use1 and will now be defined4 7#a7 ,the ,fundamental ,interval is the increase in resistance between #j ,c4 and #ajj ,c1 or ,r#ajj-,r#j4 ,it should be remembered that the increases between #bjj and #cjj1 or #hjj and #ijj1 all temperatures being taken on the gas scale1 differ from the fundamental increase4 7#b7 ,the ,fundamental ,coefficient is that fraction of the resistance at #j ,c4 by which it increases per degree between #j and #ajj1 on the average1 or 7,r#ajj - ,r#j7 4 7,r#j #ajj7 ,this figure is in reality the average temperature coefficient between #j and #ajj4 ,for pure platinum the value is or #a_/bfj or #jjjchdf4 7#c7 ,the ,fundamental ,zero is the temperature1 on the platinum scale1 at which the resistance would vanish2 it is evidently the reciprocal of 7#b71 prefaced by a minus sign1 or 7,r#j #ajj7 4 7,r#ajj - ,r#j7 ,for pure platinum this temperature would be -#bfjp1 since it is assumed that the average increase or decrease per degree holds throughout2 that is1 for every degree the metal is cooled the loss of resistance is taken to be #a_/bfj the resistance at #j4 ,hence at -#bfjp the resistance1 on this assumption1 would vanish4 7#d7 ,the ,difference ,formula is the expression which gives the relation between gas-scale and platinum-scale temperatures1 or t - p 7t_/#ajj7b - 7t_/#ajj7 ,this formula has already been fully dealt with4 7#e7 ,the ,platinum ,constant is in the above expression4 ,the value for pure platinum is about #ae1 but small quantities of impurities may alter the figure considerably4 ,the truth of the formula 7#d71 however1 is unaffected by changes in 1 as p would be correspondingly altered4 ,illustration3 ,,fig4 #cc4--,,platinum ,,resistance ,,pyrometer4 ,practical ,forms of ,resistance ,pyrometers4--,a typical form of resistance pyrometer1 made by the ,cambridge and ,paul ,instrument ,company1 is illustrated in fig4 #cc4 ,the coil of platinum wire is wound round the edges of a mica framework1 made of two strips of mica fastened at right angles so as to form a in section4 ,this method of winding is due to ,callendar1 who discovered that mica was chemically inert towards platinum1 even at high temperatures4 ,the leads1 also of platinum wire1 pass from the coil through mica washers to terminals fastened to the boxwood head4 ,a second wire1 not connected with the coil1 but identical in length and diameter with the ordinary leads1 is bent into two parallel branches1 which are passed through the mica washers side by side with the leads1 and are brought to a second pair of terminals in the head4 ,the function of this wire is to compensate for changes in the resistance of the leads when heated1 by opposing the compensating wire to the pyrometer in the measuring arrangement1 when the resistance of the leads and wire1 being equal1 will cancel1 the resistance actually measured being in consequence that of the coil only4 ,fig4 #cd shows the connections for a ,wheatstone bridge when this method of compensation is employed1 a and b representing two equal fixed resistances1 ,p the pyrometer coil1 x the leads1 ,l the compensating wire1 and d the adjustable resistance4 ,when no deflection is observed on the galvanometer1 a x ,p 1 b ,l d and since a b and x ,l1 it follows that ,p d4 ,the protecting tube used by the ,cambridge and ,paul ,instrument ,company is made of porcelain1 which is found to shield the platinum completely from the furnace gases1 but is extremely fragile1 and for workshop use should be protected by an outer iron sheath4 ,illustration3 ,,fig4 #cd4--,,wheatstone ,,bridge ,,as ,,used ,,with ,a ,,resistance ,,pyrometer4 ,resistance pyrometers made by other firms differ in detail from the foregoing4 ,in the ,siemens pyrometer the coil is wound on special fireclay1 and protected by an iron sheath1 the space between the coil and the sheath being filled with magnesia1 which effectively prevents the corrosion of the platinum2 and compensation is effected by means of a single wire passing down the centre and connected to one end of the coil1 a special form of ,wheatstone bridge being used to take the measurement4 ,in the instruments made by ,r4 ,w4 ,paul the coil is made of flat strip rolled out from wire1 wound on mica1 and protected by a silica tube and outer iron sheath4 ,the ,leeds-,northrup ,company of ,philadelphia employ a rod of obsidian on which to wind the coil1 and also make a form in which the coil is wound so as to be self-sustaining1 thus dispensing with the support4 ,in all cases the coil is wound non-inductively1 i4e4 the wire is doubled before making into a spiral4 ,the zero resistance of a given instrument depends upon the accuracy of the measuring appliances used1 and upon the degree of precision it is desired to attain4 ,if1 for example1 it is intended to read to #a;,c41 with appliances capable of measuring to #a_/ajj of an ohm1 a convenient zero resistance is #bf ohms2 it being found that with pure platinum the resistance rises from #bf ohms at #j to #cf ohms at #ajj ,c41 an increase of #a_/ajj of an ohm for #a ,c4 ,with coarser measuring arrangements1 for the same degree of precision1 a correspondingly higher zero resistance will be required2 thus if #a_/be ohm be the least amount detectable by the measuring device1 a zero resistance of #ajd ohms would enable #a ,c4 to be observed4 ,it is evident that a suitable zero resistance may be calculated similarly in all cases when the limit of the measuring appliance is known1 and the minimum temperature interval specified4 ,for work above a red heat1 the leads from the coil should always be made of platinum4 ,copper leads1 when heated1 give off vapour in sufficient quantity to attack the platinum2 and the same applies to a greater degree to all kinds of solder4 ,for low temperature work1 however1 copper leads may be used1 thus reducing the cost of the instrument4 ,mica1 above #ajjj ,c41 tends to crumble2 and most forms melt at #acjj ,c4 or lower2 hence a mica-wound instrument should not be used continuously above #ajjj ,c4 ,the fireclay winding used by ,siemens permits of occasional readings being taken up to #adjj ,c41 and the same applies to wires wound on obsidian 7melting point #aeej ,c471 or those in which the coil is self-sustaining4 ,as previously mentioned1 however1 alterations in the platinum itself render continuous readings above #ajjj ,c4 inaccurate after a short time4 ,it has been pointed out that with accurate measuring devices1 a resistance corresponding to a change of #a ,c4 can be measured2 and it might appear at first sight that the resistance method is considerably more accurate in practice than the thermo-electric4 ,if a perfectly constant temperature were to be measured1 a resistance pyrometer would undoubtedly give a closer indication2 but constancy to #aj ,c4 is seldom possible with gas-fired or coal furnaces or other hot spaces in which pyrometers are used4 ,the accuracy of a pyrometer under workshop conditions therefore depends upon the rapidity with which it responds to temperature fluctuations1 which condition will evidently be influenced by the thermal conductivity of the sheath4 ,as it is necessary to protect a resistance pyrometer with a porcelain or silica sheath1 both of which are poor conductors of heat1 this instrument is in consequence not capable of following a rapidly changing temperature4 ,the same applies to the magnesia packing used in the ,siemens form2 whereas a thermo-electric pyrometer is often sufficiently shielded by an iron tube1 which transmits heat with a fair degree of freedom4 ,the superior delicacy of the resistance method is therefore nullified by the sluggishness of its indications2 and for reading changing temperatures the thermo-electric pyrometer is at least equally accurate4 ,if1 however1 a constant temperature can be obtained1 as in the determination of melting points1 or when using experimental furnaces capable of exact regulation1 the steady temperature reading may be secured with greater precision by using the resistance pyrometer4 ,indicators for ,resistance ,pyrometers4--,all existing indicators for resistance pyrometers are in reality outfits for measuring resistance1 either by the ,wheatstone bridge1 differential galvanometer1 or other method1 the resistance being translated on the dial into corresponding temperatures4 ,typical examples will now be described4 ,illustration3 ,,fig4 #ce4--,,siemens' ,,dial ,,indicator4 ,siemens' ,indicator4--,this instrument is based upon the ,wheatstone bridge principle1 and is shown in fig4 #ce4 ,the galvanometer is mounted in the centre of the dial1 round the edge of which is fixed a ring on which the adjustable resistance is wound in spiral form4 ,suitable terminals are provided1 duly labelled1 to which the battery1 pyrometer leads1 and compensator are attached4 ,a brass arm1 movable about the centre of the dial1 terminates in a tapping-key which moves over the adjustable resistance2 the key being placed in the battery circuit4 ,the fixed known resistances are located in the interior of the indicator4 ,the adjustment consists in moving the key round the circumference until1 on tapping1 no deflection is obtained on the galvanometer4 ,the pointed end of the movable arm then indicates the temperature of the pyrometer on the dial1 which is marked in temperatures corresponding to the resistance opposed to the pyrometer for different positions of the key4 ,in taking a reading1 the operator is guided by the fact that when the temperature indicated is too high1 the movement of the galvanometer needle will be in one direction2 whereas if too low an opposite deflection will be given4 ,the intermediate position of no deflection must then be found by trial2 and the procedure should not occupy more than two minutes if the observer possess an approximate notion of the temperature to be measured4 ,illustration3 ,,fig4 #cf--,,whipple',s ,,indicator4 ,whipple's ,indicator4--,this instrument 7fig4 #cf7 is employed by the ,cambridge and ,paul ,instrument ,company1 and is also a form of ,wheatstone bridge4 ,the pyrometer leads and compensator are connected to properly labelled terminals ,t1 and the battery to other terminals at the opposite side of the box4 ,the pointer of the galvanometer is visible through the small window ,b1 and a battery of two dry cells is placed at the side of the box4 ,the fixed resistances are contained in the interior1 and the adjustable resistance consists of a continuous wire wound on a drum1 which may be rotated by the handle ,h4 ,the shaft connecting ,h with the drum is screwed1 and works in a nut1 so that the turning of ,h produces a spiral movement of the drum4 ,the adjustment consists in rotating ,h until1 on tapping the key ,f1 no deflection of the galvanometer pointer is observed4 ,the temperature of the pyrometer is then read off directly from a paper scale wound round the drum and rotating with it1 visible through the window ,a1 the reading being indicated by a fixed pointer4 ,this arrangement forms a compact and convenient indicator4 ,illustration3 ,,fig4 #cg4--,,the ,,harris ,,indicator4 ,the ,harris ,indicator4--,in the ,siemens and ,whipple indicators it is necessary1 before a reading can be taken1 to adjust a resistance until the galvanometer shows no deflection--an operation which takes up time and requires a fair amount of skill4 ,this is obviated in the ,harris indicator1 made by ,r4 ,w4 ,paul1 and shown in fig4 #cg4 ,this instrument is a special form of ohmmeter1 which automatically indicates the resistance of the pyrometer by the movement of the pointer2 the scale1 however1 being divided so as to read corresponding temperatures4 ,in this indicator the scale may be made to notify an excess temperature--say #ajj--;above a given fixed number1 and hence is capable of yielding an exact reading over the working range for which it is used4 ,it may also be connected so that the whole scale represents the complete range--say #j to #ajjj ,c4--or other specified interval4 ,the advantage possessed by this instrument is that the manipulation is much simpler than in the indicators previously described4 ,the ,leeds-,northrup ,indicator4--,in this apparatus the ,wheatstone bridge principle is employed1 but the galvanometer is provided with a scale divided or temperatures4 ,coils are provided which correspond to an increase of resistance due to a rise of #ajj ,c4 on the part of the pyrometer1 and by inserting these coils in the circuit the temperature is obtained to the nearest #ajj4 ,if the temperature were exactly at an even hundred--say #gjj--the pointer of the galvanometer would be at zero on its scale2 but if now the temperature rose1 the system would no longer be balanced1 and the galvanometer pointer would move over its scale by an amount depending upon the potential difference at its terminals4 ,a very sensitive galvanometer would give a movement to the end of its scale with a slight alteration from the correct balance of the system2 but by using a coarser instrument the pointer would remain within bounds2 and the greater the increase of resistance1 the larger would be the deflection4 ,it is possible1 in such a case1 to divide the galvanometer scale to read temperatures corresponding to a given increase above that of the coils placed in the circuit4 ,in one form of the ,leeds-,northrup indicator1 the whole scale is thus divided to read #ajj1 and the reading is obtained by adding the figure shown on the galvanometer to the hundreds represented by the coils inserted4 ,in another form the galvanometer has a central zero1 and its scale is divided both right and left1 one side giving the number of degrees above1 and the other below1 the nearest hundred4 ,the observations are thus much simpler than in the case where adjustment to the condition of no deflection is requisite4 ,siemens' ,differential ,indicator4--,this form of indicator is still in use1 and consists of a differential galvanometer and box of resistance coils1 connected as shown in fig4 #cj4 ,by adjusting the coils until no deflection is produced1 the resistance of the pyrometer is obtained1 and the corresponding temperature read off from tables provided4 ,this form of indicator is preferred by some users1 but it is less sensitive than the more recent ,wheatstone bridge indicator made by this firm 7fig4 #ce71 and equally difficult to manipulate4 ,recorders for ,resistance ,pyrometers4--,the value of records in high-temperature work has led to the invention of recording mechanisms for use with resistance pyrometers4 ,the form in common use in ,britain is that devised by ,callendar1 shown in fig4 #ch1 and consists of a mechanism for restoring automatically the balance of the resistances in a ,wheatstone bridge circuit1 in such a manner as to indicate the existing resistance on a chart4 ,to this end the moving coil of the galvanometer carries a boom1 or contact-arm1 which1 on swinging to the right or left1 completes one of two electric circuits4 ,the closing of either circuit brings into action a clockwork mechanism1 which causes a slider carrying a pen to move over the bridge wire until the balance is restored1 and incidentally to produce a mark in ink on a paper wound on a drum1 which rotates at a known speed4 ,when the resistance of the pyrometer is balanced1 the galvanometer boom will be in a central position1 and the slider at rest2 whereas a rise in temperature causing an increase in the resistance of the pyrometer1 will result in the boom swinging over and completing the circuit1 which introduces more resistance in opposition to the pyrometer4 ,a fall in temperature will similarly result in the liberation of the second mechanism1 owing to the boom swinging in the opposite direction1 with the result that the slider moves so as to oppose a less resistance to the pyrometer4 ,if the chart be divided horizontally into equal spaces1 representing equal increments or decrements of resistance1 they may be marked to represent degrees on the platinum scale1 which may be translated into ordinary degrees by reference to a conversion table4 ,in careful and skilled hands this recorder gives excellent results1 and the value of the records obtained is clearly shown by an inspection of the example shown in fig4 #ci1 which represents the fluctuations of an annealing furnace during a period of nine hours4 ,it will be noted that during the period covered by workman ,a the furnace has received constant and careful attention2 but workman ,b has evidently neglected his duty conspicuously at two separate times4 ,illustration3 ,,fig4 #ch4--;,,callendar',s ,,recorder4 ,illustration3 ,,fig4 #ci4--,,record ,,obtained ,,with ,,callendar',s ,,recorder4 ,the ,leeds-,northrup ,recorder4--,in the ,callendar recorder the boom which completes the electric circuits is pressed against the contact-surface merely by the small force due to the axial twist of the galvanometer coil1 which necessitates the use of delicate mechanism if certainty of action is to be secured4 ,a surer contact is secured in the instrument made by the ,leeds-,northrup ,company of ,philadelphia1 by means of an intermittent action which will be understood from the annexed drawing 7fig4 #dj74 ,the boom from the galvanometer terminates in a platinum tip1 ,p1 which moves between two blocks1 the upper of which consists of two pieces of silver1 ,a and ,b1 separated by a strip of ivory1 ,i1 whilst the lower block1 ,c1 is another piece of silver1 which is moved periodically up and down by an electro-magnetic contrivance not shown in the drawing4 ,when the galvanometer is at the position of balance1 the tip of the boom is beneath the ivory piece ,i2 and when ,c ascends the tip ,p is then squeezed on to the ivory1 and no current will then pass from the battery through either of the circuits ,e or ,f4 ,if1 however1 the point of the boom be beneath ,a1 owing to an alteration in the temperature of the pyrometer1 then on ,c rising the circuit through ,e will be completed2 and1 similarly1 if beneath ,b the circuit through ,f will be established4 ,the result in either case is to bring into action a mechanism which moves a slider1 carrying a pen1 over a resistance wire opposed to the pyrometer in such a manner as to restore the balance4 ,certainty of contact is thus secured1 which enables all the parts to be strongly made4 ,the actual recorder is shown in fig4 #da1 in which it will be seen that the slider carries an ordinary stylographic pen in contact with the chart4 ,this recorder is worked on the differential galvanometer method2 and the adjusting resistance1 over which the slider moves1 consists of a manganin wire wound on a tapered core1 such that horizontal movements represent equal changes of temperature1 and not of resistance1 thus obviating the necessity of translating platinum-scale readings into ordinary degrees4 ,concordant and accurate results1 coupled with robust construction1 are claimed for this instrument by the makers4 ,the other type of recorder made by this firm 7fig4 #bf7 may also be used in conjunction with a resistance pyrometer4 ,in this case the movements described introduce or cut out resistance opposed to the pyrometer in a ,wheatstone bridge circuit1 until the balance is restored4 ,illustration3 ,,fig4 #dj4--,,principle ,,of ,,leeds-,,northrup ,,recorder4 ,illustration3 ,,fig4 #da4--,,leeds-,,northrup ,,recorder4 ,paul's ,recorder4--,this instrument1 as used for thermo-electric pyrometers1 has already been described4 ,by replacing the galvanometer by a ,harris indicator1 and using a suitable chart1 the same mechanism serves to record the indications of a resistance pyrometer4 ,installations of ,resistance ,pyrometers4--,the resistance method cannot be so readily applied to the purpose of a centrally controlled installation as the thermo-electric1 owing to the difficulty of producing a set of pyrometers exactly equal in resistance4 ,the introduction of the ohmmeter method of measuring resistances1 as in the ,harris indicator 7page #abb71 has1 however1 rendered this project feasible1 as it is possible in this arrangement to bring a set of pyrometers to a common resistance by adding the requisite amount in the form of a wire of negligible temperature coefficient4 ,several instruments1 brought thus to a zero resistance of #c ohms1 for example1 may then be wired up to a ,harris recorder1 and will give closely identical results4 ,for various reasons1 however1 a thermo-electric installation is preferable4 ,management of ,resistance ,pyrometers4--,it is not advisable to use resistance pyrometers continuously above #ijj ,c4 7#afej ,f471 although an occasional reading may be taken up to #abjj ,c4 7#baij ,f474 ,great care must be taken that metallic vapours or furnace gases do not find access to the interior1 and for this reason a cracked or defective sheath should immediately be replaced4 ,as the resistance gradually changes1 even when #ijj ,c4 is not exceeded1 a reading should be checked at a fixed point in the neighbourhood of the working temperature1 and allowance made for the observed error4 ,another method of correction recommended by some makers is to measure the resistance in ice1 and to note how much this differs from the zero resistance noted when the indicator was marked1 and to correct by simple proportion4 ,thus1 if the observed resistance in ice were #ajb ohms1 the original having been #ajj the reading on the indicator would be multiplied by #ajj_/ajb #jih1 a correction which assumes a linear relation between resistance and temperature1 and is therefore only approximate4 ,generally speaking1 any serious defect entails the sending of the instrument to the maker1 as a special degree of skill is required to execute the necessary repairs4 ,as the indicators are usually not automatic in action1 care should be taken in the manipulation not to damage any part1 particularly the galvanometer2 and it is advisable not to trust the instruments to unskilled observers4 ,the remarks applying to recorders and protecting sheaths in relation to thermo-electric pyrometers 7page #ib7 apply equally in this case4 ,special ,uses of ,resistance ,pyrometers4--,in all cases in which an exact reading is required1 and a steady temperature can be secured1 the resistance pyrometer can be used to advantage4 ,thus for accurate determinations of melting points and boiling points1 or for exact readings of temperatures in experimental furnaces1 a resistance pyrometer is superior to appliances of other kinds4 ,on the other hand1 it is not capable of responding to changes with the same rapidity as a thermal junction1 and is therefore inferior for such purposes as the determination of recalescence points1 or the temperature of exhaust gases from an internal combustion engine4 ,the resistance method may be applied to atmospheric and very low temperatures 7liquefied gases1 etc471 to measure steady conditions with accuracy1 nickel wire being sometimes used instead of platinum below #djj ,c4 ,many cold stores are fitted with resistance thermometers1 the temperature being read directly on the galvanometer1 which is placed across a ,wheatstone bridge1 and shows a deflection which depends upon the amount by which the bridge is thrown out of balance4 ,changes in the temperature of the resistance element may thus be read accurately4 ,whether the resistance method is suitable to a given purpose must be decided by the three factors3 7#a7 temperature to be measured1 which must not exceed #ajjj ,c4 continuously2 7#b7 degree of accuracy required 7a thermo-electric pyrometer giving results to #aj ,c472 7#c7 stability of the temperature measured1 rapid changes not being readily shown by resistance pyrometers4 ,one advantage of resistance pyrometers is that the readings are independent of the resistance of the wires used to connect the pyrometer with the indicator1 as such wires are duplicated and opposed to each other in the measuring device1 their resistance being thereby cancelled4 ,hence the same reading is obtained at any distance1 and1 in addition1 the head of the pyrometer may vary in temperature to any extent without altering the reading4 ,these are points of superiority over the thermo-electric method2 but1 on the other hand1 resistance pyrometers and indicators are more costly1 more fragile1 more difficult to repair1 require more skilled attention1 and are more liable to get out of order when used for industrial purposes4 ,these drawbacks have resulted in restricting the use of resistance pyrometers to special purposes1 the general run of observations being conducted by means of thermo-electric pyrometers4 ,,chapter ,v ,,radiation ,,pyrometers ,general ,principles4--,it is a common experience that the heat radiated by a substance increases as its temperature rises2 and it would obviously be an advantage if the temperature of a hot body could be deduced from the intensity of its radiations1 as the measurement could then be made from a distance1 without the necessity of placing a pyrometer in contact with the heated substance4 ,at temperatures above #ajjj ,c41 when difficulties are experienced either with the metals or protecting sheaths of thermo-electric or resistance pyrometers1 the advantage gained would become more conspicuous as the temperature increased4 ,a brief survey of our knowledge of the relations between radiant energy and temperature will indicate how this desired end may be achieved4 ,any substance at a temperature above absolute zero 7-#bgc ,c47 radiates energy to its surroundings by means of ether waves4 ,below #djj ,c4 these waves produce no impression on the retina of the eye1 and the radiating body is therefore invisible in a dark room4 ,above #djj ,c41 however1 a proportion of visible waves are emitted2 and as the temperature rises the effect on the retina is enhanced1 and the body increases in brightness4 ,the difference between the non-luminous and luminous waves is merely one of wave-length1 the shorter wave-lengths being visible to the eye2 and both represent radiant energy4 ,in addition to giving out radiant energy1 a substance receives waves from its surroundings1 which it absorbs in greater or less degree1 and which when absorbed tend to raise the temperature of the receiving substance4 ,a number of objects in a room1 all at the same temperature1 are therefore radiating energy to one another1 and equality of temperature is established when each object receives from its surroundings an amount of energy equal to that which it radiates4 ,a hot substance radiates more energy than a cold one2 thus if a hot iron ball be hung in a room it will radiate more energy to its surroundings than it receives from them1 and will therefore cool until the outgoing energy is balanced by the incoming1 when its temperature will be equal to that of the other objects in the room4 ,the rate at which a substance emits or takes up radiant energy depends upon the nature of its surface4 ,a rough1 black surface1 such as may be obtained by holding an object in the smoke from burning camphor1 radiates and absorbs heat with greater freedom than any other2 whilst a polished1 metallic surface1 which acts as a reflector1 is worst of all in these respects4 ,even a surface of finely divided soot1 however1 does not completely absorb all the radiations which fall upon it1 but exhibits a small degree of reflection4 ,an 8absolute black surface10 if such could be found1 would be totally devoid of reflecting power1 and would absorb all the radiant energy incident upon it2 and conversely would radiate all energy reaching it from its under side1 without reflecting any back1 or allowing any to pass through in the manner that light waves are transmitted through a transparent substance4 ,no such perfect surface is known2 but1 as ,kirchoff showed1 it is possible to make a radiating arrangement which will give the same numerical result for the energy radiated as would be obtained by a perfect surface at the same temperature4 ,such an arrangement is termed a 8black body10 and radiations from it are designated 8black-body radiations40 ,illustration3 ,,fig4 #db4--;,,black-,,body ,,radiations4 ,any enclosure1 if opaque to radiant energy1 and kept at a constant temperature1 constitutes a black body1 and radiations received from the interior through a small opening in the side are black-body radiations4 ,fig4 #db represents such an enclosure2 in which1 to show the application to pyrometry1 a body ,a is indicated opposite to an opening in the side1 through which radiations escape from the surface of ,a4 ,if this surface were 8perfect10 all the waves falling upon it would be completely absorbed and completely radiated2 but to prevent change of temperature the energy radiated must balance the energy received4 ,if1 on the other hand1 the surface of ,a were a polished metal1 the waves falling upon it from the sides of the enclosure would in the main be reflected2 but here again the energy leaving the surface must equal the amount received if the temperature be constant4 ,it follows1 therefore1 that if no alteration in temperature occur1 the energy leaving the surface of ,a is independent of the nature of that surface2 and the amount escaping through the opening will therefore be the same1 whatever be the character of the surface opposite the opening4 ,with a good radiating surface the rays from the enclosure will first be absorbed and then radiated through the opening2 in the case of a poor radiating surface1 the rays will be directly reflected through the opening2 the total energy escaping being the same in either case4 ,it will be seen later that radiation pyrometers are based upon black-body radiations2 and it is important to note that the arrangement under discussion is realised in a furnace at a constant temperature1 in which ,a might represent an object such as a block of steel4 ,it happens1 therefore1 that the condition of perfect radiation is attained by the appliances in everyday use2 and1 moreover1 black-body radiations can always be secured by placing a tube1 closed at one end1 in the heated space1 and receiving the radiations through the open end2 for this again represents an enclosure at a constant temperature4 ,similarly1 radiations from a solid in the interior of the tube of the electric furnace shown in fig4 #bi will be of the same description1 and we can therefore apply with accuracy any instrument based upon black-body radiations1 knowing that the same may be readily realised in practice4 ,the law connecting the energy radiated by a substance1 under given conditions1 with its temperature1 was variously stated by different observers until ,stefan1 in #ahgi1 deduced the true relation from certain experimental data obtained by ,tyndall4 ,stefan concluded that the figures given by ,tyndall indicated that the energy radiated by a given solid varied as the fourth power of its absolute temperature4 ,numerous experiments1 under different conditions1 showed that the fourth-power law did not apply to all kinds of surfaces or circumstances2 but a strong confirmation of its truth when applied to black-body radiations was forthcoming in #ahhd1 when ,boltzmann showed1 from thermodynamic considerations1 that the quantity of energy radiated in a given time from a perfect radiator must vary as the fourth power of its absolute thermodynamic temperature4 ,certain assumptions made by ,boltzmann in this investigation were subsequently justified by experiment2 and numerous tests under black-body conditions have since amply verified the law4 ,it is upon the ,stefan-,boltzmann law that radiation pyrometers are based2 the energy received by radiation from the heated substance1 under black-body conditions1 being measured by the instrument1 and translated into corresponding temperatures on its scale4 ,expressed in symbols1 the fourth-power law takes the form-- ,e ,k7,t#ad - ,t#bd71 where ,e is the total energy radiated2 ,t#a the absolute temperature of the black body2 ,t#b the absolute temperature of the receiving substance1 and ,k a constant depending upon the units chosen4 ,if ,e be expressed as watts per square centimetre1 the value of ,k is #ef #aj-ab2 if in calories per square centimetre per second1 the value is #acd #aj-ab4 ,the introduction of the temperature of the receiving substance1 ,t#b1 is rendered necessary by the fact1 previously cited1 that energy will be radiated back to the hot body1 and the net loss of energy will evidently be the difference between that which leaves it and that which returns to it from the receiving substance4 ,if ,t#b were absolute zero1 the energy leaving the black body would be ,,kt#ad2 whereas if ,t#b were equal to ,t#a1 the loss of energy would be nil1 as a substance cannot cool by radiation to a lower temperature than its surroundings4 ,the temperatures ,t#a and ,t#b refer to the thermodynamic scale 7page #i71 but as the gas scale is practically identical1 ,centigrade degrees may be used1 measured from absolute zero1 or -#bgc4 ,an example is appended to illustrate the application of the law3-- ,example4--,to compare the energy radiated through an opening in the side of a furnace at temperatures of #ebg1 #gbg1 and #ibg ,c4 respectively1 to surroundings at #bg ,c4 ,the quantities will be as ,k7#hjjd - #cjjd7 3 ,k7#ajjjd - #cjjd7 3 ,k7#abjjd - #cjjd74 since #bgc must be added to each temperature to convert into absolute degrees4 ,dividing each by ,k1 and expanding in each case1 the ratio becomes 7#djif - #ha7 #ajh 3 7#ajjjj - #ha7 #ajh 3 7#bjgcf - #ha7 #ajh4 ,dividing each by #ajh and subtracting1 the result is #djae 3 #iiai 3 #bjfee1 or #a 3 #bdg 3 #eab4 ,it will be noted in the above example that the effect of the surrounding temperature1 taken as #bg ,c41 is small in quantity1 and becomes proportionately less as the temperature of the furnace increases4 ,if ,t#b had been ignored in the calculation1 the amounts of energy radiated would have appeared as #a 3 #bdd 3 #ejf4 ,it will be seen later1 that in calculating the temperature scale of a radiation pyrometer1 the temperature of the surroundings is for this reason not taken into account4 ,fig4 #dc is a graphic illustration of the fourth-power law4 ,illustration3 ,,fig4 #dc4--;,,energy ,,radiated ,,by ,a ,,black ,,body ,,at ,,different ,,temperatures4 ,when the relation between temperature and quantity of energy radiated is known1 any instrument which will indicate the amount of the radiations it receives may be used to measure temperatures4 ,the ray1 for example1 may be focused on a thermal junction1 which will be heated in proportion to the amount of energy incident upon it1 and when connected to a millivoltmeter will cause deflections proportional to the energy it receives4 ,a thin strip of metal might be used in place of a junction1 and by measuring its resistance the heating effect of the radiations1 and hence the amount thereof1 may be deduced4 ,a third method would be to focus the rays on to a compound strip of two metals1 which by altering in shape could be made to furnish a clue to the quantity of energy received by it4 ,in theory1 it is only necessary to allow the radiations to fall on the working part of any instrument for measuring low temperatures1 when the rise in temperature produced may be taken as proportional to the energy received1 and the thermal condition of the radiating body deduced from the fourth-power law4 ,in practice1 however1 it is desirable that the receiving thermometer should be small in size2 of low thermal capacity1 so as to respond rapidly2 and capable of giving a sensitive indication--hence an ordinary mercury thermometer would be unsuitable for this purpose4 ,a thermopile1 placed at a fixed distance1 would fail owing to the cold junctions gradually warming up by conduction through the pile4 ,the part receiving the radiations should be coated with lamp-black1 so that practically all the waves impinging upon it1 whether luminous or non-luminous1 may be absorbed1 and the energy they represent utilised in producing a rise in temperature4 ,practical ,forms of ,radiation ,pyrometers3 ,fry's ,instruments4--,in the year #aijb ,fry introduced a pyrometer in which the rays were focused by the aid of a lens upon a small1 blackened thermal junction1 in the same way that the rays of the sun may be focused by a burning-lens4 ,the junction was connected to a special form of d',arsonval galvanometer1 which recorded the ,e4,m4,f4 developed4 ,by taking the readings of the galvanometer as proportional to the temperature of the junction--that is1 to the radiant energy impinging upon it--the temperature of the source could be calculated from the fourth-power law4 ,the drawback to the use of this instrument was the fact that a proportion of the rays was absorbed by the glass1 this proportion1 moreover1 varying at different temperatures1 so that the fourth-power law could not be applied with accuracy4 ,by using a fluorspar lens in place of glass1 this error was overcome1 but the cost of a good lens of this material being high1 its use in ordinary workshop practice was rendered prohibitive on account of the price4 ,a number of these pyrometers1 furnished with glass lenses1 and calibrated by comparison with a standard possessing a fluorspar lens1 were placed on the market1 but were superseded in #aijd1 when ,fry hit upon the plan of focusing the rays by means of a concave mirror1 thus overcoming the error due to absorption by the glass lens4 ,this plan1 which serves admirably1 has since been adopted in most radiation pyrometers4 ,fry's ,mirror ,pyrometer4--,this instrument is shown in longitudinal and also in cross section in fig4 #dd4 ,a concave mirror1 ,m1 which has a gilt reflecting surface1 is placed at one end of a metal tube1 and is fastened to a rack which engages in a pinion moved by the milled-head1 ,p1 so that on turning ,p1 a longitudinal movement is imparted to the mirror4 ,a small1 blackened thermal junction1 shown at the centre of the cross section1 and consisting of a copper disc to which wires of copper or iron and constantan are fastened1 receives the rays after reflection1 and may be brought into focus by suitably moving the mirror4 ,illustration3 ,,fig4 #dd4--;,f,,ry',s ,,mirror ,,pyrometer4 ,,section4 ,illustration3 ,,fig4 #de4--;,f,,ry',s ,,mirror ,,pyrometer4 ,,end ,,view4 ,the wires pass to terminals b and b on the outside of the tube1 from which leads are taken to the indicator4 ,in order to discover when the junction is in the focus of the mirror1 an eye-piece1 ,o1 is fitted in the end of the tube1 which enables the junction to be seen1 magnified1 through a hole in the centre of ,m4 ,by means of an optical device placed near the junction1 the image of the sighted object1 produced by ,m1 is reflected in two portions to the eye-piece ,o4 ,when the junction is exactly in the focus of ,m1 a circular image is seen round the junction2 when out of focus1 the appearance presented is that of two semi-circles not coinciding laterally4 ,the adjustment consists in moving the mirror until the separate semi-circles produce a continuous circle2 a method at once simple and definite4 ,the front end of the pyrometer is shown in fig4 #de1 in which it will be seen that the entrance may be partially closed by a diaphragm1 or left entirely open1 as required4 ,the diaphragm is used to cut off a definite proportion of the radiations1 and is used for very high temperatures1 at which1 with full aperture1 the indicator needle would be urged beyond the limits of the scale4 ,on the indicator two separate temperature scales are provided1 one referring to full1 and the other to partial aperture4 ,the same end might be achieved by inserting a suitable resistance in series with the indicator3 but in this case the junction might be unduly heated1 and possibly damaged thereby4 ,the proportions of the pyrometer are such that at the highest temperatures measured the heat incident on the junction never raises it above #aaj ,c4 ,although the intensity of radiations diminishes as the square of the distance1 the quantity impinging on the junction is1 within limits1 independent of the distance3 ,this arises from the property of concave mirrors with respect to the relation between the size of an image and the distance of the object producing it4 ,if r the radius of the mirror1 u the distance of the object1 and v the distance of the image1 both measured from the centre of the mirror1 the relation #a_/u #a_/v #b_/r holds for a concave mirror1 and when two of these are known the third may be calculated4 ,further1 if d be the linear dimension of an object1 and d#a that of its image1 the relation d_/d#a u_/v also holds1 and from these two expressions all the points arising in connection with the ,fry pyrometer may be determined1 as will best be made clear by examples4 ,example ,i4--,to find the position of the image of an object formed by a mirror of #f inches radius1 with object at distance 7a7 #aj feet1 7b7 #bj feet4 ,reducing to inches1 and applying in the formula #a #a #b #a #a #a 1 u v r #abj v #c and #a #a #a #bdj v #c from which the values of v are #c-a_/ac inches and #c-a_/bf inches respectively1 a difference of only #a_/bf of an inch4 ,if u were #f inches1 v would also be #f inches2 if u were infinity1 v would be #c inches4 ,the movement of the image1 when an object is brought towards it from a great distance1 would in the mirror under notice be from #c inches away to #f inches away1 and at distances of #aj feet and upwards would only differ in position by small fractions of an inch4 ,example ,,ii4--,to find the area of the image of a circular opening1 #a foot in diameter1 formed by a mirror of #f inches radius distant from the opening 7a7 #aj feet2 7b7 #bj feet4 ,since d u 2 then1 from the results of d#a v ,example #ab #abj at #aj feet distance1 d#a 7#c-a_/ac7 and #ab #bdj at #bj feet4 d#a 7#c-a_/bf7 ,hence the linear dimensions1 i4e4 the diameters of the circular images1 will be #jcjh and #jaeb inch respectively2 and the areas #jjgd and #jjahb square inch4 ,these areas are to each other practically as #d 3 #a4 ,that is1 the area of the image decreases in size directly as the square of the distance of the object2 the squares of the distances being #ajj and #djj1 or as #a 3 #d2 whereas the areas of the images are as #d 3 #a4 ,example ,,iii4--,to find1 for a #f-;inch mirror1 and a junction of #a_/ajth of an inch in diameter1 the greatest distance at which the mirror may be placed from an opening #a foot in diameter1 so as to give an image not less than the junction4 ,from ,example ,i it is evident that at any distance exceeding #bj feet the position of the image will only be a minute and negligible fraction over #c inches2 hence v may be taken as #c4 ,applying values in the formula d u and d#a v taking d#a as equal to the diameter of the junction1 #ja inch1 #ab u 1 and u #cfj inches1 or #cj feet4 #ja #c ,beyond this distance the image would be less than the junction4 ,the conclusions to be drawn from the foregoing examples are3 7#a7 that the amount of energy received by the junction does not vary1 provided the image overlaps it2 and 7#b7 that the limiting distance at which a correct reading can be secured is that at which the size of the image is equal to that of the junction4 ,thus1 taking distances of #aj and #bj feet1 as in ,example ,,ii2 at the former distance the energy striking the mirror is four times as great as with the latter2 but1 on the other hand1 the area of the image at #aj feet distance is four times as great as that obtained at #bj feet4 ,hence1 at the greater distance1 the proportion of the image impinging on the junction is four times as great1 and the fact that only the amount of energy strikes the mirror is thus counterbalanced4 ,all the reflected rays which fail to strike the junction are ineffective1 and pass out through the entrance of the tube4 ,illustration3 ,,fig4 #df4--;,f,,ry',s ,,spiral4 ,the two-scale form of instrument described above is extremely useful for general purposes1 but when all the temperatures to be controlled fall within the limit of one of the scales1 it is simpler and cheaper to dispense with the diaphragm1 and to use an indicator furnished with one scale only4 ,the single-scale mirror pyrometer is for this reason more generally employed for industrial purposes2 and the ,cambridge and ,paul ,instrument ,company now make a pivoted indicator for use with full aperture1 which is less liable to damage than one which possesses a suspended coil4 ,illustration3 ,,fig4 #dg4--;,f,,ry',s ,,spiral ,,pyrometer4 ,,section4 ,fry's 8,spiral0 ,radiation ,pyrometer4--,this instrument differs from the preceding merely in the fact that the rays are focused on a small spiral1 formed of a compound strip of two metals1 fixed at one end and furnished with a pointer at the free-moving end 7fig4 #df74 ,the effect of alterations of temperature on this spiral are to cause it to coil up or uncoil1 according to whether the temperature rises or falls4 ,this movement is magnified by the pointer1 the end of which moves over a dial graduated to read temperatures directly4 ,this arrangement is shown in section in fig4 #dg1 where ,c is the mirror1 ,e the eye-piece1 ,s the spiral1 ,p the pointer1 and ,d the dial1 viewed through the window ,w4 ,the appearance of the apparatus when viewed from the front is shown in fig4 #dh4 ,the advantage gained by the use of the spiral is that the instrument is self-contained1 no galvanometer being necessary2 but1 on the other hand1 the indications are not so exact1 an error of #bj ,c4 being probable at temperatures over #ajjj ,c4 ,in using this pyrometer1 it is observed that after focusing the hot substance1 the pointer moves rapidly for a time and then pauses1 after which it again commences to creep along the scale4 ,the temperature indicated at the moment the pause occurs is generally taken as the reading1 but this is not always correct4 ,illustration3 ,,fig4 #dh4--;,f,,ry',s ,,spiral ,,pyrometer4 ,,front ,,view4 ,the creeping movement is probably due to the whole instrument1 and the air in the interior1 becoming heated by the entering rays1 and by proximity to the hot source4 ,in a number of trials made by the author1 it was noticed that when the instrument was allowed to stand near the furnace for some time before using1 thereby attaining the temperature existing in the vicinity1 the 8creep0 almost entirely vanished4 ,all things considered1 the spiral form of ,fry's pyrometer must be regarded as more portable but less accurate than that in which the rays are received on a thermal junction4 ,foster's ,fixed-,focus ,radiation ,pyrometer4--,the necessity for focusing1 common to all ,fry's radiation pyrometers1 is obviated in ,foster's pyrometer1 which1 however1 cannot be used from so great a distance4 ,the principle involved in the fixed-focus pyrometer is that the amount of energy received by a concave mirror and focused on a thermal junction will not vary so long as the area of the surface sending rays to the mirror1 through a fixed opening1 increases as the square of the distance4 ,this will be understood from fig4 #di1 in which ,c is the mirror1 ,d a thermal junction fixed so as to be in the focus of the opening ,e ,f1 and ,a ,b the heated surface4 ,the lines joining ,e and ,f to the edge of the mirror intersect in a point ,g1 and provided the lines ,g ,e and ,g ,f1 if produced1 fall within the heated surface ,a ,b1 the quantity of energy falling on ,d will always be the same4 ,a cross section of the cone ,g ,a ,b is a circle2 and if ,a ,b be twice as far away from ,g as ,e ,f1 the areas of the circles of which ,a ,b and ,e ,f are diameters will be in the ratio #d3 #a4 ,but as ,a ,b is twice as far from ,g as ,e ,f1 the intensity of its radiations will be as #a3 #d2 and hence loss of radiating power is exactly balanced by increase in area4 ,illustration3 ,,fig4 #di4--,,principle ,,of ,,foster',s ,,fixed-,,focus ,,pyrometer4 ,in the actual instrument the tube in which the mirror is placed is blackened internally1 so that no rays reach the mirror by reflection from it4 ,the diameters of the opening ,e ,f and the mirror ,c are such that the perpendicular from ,g on to ,a ,b is ten times the length of ,a ,b4 ,hence1 if the heated object be #f inches in diameter1 the limiting distance of ,g is #aj #f #fj inches4 ,the position of the point ,g is indicated by a ring on the outside of the tube1 and in taking a measurement the tube is brought well within the distance prescribed1 which is in all cases ten times the diameter of the heated object4 ,temperatures are read from a galvanometer connected to the thermal junction1 the whole arrangement being portable1 as shown in figs4 #ej and #ea1 which represent the instrument in use4 ,illustration3 ,,fig4 #ej4--;,,foster',s ,,pyrometer1 ,,mounted ,,on ,,stand4 ,the advantages derived from the use of a fixed focus instrument are simplicity and cheapness2 but1 as many occasions arise in practice in which focusing on an object is a necessity1 ,foster's pyrometer must be regarded as a simplified apparatus not capable of the wider applications of ,fry's instruments1 but of great service in many cases4 ,whipple has recently adapted the ,fry spiral pyrometer to produce an instrument with a fixed focus1 by fastening the instrument to a fireclay tube1 on the closed end of which the pyrometer is permanently focused4 ,this form is specially useful for determining the temperature of molten metals1 into which the end of the fireclay tube is plunged1 thus giving true black-body conditions4 ,illustration3 ,,fig4 #ea4--;,,foster',s ,,pyrometer1 ,,in ,,use4 ,paul's ,radiation ,pyrometer4--,thwing1 in ,america1 has introduced a radiation pyrometer in which the rays from the furnace enter the wide end of a cone1 and by internal reflection are brought to the apex1 at which a thermal junction is located4 ,paul1 in this country1 has marketed a similar instrument1 the action of which is shown in fig4 #eb1 where ,e is a tube containing a polished cone1 ,c1 at the apex of which is fixed a thermal junction1 ,t4 ,rays from the hot source ,a ,a enter the tube at ,d1 and pass into the cone1 being finally reflected on to ,t1 which is connected to the indicator4 ,so long as the lines joining the outside of the cone with the extremities of the entrance ,d1 crossing at ,o1 fall within the hot source1 ,a ,a1 the reading will be the same at all distances4 ,fig4 #ec shows the actual pyrometer1 mounted on a tripod4 ,illustration3 ,,fig4 #eb4--,,principle ,,of ,,paul',s ,,radiation ,,pyrometer4 ,indicators for ,radiation ,pyrometers4--,when the radiations are focused on a thermal junction1 the temperature of which is raised in consequence1 the ,e4,m4,f4 developed is in accordance with the laws discussed in ,chapter ,,ii1 and any thermo-electric indicator1 if sufficiently sensitive1 will serve for the purposes of a radiation pyrometer4 ,the effect on the galvanometer is influenced by3 7#a7 the nature of the junction2 7#b7 the size of the mirror or cone2 and 7#c7 the highest temperature attained by the junction4 ,the indicators used in connection with radiation pyrometers are of the pivoted type1 which can now be made sufficiently sensitive to give full-scale deflection for a rise of #ajj ,c4 in the temperature of the junction4 ,for the junction itself1 ,heil's alloy 7zinc and antimony in atomic proportions7 partnered with constantan has been used1 owing to the high ,e4,m4,f4 developed2 but cases of deterioration of this alloy have been noted1 causing it to be replaced by some makers by iron4 ,two iron or copper constantan junctions in series give an ,e4,m4,f4 for a rise of #ajj ,c41 sufficient to work a pivoted indicator1 and are preferable to ,heil's couple for a radiation pyrometer4 ,illustration3 ,,fig4 #ec4--,,paul',s ,,radiation ,,pyrometer4 ,calibration of ,indicators for ,radiation ,pyrometers4--,the deflections on the indicators are due to the ,e4,m4,f4 generated1 which is proportional to the difference in temperature between the hot and cold junctions4 ,if both these are at the same temperature--say1 #bj ,c4--the deflection is zero2 and on allowing the radiations to fall on the hot junction its temperature is raised by an amount depending upon the intensity of the radiations--say1 to #ij ,c4 ,the deflection produced is then due to a difference of 7#ij-bj7 #gj1 the radiations having raised the temperature of the hot junction #gj above its surroundings4 ,if the surroundings 7including the cold junction or junctions7 had been at #ae to commence with1 the hot junction under the same conditions would have risen to #he1 giving again a difference of #gj1 and thus causing the same deflection as before4 ,provided both hot and cold junctions are located so as to attain the same atmospheric temperature in the absence of radiations1 a given quantity of energy impinging on the hot junction will always produce in it the same excess temperature1 and will therefore give rise to the same deflection at all ordinary atmospheric temperatures4 ,as the junctions are so arranged in radiation pyrometers as to fulfill this condition1 no correction for fluctuations in the cold junctions is necessary4 ,the deflections1 therefore1 correspond to excess temperatures of the hot junction1 which in turn are directly proportional to the energy received by the junction4 ,readings in millivolts on the indicator thus represent directly the proportions of energy received by the hot junction1 #d millivolts corresponding to twice the energy1 which produces #b millivolts1 and so on2 and hence the millivolt scale becomes an energy scale4 ,in order to translate energy into corresponding temperatures1 the fourth-power law must be applied4 ,if ,e#a correspond to an absolute temperature ,t#a on the part of the black body from which radiations are received1 and ,e#b correspond to another temperature ,t#b1 the following relations will hold good3 ,e#a ,k7,t#ad - x#d71 and ,e#b ,k7,t#bd - x#d71 where x is the temperature of the surroundings receiving the radiations4 ,as previously pointed out 7see ,example on page #adj71 the term x#d may be ignored for the range of high temperatures measured by a radiation pyrometer1 hence ,e#a ,,kt#ad1 and ,e#b ,,kt#bd2 and therefore ,e#a_/;,e#b ,t#ad_/,t#bd4 ,but1 as shown above1 readings in millivolts on the indicator are directly proportional to the energy received1 and if ,r#a and ,r#b millivolts due to ,e#a and ,e#b1 the relation ,r#a_/,r#b ,t#ad_/,t#bd is then obtained4 ,in order to prepare a temperature scale from this relation1 it is necessary to take one correct reading at a known temperature1 after which the remainder of the scale may be marked by calculation1 as shown in the example appended3-- ,example4--,a tube closed at one end is at #ibg ,c4 7#abjj abs471 and gives a deflection corresponding to #b millivolts on the indicator4 ,to find the temperatures which would yield deflections due to #a1 #c1 #d1 and #e millivolts4 ,taking the case of #a millivolt and applying in the formula ,r#a ,t#ad #b #abjjd 1 2 ,r#b ,t#bd #a ,t#bd #abjjd from which ,t#bd and ,t#b #ajji abs4 #b #gcf ,c4 ,similarly1 #c millivolts represent #ajee ,c42 #d millivolts #aaed ,c42 and #e millivolts #abcf ,c4 ,these values are readily obtained by the use of four-figure logarithms4 ,having calculated the temperature corresponding to each whole millivolt1 a curve may be plotted to represent millivolts against corresponding temperatures1 and intermediate values deduced from it4 ,evidently1 the standard reading must be taken with great accuracy1 as the whole scale hinges upon it2 and for this purpose an accurate resistance or thermo-electric pyrometer may be used1 placed inside the tube of an electric furnace1 and the radiation pyrometer sighted on a thin sheet of iron placed just in front of the naked junction4 ,a check at the higher readings of the scale is necessary1 as an exact realisation of the fourth-power law is seldom obtained in practice4 ,this may be taken in the same manner1 as thermocouples may now be calibrated directly against the gas scale up to #aeej ,c41 thus enabling the gas-scale reading to be transferred to the radiation pyrometer4 ,for delicate readings over a given range1 the scale of a mirror galvanometer may be calibrated in this manner1 sufficient resistance having first been added in series to ensure that at the highest temperature employed the spot of light will remain on the scale4 ,illustration3 ,,fig4 #ed4--,,record ,,obtained ,,with ,,radiation ,,pyrometer4 ,recorders for ,radiation ,pyrometers4--,any of the thermo-electric recorders described in ,chapter ,,ii may be applied to radiation pyrometers1 the chart being suitably divided according to the fourth-power law4 ,when taking a record1 the pyrometer is fixed on a stand or bracket and focused on the desired spot4 ,fig4 #ed is an example of a record taken with a ,thread recorder and ,fry pyrometer1 in which the division of the temperature scale according to the fourth-power law will be noticed4 ,it is possible to arrange that the working temperature shall lie on the open part of the scale1 by adjusting the sensitiveness of the galvanometer accordingly before calibrating4 ,management of ,radiation ,pyrometers4--,it is not advisable to place a radiation pyrometer in the hands of an unskilled observer1 as intelligent oversight is required if good results are to be secured4 ,care must be taken to adjust the galvanometer needle to zero before taking a reading1 and the needle should always be locked during transit4 ,when focusing on an object in a furnace it is necessary to make certain that the red image seen is actually that of the object1 which may be done by moving the pyrometer until the side of the object1 or some special feature1 is visible in the eye-piece1 when the pyrometer may be moved until the image surrounds the junction4 ,occasions may arise1 as in taking the temperatures of various zones of a rotary cement-kiln or other furnace1 in which it is required to focus the mirror for a specified distance2 in which case the author has adopted the plan of placing a fixed pointer opposite the milled head which controls the mirror 7,p1 fig4 #dd7 and focusing the bars of a window at measured distances1 marking the same on the milled head opposite the pointer2 and it would be a convenience if all radiation pyrometers were thus marked initially4 ,a good check to correct focusing in the case of a heated object is to alter the focus in both directions1 and finally to adjust to the maximum reading1 which should correspond to the true focus4 ,great care should be taken not to damage the mirror4 ,if1 in a workshop1 the surface become covered with dirt1 this should be removed by gentle brushing with a camel-hair brush or by blowing air over the mirror4 ,the focusing device should never be strained beyond its working limits2 when these are reached1 the pyrometer should be moved bodily until the object can be correctly sighted within the ordinary limits of the movement of the milled head4 ,if metallic fumes or dense smoke intervene between the furnace and the pyrometer1 the radiations will be impeded and the temperature recorded will be too low2 and in such cases the pyrometer should be placed at the open end of a tube and sighted upon the closed end1 which should terminate at the spot under observation4 ,in all cases it must be borne in mind that the indications only apply to black-body conditions4 ,if a block of steel be sighted inside a furnace1 and then be removed to the exterior and again sighted1 the external reading will be much less than the internal1 owing to the inferior radiating power of the surface1 which now derives no assistance from the furnace4 ,all readings should therefore be taken whilst the object is still in the furnace1 or 7as in taking the temperature of molten metal in a ladle7 a fireclay tube with a closed end inserted in the mass may be used1 and readings taken through the open end4 ,statements are sometimes made that the difference between external readings and black-body readings is constant for a given surface1 and that the one may be translated into the other2 but this is true only for unchanging surfaces1 such as platinum1 and seldom applies to ordinary working surfaces4 ,as black-body conditions are so easy to ensure1 it is simpler and safer always to arrange to take observations under such conditions1 rather than to trust a relation seldom constant in practice4 ,when using a radiation pyrometer for a number of furnaces1 fireclay tubes1 closed at one end1 may be inserted in each1 so that the closed end terminates at the working spot1 the open end being left flush with the exterior of the furnace4 ,the diameter of such tubes will depend upon the length and also upon the make of the pyrometer2 in all cases the image of the closed end must be large enough to overlap the receiving junction or spiral4 ,information on this point can always be obtained from the makers1 or can be discovered by trial with openings of known diameter4 ,when using the pyrometer to obtain temperatures in the interior of the tube of an electric furnace1 such as that illustrated in fig4 #bi1 a solid object1 such as a short fireclay cylinder1 or a piece of graphite1 should be placed in the middle of the tube1 and focused on the junction4 ,special ,uses of ,radiation ,pyrometers4--,for regular use at temperatures above #ajjj ,c4 or #ahej ,f4 the radiation pyrometer will be found to be more useful than instruments of the thermo-electric or resistance type as the latter undergo deterioration owing to the continuous action of the furnace gases1 which becomes more marked as the temperature increases4 ,examples of industrial processes in which #ajjj ,c4 is considerably exceeded are the manufacture of glass1 pottery1 and cement1 the treatment of special steels1 and the casting of metals and alloys4 ,even for temperatures between #gej and #ajjj ,c4 a radiation pyrometer may be used1 but is not so convenient for this range as a thermo-electric instrument4 ,there is no upper limit to the instrument1 which may be calibrated by the fourth-power law to the highest temperature attainable1 that of the electric arc1 which has been found to be #cgbj ,c4 by the use of a ,fry radiation pyrometer4 ,measurements may therefore be made beyond the limits of thermal junctions1 such as the temperature of electric furnaces and of thermit in the mould1 and of molten steel before pouring1 thus opening out the possibility of accurate control at extremely high temperatures4 ,there is always a danger1 however1 of the cold junction becoming unduly heated when near to large masses at very high temperatures1 and serious errors may arise from this cause4 ,two examples may be cited to illustrate the usefulness of the radiation pyrometer in practice3 7#a7 the hardening of steel projectiles2 and 7#b7 the determination of the temperature of the clinkering zone in a rotary cement kiln4 ,in 7#a7 the projectile is brought to a given spot near the brink of the furnace1 where it is in the focus of a radiation pyrometer1 and when at the specified temperature is raked out of the furnace and drops into an oil-trough4 ,it has been found that a difference of #aj ,c4 from the standard temperature at which the projectiles should be quenched may cause a serious lowering of the penetrative power of the finished projectile2 and hence a radiation pyrometer1 which may readily be sighted on each individual shell1 is the best to use for this purpose4 ,in 7#b7 the hottest spot may be found by focusing the pyrometer to different distances up the kiln1 and1 by taking a record1 any fall in temperature due to defect of coal or air supplies1 or to excessive feed of raw material1 may be detected1 thus furnishing information from which the process may be regulated to the best advantage4 ,at the temperatures prevailing in such kilns--#acjj to #adej ,c41 or #bcgj to #bfdj ,f41 according to the nature of the kiln--a ,fry radiation pyrometer is quite sensitive to changes of #aj ,c4 or #ah ,f41 and the author has found it to be entirely satisfactory in this connection4 ,the adaptability of radiation pyrometers to all temperatures above a red heat1 combined with the absence of deterioration1 renders these instruments of great value1 and the possibility of obtaining records is a further recommendation4 ,the radiation method1 however1 is not suited to the purposes of an installation1 as even if mirrors and junctions could be constructed so as to be identical1 the arrangement would be very costly4 ,a cheap adaptation of the radiation principle1 by means of which a number of furnaces1 such as a set of cement-kilns1 could be controlled from a centre1 would be of great advantage1 and would add further to the general utility of this class of pyrometer4 ,,chapter ,,vi ,,optical ,,pyrometers ,general ,principles4--,when a solid is heated to #dej ,c41 it commences to send out luminous radiations and appears a dull-red colour in a darkened room4 ,as the temperature rises1 the luminous radiations become more intense2 the colour changes to a lighter red1 then to orange1 yellow1 white1 and finally to a dazzling white4 ,attempts have been made to assign temperatures to specified colours1 and ,pouillet1 in #ahcf1 introduced a table which purported to give the relation between colour and temperature4 ,the following table1 published by ,howe in #aijj1 differs considerably from that of ,pouillet1 who had no accurate means of measuring the temperatures he assigned to the colours3-- ,,howe',s ,,table4 ,description4 ,temp4 ,deg4 ,c4 ,temp4 ,deg4 ,f4 ,lowest red visible in darkness #dgj #hgh 0 0 0 daylight #dge #hhg ,dull red #eej to #fbe #ajbb to #aaeg ,full cherry #gjj #abib ,light red #hej #aefb ,full yellow #iej to #ajjj #agdb to #ahcb ,light yellow #ajej #aibb ,white #aaej #bajh ,if it were possible for all observers to detect exactly the colours to which these temperatures refer1 the table would be of great utility2 but in practice any two persons might differ in judgment to the extent of #ej ,c4 below a yellow2 and when the white is reached1 and becomes dazzling1 accurate discrimination is impossible4 ,at the same time1 a trained workman1 used to quenching steel at a fixed temperature1 say #hej ,c41 acquires a high degree of judgment with constant practice1 and may not vary by more than #bj ,c4 at temperatures below a light yellow4 ,the personal equation1 however1 is too great for colour judgment by the unaided eye to be taken as an accurate guide to temperature4 ,a fairly close approximation1 however1 may be obtained by matching the colours against prepared standards1 as will be referred to later4 ,the determination of the intrinsic brightness of the heated substance by a photometric method naturally suggests itself as a possible means of ascertaining temperatures by optical means1 and it will be found that all the optical pyrometers used for industrial purposes are based on this procedure4 ,the law connecting the intensity of the whole of the light waves emitted with temperature1 for a given solid1 is approximately given by ,rasch's formula3-- x ,i#a ,t#a ,i#b ,t#b where ,i#a and ,i#b are the intensities corresponding to absolute temperatures ,t#a and ,t#b2 and the exponent #bejjj x 4 ,t#a ,hence at #abej abs4 the brightness increases as the #bjth power1 and at #bejj abs4 as the #ajth power of the temperature4 ,this rapid increase in brightness for a small rise in temperature enables small increments to be readily observed2 but a difficulty arises in practice owing to vast differences in brightness displayed by different substances at the same temperature4 ,for example1 the light emitted by an incandescent gas-mantle1 which consists of thorium oxide1 is vastly greater than that given out by a metal1 such as platinum1 at the same temperature2 and it is therefore evident that the luminosity of a substance depends not merely upon its temperature1 but also upon its nature4 ,it is possible1 however1 to obtain indications for any substance in terms of a black body2 thus if a heated solid possessed the same intrinsic brightness as a black body at a temperature of ,t1 the 8apparent0 or 8black-body0 temperature of the solid would also be called ,t4 ,all that this would signify would be that the condition of the solid was such that the light radiated was equal in intensity to that emitted by a black body at temperature ,t2 and to obtain the true temperature of the solid1 ,t must be multiplied by a factor which expresses the ratio of its emissive power to that of a black body4 ,in all photometric methods a standard light is employed1 which should not vary in brightness1 and with which the light from the source is compared4 ,in optical pyrometers no attempt is made to measure the illumination in terms of candle-power2 all that is necessary is to bring the standard and the source to the same degree of brightness by suitable adjustments4 ,amongst the standards employed are carbon-filament electric lamps1 amyl-acetate lamps1 and for higher temperatures the centre of an acetylene gas-flame2 each of which is capable of producing a fixed degree of brightness when used under specified conditions4 ,a black body1 at known temperatures1 is compared with the standard used1 thus furnishing a scale of 8black-body0 temperatures to which the indications of a given source may be referred1 as explained in the previous paragraph4 ,above #ajjj ,c41 however1 the light becomes too dazzling to enable a proper comparison of the standard and source to be made1 and absorbing glasses must then be used to reduce the brightness4 ,any coloured glass1 taken at random1 might not reduce the standard and source equally2 but if a monochromatic glass be used--that is1 a glass which transmits light of one wave-length only--a well-defined relation is found to exist between the intensity of the transmitted light and the temperature of the source4 ,as optical pyrometers are used for temperatures above #ajjj ,c4 in most cases1 involving the use of such glass1 it will be necessary briefly to consider the relations between the wave-lengths of light and the temperature of the radiating substance1 which in all cases will be assumed to be a black body4 ,wien's ,law4--,when the temperature of a substance increases1 the enhanced brightness which results is shared by all parts of its spectrum2 and if the substance were viewed through a glass prism1 it would be noticed that every portion was brighter than before4 ,taking a ray of wave-length 1 the relation between its intensity and the temperature of the 7black-body7 source is given by ,wien's formula3-- ,j c#a-e e-c#b_/,t 7#a7 where ,j energy corresponding to wave-length 2 e the base of the natural system of logarithms2 ,t absolute 7thermodynamic7 temperature of the black-body source1 and c#a and c#b are constants1 the values of which may be found by measuring ,j at two known temperatures for light of a known wave-length4 ,experiment has shown that this formula is correct for wave-lengths which lie in the visible spectrum1 but does not hold for longer waves2 and modifications of ,wien's equation have been given by ,planck and others which are of more extended application4 ,for the purposes of optical pyrometry1 however1 using red light of wave-length about #fe millionths of a centimetre1 ,wien's law may be applied with great accuracy2 and a calibration based upon this law agrees closely with the values obtained by other pyrometric methods4 ,wien's formula may be written in the form-- log#aj;,j ,k#a ,k#b7a_/,t7 7#b7 where ,k#a log c#a-e log and ,k#b c#b7log e_/74 ,this simplified expression shows a linear relation between log ,j and #a_/,t2 and hence if the temperatures corresponding to two intensities be known1 the results may be plotted on squared paper in the form of a straight line connecting ,t and ,j1 from which line intermediate or extraneous readings of temperatures may be obtained for any given intensity4 ,another useful form of ,wien's equation1 referring to the ratio of two intensities ,j#a and ,j#b1 is as under3-- ,j#a c#b log e #a #a log - 7#c7 ,j#b ,t#b ,t#a where ,t#b and ,t#a are the absolute temperatures corresponding to ,j#b and ,j#a ,the value of c#b is #adejjjj1 when is expressed in millionths of a centimetre4 ,evidently1 if the ratio ,j#a_/;,j#b and the value of c#b1 1 and ,t#b be known1 ,t#a may be calculated4 ,when is not known1 as in the case of a piece of red glass for which its value has not been determined1 two readings at known temperatures will establish the value of 7c#b log e7_/1 and all other results may then be calculated4 ,examples illustrating the application of the formula will now be given4 ,example ,i4--,a black body at an absolute temperature ,t#a is found to give twice the intensity observed at #abjj abs41 the comparison being made with red glass transmitting wave-length #fe #aj-f cms4 ,to find the value of ,t#a4 ,applying values to formula 7#c7 #adejjjj #a #a log #b log #bgahc - #fe #abjj ,t#a and #adejjjj #jdcdc #a ,t#a #jcjaj - #fe #abjj #a from which ,t#a #abcg abs4 ,example ,,ii4--,the intensity of the radiations from a black body at #bjjj abs4 are found to be equal to those from a given standard1 taken as unity4 ,to find the intensity at #cjjj abs41 compared with the same standard4 #fe #aj-f cms4 ,applying in 7#c7 as before1 ,j#a #adejjjj #jdcdce #a #a log - #a #fe #bjjj #cjjj from which log ,j#a #afae1 and ,j#a #ade4 ,in applying ,wien's law to the calibration of an instrument in which the intensity of a source may be measured photometrically against that of a standard1 an electric furnace 7fig4 #bi7 may be used1 with a piece of iron in the centre1 coated with oxide1 which gives black-body radiations4 ,a thermo-electric pyrometer in contact with the oxide may be used to measure the standard temperatures1 and brightnesses may then be compared with that of an amyl-acetate or other lamp giving a flame of constant luminosity4 ,temperatures corresponding to other intensities may then be deduced by calculation1 as previously shown4 ,practical ,forms of ,optical ,pyrometers4--,the instruments used in practice fall under the following heads3-- #a4 ,the standard light is constant1 and the intensity of the light from the source varied in the instrument until equal to the standard4 7,fry1 ,le ,chatelier1 ,wanner1 and ,cambridge47 #b4 ,the standard is varied until equal to that of the source1 which may be reduced in intensity if this exceed that of the standard4 7,holborn-,kurlbaum1 made in commercial form by ,siemens47 #c4 ,the colour of the source is matched against a standard colour1 made to agree with that obtained in a given operation 7,lovibond72 or the source may be made to produce a standard colour by a polarising device 7,mesur and ,nouel72 or the colour of the source is extinguished by suitable absorbents 7various forms74 ,examples of each type will now be described4 ,illustration3 ,,fig4 #ee4--;,f,,ry',s ,,optical ,,pyrometer4 ,,section4 ,illustration3 ,,fig4 #ef4--;,f,,ry',s ,,optical ,,pyrometer4 ,,external ,,view4 ,fry's ,optical ,pyrometer4--,this instrument 7shown in figs4 #ee and #ef7 consists of a telescope furnished with a side-branch1 in which a standard lamp ,e is placed4 ,light from ,e is focused upon a piece of transparent glass ,f1 inclined at an angle of #de to the axis of the telescope1 from whence it is reflected into the eye-piece4 ,to render the light received from the lamp monochromatic1 a piece of red glass is interposed between ,e and the mirror4 ,the telescope is sighted on the hot substance1 rays from which pass through a piece of red glass ,d1 and thence through two wedges of darkened glass1 which diminish the intensity to a greater or less degree according to the thickness of absorbent glass interposed1 which is reduced by sliding the wedges apart1 and increased by the contrary movement4 ,after passing through the wedges1 the light proceeds through the inclined mirror to the eye-piece2 consequently1 the appearance presented to the eye is that of a field illuminated one-half by the standard lamp1 and the other by the hot source4 ,the adjustment consists in sliding the wedges1 by a screw movement1 until both portions of the field are equally illuminated4 ,a temperature scale is provided on the moving piece which actuates the wedges1 and is derived by ,wien's equation from the thickness of the wedges interposed when equality is obtained4 ,calibration is effected by noting the thickness of the wedges corresponding to two known temperatures1 from which a straight line connecting thickness with the reciprocal of the absolute temperatures may be drawn1 and a table formed giving values of ,t in terms of the thickness of the wedges4 ,the calibration may be extended indefinitely1 the accuracy of the readings depending upon the truth of ,wien's law4 ,fry's optical pyrometer is a convenient instrument for occasional readings of high temperatures1 combining simplicity with portability4 ,le ,chatelier's ,optical ,pyrometer4--,this pyrometer was the original form of instrument in which the temperature of a luminous source was deduced by photometric comparison with a standard light2 and ,fry's apparatus1 described above1 is merely a convenient modification of the original4 ,instead of the absorbent glass wedges1 ,le ,chatelier employed an iris diaphragm to reduce the quantity of light entering the telescope2 the adjustment being carried out by altering the size of the opening in the diaphragm until the brightness of the source agreed with that of the standard4 ,the intensity of the light received in the telescope will vary as the square of the diameter of the opening2 and calibration at two known temperatures with a given monochromatic glass enables a temperature scale corresponding to diameter of opening to be computed by ,wien's law4 ,le ,chatelier's pyrometer is a valuable implement for research work in the laboratory1 but is not so convenient for workshop purposes as ,fry's modification4 ,illustration3 ,,fig4 #eg4--,,wanner',s ,,pyrometer4 ,,section4 ,wanner's ,pyrometer4--,the principle of this pyrometer is the comparison of the brightness of a red ray from the standard with that of the ray of some wave-length obtained from the source1 both rays being produced spectroscopically and therefore being truly monochromatic4 ,the brightness is compared by the aid of a polarising device1 resulting in a somewhat complicated optical arrangement1 which is shown in fig4 #eg4 ,light from a standard electric lamp passes through the slit ,s#a1 and from the hot source through ,s#b4 ,both beams are rendered parallel by means of an achromatic lens ,o#a1 which is placed at a distance equal to its focal length from the slits4 ,the parallel beams are dispersed by the direct-vision spectroscope ,p2 and then pass through the polarising prism ,r1 which separates each beam into two beams1 polarised in planes at right angles4 ,a biprism1 ,b1 placed in contact with a second achromatic lens1 ,o#b1 is made of such an angle that two fields of red light1 polarised in planes at right angles1 one from the source and the other from the standard1 are focused on the slit ,d4 ,these fields are viewed through an analyser ,a1 and are brought to equal brightness by rotating the analyser1 to which a graduated scale is attached1 the temperature being deduced from the angle through which the analyser is turned4 ,the calibration is effected by ,wien's law 7equation 7#c7 page #agb71 the intensities of standard and source being related to the angle of rotation as indicated by the equation4 ,j#b_/;,j#a tan#b where ,j#b and ,j#a represent the intensities of source and standard respectively1 and angle of rotation4 ,introducing this value into ,wien's equation 7page #agb71 the relation between and ,t may be shown to take the form log tan a b_/,t1 where a and b are constants4 ,hence1 if log tan be plotted against #a_/,t a straight line is obtained1 and hence by a few observations at known temperatures a calibration curve may be drawn from which intermediate and extraneous readings may be obtained4 ,messrs ,hadfield have introduced a special chart1 divided so that actual readings in degrees ,c4 may be taken directly by observing the angle 4 ,as sent out for use1 the temperature scale is prepared beforehand1 so that direct readings may be taken4 ,as the standard electric lamp will vary in brightness with repeated use1 means must be provided to restore it to its proper value4 ,this can be done by placing a rheostat in the circuit of the lamp1 and adjusting the current until the brightness1 as viewed through the pyrometer1 exactly agrees with that of a ground-glass surface illuminated by a standard amyl-acetate lamp4 ,the flame of this lamp really constitutes the standard2 but as it would be blown about by air-currents when used in a workshop1 the electric lamp1 lighted by a portable battery1 is brought to equality and used for general measurements4 ,cambridge ,optical ,pyrometer4--,during the recent war the manufacture of pyrometers of this type was taken up by the ,cambridge and ,paul ,instrument ,company4 ,the external form of the ,cambridge optical pyrometer is shown in fig4 #eh1 in which an observer is shown using the instrument1 the accessories consisting of a #d-volt accumulator1 an ammeter1 and an adjustable resistance for regulating the current through the electric lamp used for comparison2 and a standard amyl-acetate lamp for adjusting the electric lamp to the correct brightness4 ,the scale is marked on a circular disc1 and direct readings are obtained from the position of a pointer which rotates with the analyser4 ,by interposing a monochromatic glass to dim the source1 the range of the pyrometer can be modified2 and instruments are provided in four ranges3 #gjj-adjj ,c42 #ijj-bjjj ,c42 #abjj-bejj ,c41 and #adjj-djjj ,c4 ,the ,cambridge optical pyrometer has proved a useful instrument in skilled hands1 and has been found of great service in the steel1 glass1 and pottery industries4 ,trained observers have found it possible to detect a difference of #aj ,c4 at the region of #aijj ,c4 ,the adjustment of the two fields to equality1 however1 involves a judgment which varies with different observers1 and in practice it is advisable for one individual to be entrusted to take all readings4 ,illustration3 ,,fig4 #eh4--;,,cambridge ,,optical ,,pyrometer4 ,holborn-,kurlbaum ,pyrometer4--,in the optical pyrometers previously described a constant standard is used1 and the brightness of the light from the source varied until equality is obtained4 ,the idea of varying the brightness of the filament of an electric lamp until its colour matched that of the source1 and deducing the temperature from the current taken by the lamp1 was due to ,morse1 who used a filament in the form of a flat spiral1 heated by a battery of ,e4,m4,f4 #dj volts4 ,this spiral was placed in a metal tube and interposed between the eye and the heated object4 ,the ,holborn-,kurlbaum pyrometer1 as made by ,siemens1 is a refinement of that of ,morse1 and capable of reading over a more extended range4 ,in fig4 #ei1 ,l is a small electric lamp with a hairpin filament1 as shown at ,a4 ,this lamp is placed in a telescope1 so that the filament is in the focus of the eye-piece and is lighted by a #d-volt accumulator1 in series with which is a rheostat1 ,r1 and a milliammeter1 ,m4 ,the heated source is focused by moving the object-glass of the telescope1 and both lamp and source are viewed through red glass placed in front of the eye-piece1 ,d4 ,the rheostat1 ,r1 is then adjusted until the tip of the filament is indistinguishable from the background1 which is illuminated by the source4 ,if the lamp be too bright1 the filament will appear as a bright line2 if duller than the source1 as a dark line2 and when equal to the source it will merge into the background4 ,when equality is obtained1 the milliammeter is read1 and the temperature deduced from the current taken by the lamp4 ,illustration3 ,,fig4 #ei4--;,,holborn-,,kurlbaum ,,pyrometer4 ,,section4 ,the relation between current and the temperature of the filament varies with each lamp1 but is in all cases represented by a formula of the type ,c a bt ct#b where ,c current1 t temperature in degrees ,c41 and a1 b1 and c are constants depending upon the lamp used1 and which can be determined by making a number of observations at known temperatures4 ,the instrument is calibrated in this manner by the makers1 and a scale affixed from which temperatures may be read corresponding to observed currents4 ,when the temperature of the source exceeds that of the standard at maximum current1 an absorbing device1 ,e1 consisting of two prisms of darkened glass1 with their reflecting faces parallel1 is placed over the end of the telescope1 so as to reduce the intensity of the source below that of the lamp4 ,a separate calibration is performed with the absorber in position1 and a second temperature scale provided1 from which readings are taken when the absorbing device is used4 ,fig4 #fj represents the instrument as made by ,messrs ,siemens1 for use in a fixed position1 the telescope1 milliammeter1 and rheostat being mounted on an upright supported by a tripod1 and the current obtained from a portable accumulator4 ,a second form 7fig4 #fa7 is designed for use in cases when observations at a number of different places are required1 the rheostat being mounted on the telescope1 and the milliammeter contained in a leather case provided with shoulder-straps4 ,illustration3 ,,fig4 #fj4--,,siemens' ,,optical ,,pyrometer1 ,,on ,,stand4 ,illustration3 ,,fig4 #fa4--,,siemens' ,,optical ,,pyrometer1 ,,portable ,,form4 ,the adjustment in this pyrometer is simple1 and the condition of equality sharply defined4 ,whereas1 in matching the colours of two contiguous fields1 separate observers may disagree to an extent representing #dj ,c4 or more1 a divergence of #aj ,c4 is seldom exceeded when different operators adjust the tip of the filament to extinction4 ,in a special test to decide this point1 the author compared the observations of five persons1 some trained and others untrained1 with the result that all agreed to within #aj at a steady temperature in the vicinity of #abjj ,c42 and in this respect the ,holborn-,kurlbaum pyrometer is superior to other forms of optical pyrometer4 ,the continuous accuracy of the readings depends upon the permanence of the standard lamp1 which is ensured by over-burning for #bj hours1 after which the lamp may be used at its proper voltage for a long period without further change4 ,as used for occasional readings in the workshop1 such a lamp will last for a year or more without varying in brightness by an amount representing #aj ,c4 at a temperature of #ahjj ,c4 ,when a new lamp is used1 a fresh calibration is necessary2 the makers1 however1 in such case send out a new temperature scale with the lamp4 ,lovibond's ,pyrometer4--,it is possible1 by the use of coloured glasses superposed1 to match closely any given colour2 and ,lovibond1 whose tintometer for this purpose is well known1 has applied this method to temperature measurement4 ,taking the case of a block of steel in a furnace1 it is possible to arrange combinations of glasses which1 when illuminated by a standard light1 will give the same tint as the steel at any specified temperature4 ,if it be desired to work the steel at #hej ,c41 for example1 glasses are provided which1 when viewed by the light transmitted from a #d-volt glow-lamp1 using a constant current1 represent the tint of steel at #hdj1 #hej1 and #hfj respectively4 ,the image of the steel is reflected by a mirror through one hole in a brass plate1 which forms the end of a wooden box1 at the opposite end of which an eye-piece is placed4 ,a second hole in the brass plate receives light from the standard lamp1 after passing through the glasses2 and the appearances of the two lights are then compared4 ,a skilled eye can readily detect a disagreement in the two fields corresponding to #aj ,c42 and by introducing the glasses in turn it can be observed whether the steel is within #aj ,c4 of the temperature required4 ,this instrument is cheap and simple1 but is obviously only useful in deciding a pre-arranged temperature1 as to take a measurement at an undefined temperature would involve an unwieldy number of glasses1 and absorb a considerable time4 ,the correct glasses to use for a given operation are decided under working conditions at temperatures measured by a standard pyrometer2 after which any number of instruments may be made from glasses of the same colour and absorptive power as those used in the calibration4 ,correct matching is difficult below #gjj ,c4 ,mesur and ,nouel's ,pyrometer4--,this instrument1 shown in fig4 #fb1 consists of two ,nicol prisms1 between which is placed a piece of quartz cut perpendicularly to its axis4 ,light from the source1 in passing through the first ,nicol prism1 is all polarised in the same plane2 but on passing through the quartz is polarised in various planes1 according to the wave-length4 ,the colour seen after passing through the second prism1 used as analyser1 will depend upon the angle between this and the first or polarising prism4 ,the analyser is connected to a rotating disc1 divided into angular degrees2 and on viewing the heated source the colour will appear red if the analyser be turned in one direction1 and green if rotated in the opposite4 ,the intermediate colour is a lemon-yellow2 and the adjustment consists in rotating the analyser until this tint is obtained4 ,the angular reading is then taken1 and the temperature read off from a table prepared by making observations at known temperatures4 ,observers may disagree by as much as #ajj ,c4 in using this pyrometer1 owing to differences in eyesight and judgment of the lemon-yellow tint2 but a given operator1 who has trained himself to the use of the instrument1 may obtain much closer results with practice4 ,the chief use of this device is to enable a judgment to be formed as to whether a furnace is above or below an assigned temperature1 within limits of #be ,c4 on either side at the best2 and hence it is convenient for a foreman or metallurgist to carry about for this purpose when other pyrometers are not in use4 ,a great advantage is that the instrument is always ready for use1 and has no accessories4 ,illustration3 ,,fig4 #fb4--,,mesur ,,and ,,nouel',s ,,pyrometer4 ,colour-extinction ,pyrometers4--,various attempts have been made to produce superposed glasses1 or cells of coloured fluids1 which will have the effect of extinguishing the colour of a heated source4 ,as an example1 three cells containing various dyes in solution may be prepared which1 when looked through1 will extinguish the colour at #hdj1 #hej1 and #hfj ,c4 respectively4 ,if it be desired to work at #hej1 a difference of #aj on either side may be detected by a trained eye2 but to follow a changing temperature a large number of cells would evidently be necessary4 ,heathcote's extinction pyrometer1 in its early form1 consisted of an eye-shade in front of which two pairs of cells containing coloured fluid were mounted4 ,in bringing a furnace to an assigned temperature1 observation was made from time to time until a faint red image was perceived through one pair of cells1 when the heat supply was regulated so as to maintain the existing temperature4 ,when viewed through the second pair of cells1 which contained a slightly darker fluid1 no red image was to be seen at the correct temperature4 ,with training1 a workman could control a furnace to a fair degree of accuracy by this means1 but the operation was tedious1 and useful only for the attainment of a single temperature4 ,in a later instrument1 known as the 8,pyromike0 7fig4 #fc71 ,heathcote employs a single cell with flexible walls1 so that by turning the screw-end1 the length of the column of fluid interposed between the eye and the furnace can be altered4 ,in taking a reading1 the furnace is sighted and the screw turned so as to increase the length of the column of coloured fluid1 until the image is no longer visible4 ,a direct reading of the temperature is then obtained on a spiral scale marked on the cylindrical body of the instrument1 over which the screwed portion rotates4 ,this forms a simple and convenient temperature gauge for workshop use4 ,illustration3 ,,fig4 #fc4--;,,heathcote',s ,,extinction ,,pyrometer ,,or 8,,pyromike40 ,illustration3 ,,fig4 #fd4--8,,wedge0 ,,pyrometer4 ,the 8,wedge0 ,pyrometer1 designed by ,alder and ,cochrane 7fig4 #fd71 consists of a small telescope through which a prism of darkened glass may be moved1 and which is focused on the heated object4 ,by turning a head the wedge may be moved so as to interpose a thicker layer of dark glass between the eye and the furnace1 and the same operation causes a temperature scale to pass in front of a fixed pointer4 ,when the image of the hot source is just extinguished1 the temperature is read from the mark opposite the fixed point4 ,training is needed to enable an observer to judge the exact point of extinction1 when it becomes possible to obtain results of #bj ,c4 in the region of #acjj ,c4 ,on the other hand1 when used by one unaccustomed to the instrument1 the reading may be wrong by #ej ,c4 or more4 ,as an aid to the judgment near the extinction point1 the hand may be interposed between the telescope and furnace1 when1 if extinction be complete1 no alteration should be observed in the field of view4 ,the simple construction of this pyrometer is an advantage1 no accessories being needed2 and when used with the precautions stated above1 readings sufficiently close for many processes can easily be obtained4 ,management of ,optical ,pyrometers4--,careful usage is essential with optical pyrometers1 which are liable to get out of adjustment with rough handling2 and for this reason a trained observer should be in charge of such instruments4 ,skilled attention is equally requisite in taking readings1 as the matching of tints correctly is an operation which demands a high degree of judgment4 ,careful attention must be paid to the standard lights2 if flames1 regulation to the standard height is essential2 if electric lamps1 care must be taken not to use them for a longer period than necessary1 in order to increase the useful life4 ,accumulators should be recharged regularly--say once in two weeks--to keep in good order4 ,separate parts1 such as absorption glasses1 should be kept in a place of safety1 as their destruction may involve a new calibration4 ,it should be kept in mind that the temperatures indicated by optical pyrometers are 8black0 temperatures2 that is1 they correspond to the readings that would be given by a black-body of the same degree of brightness4 ,in consequence1 readings should always be taken under black-body conditions1 the precautions in this respect being identical with those necessary for total-radiation pyrometers1 given on page #afc4 ,in some special cases the connection between the apparent and true temperatures has been worked out for a given type of pyrometer1 but1 owing to the different emissive powers of different substances1 no general relation can be given4 ,special ,uses of ,optical ,pyrometers4--,the advantageous use of optical pyrometers is restricted to observations at temperatures beyond the scope of instruments which have the working part in the furnace2 or to cases in which occasional readings of temperature suffice4 ,to follow a changing temperature continuous adjustment is necessary1 involving labour1 and therefore costly4 ,amongst workshop uses may be mentioned3 7#a7 ascertaining the temperature of pottery kilns and glass and steel furnaces2 7#b7 in the treatment of steels at very high temperatures1 to which end the pyrometer may be set to a given reading1 and the process carried out when the steel is observed to attain such assigned temperature2 7#c7 to take casual readings when a number of furnaces are in use1 or when a number of sighting-holes are provided1 as in large brickmaking furnaces2 and 7#d7 for occasional observations of the firing end of rotary cement kilns4 ,as an instrument of research in the laboratory1 a good form of optical pyrometer is very useful1 as1 for example1 in investigating the working temperatures of electric lamps1 and taking observations in electric furnaces4 ,it is a great drawback that records cannot be taken by optical pyrometers1 as much valuable information can be gathered from an accurate knowledge of temperature fluctuations in most operations4 ,this disadvantage must always militate against the general use of these instruments4 ,,chapter ,,vii ,,calorimetric ,,pyrometers ,general ,principles4--,if a piece of hot metal1 of known weight and specific heat1 be dropped into a known weight of water at a temperature t#a1 which rises to t#b in consequence1 the temperature of the hot metal1 t#j1 can be obtained by calculation1 as shown by the following example3-- ,example4--,a piece of metal weighing #ajj grams1 and of specific heat #ja1 is heated in a furnace and dropped into #dge grams of water1 contained in a vessel which has a capacity for heat equal to #be grams of water4 ,the temperature of the water rises from #e to #be ,c4 ,to find the temperature of the furnace4 ,the heat lost by the metal is equal to that gained by the water and vessel4 ,equating these1 #ajj #ja 7t#j - #be7 7#dge #be7 7#be - #e7 from which t#j #ajbe ,c4 ,the above calculation1 which applies generally to this method1 depends for its accuracy upon a correct knowledge of the specific heat of the metal used4 ,this value is far from constant1 increasing as the temperature rises1 and the result will only be correct when the average value over a given range is known4 ,the metal used in the experiment should not oxidise readily1 and should possess a high melting point4 ,platinum is most suitable1 but the cost of a piece sufficiently large would considerably exceed that of a thermo-electric or other outfit4 ,nickel is next best in these respects1 and is now generally used for the calorimetric method1 up to #ajjj ,c4 ,the specific heat varies to some extent in different specimens1 but can be determined for the ranges involved in practical use4 ,this may be done by heating a given weight to known temperatures and plunging into water1 the result being obtained as in the foregoing example1 t#j in this case being known and the specific heat calculated4 ,from a series of such determinations1 a curve may be plotted connecting specific heat and temperature range1 from which intermediate values may be read off4 ,illustration3 ,,fig4 #fe4--,,specific ,,heat ,,of ,,nickel ,,over ,,ranges ,,from #j ,c4 ,regnault1 who first suggested the calorimetric method for high temperature measurement1 attempted to measure the specific heat of iron over different ranges1 with a view to using this metal in the process4 ,owing to the absence of reliable means of determining the experimental temperatures1 however1 ,regnault's values were considerably in error4 ,for the range #j to #ajjj ,c4 he gave the average specific heat of iron as #jabf1 a figure much below the truth4 ,thus1 if a piece of iron be heated to #igj ,c41 as measured by the thermo-electric method1 and dropped into water1 the temperature calculated from an assumed specific heat of #jabf will be found to be #abaj1 or #bdj too high4 ,the values now employed are obtained by experiments with a thermo-electric pyrometer1 so that temperatures deduced by the calorimetric method agree1 within the limits of manipulative error1 with those of the standard scale4 ,the accompanying curve1 fig4 #fe1 shows the average specific heat of nickel over all ranges between #j and #ajjj ,c41 and from this curve the correct figure to use in the calculation for any range may be determined4 ,thus for a furnace between #hjj and #ijj ,c4 the specific heat would be taken as #jacf2 and although the choice of the value to be taken involves a knowledge of the temperature within #ajj1 no difficulty arises in practice1 as it is easy to judge this limit by experience at temperatures below #ajjj ,c ,in the most approved forms of calorimetric pyrometers for industrial purposes the temperature of the hot metal may be read directly from a scale1 prepared in accordance with the value applying to the specific heat at various ranges4 ,copper and iron are still used to a limited extent in these pyrometers1 but lose continuously in weight by oxidation1 the scales of oxide falling off when quenched1 necessitating weighing before each test to ensure accuracy4 ,nickel oxidises very little below #ajjj ,c41 and as the thin film of oxide which forms does not readily peel off1 the weight may increase slightly4 ,quartz would probably be more suitable than metals1 not being altered by heating and quenching1 but does not appear to have been tried for this purpose4 ,another possible material is nichrom1 which resists oxidation below #ajjj ,c4 ,the weight of the solid should be at least #a_/bj of that of the water1 in order to ensure a tangible rise in temperature1 and the thermometer should be capable of detecting #a_/bj of a degree ,c4 ,the rise in temperature should not be so great as to cause the water to exceed the atmosphere in temperature by more than #d or #e ,c41 as otherwise radiation losses would have a marked effect4 ,the limits of accuracy of the method will be shown by reference to examples4 ,example ,i4--,a piece of nickel1 weighing #ajj grams1 is placed in a furnace1 and after heating dropped into #bjjj grams of water at #aj ,c41 contained in a vessel of water equivalent #ej grams4 ,the temperature rises to #afbe ,c4 ,the specific heat of nickel for the range is #jacg4 ,to find the temperature of the furnace and the limits of accuracy1 the thermometer being readable to #a_/bj ,c4 ,equating heat lost by the nickel to that gained by the water and vessel3-- #ajj #jacg 7x - #afbe7 #bjej 7#afbe - #ajj7 from which x #ieb ,c4 ,if the error in each thermometer reading amounted to #a_/dj the maximum difference in the above calculation is obtained by introducing the altered values as under3-- #ajj #jacg 7x - #afbbe7 #bjej 7#afbbe - #ajjbe7 when x #idd ,c4 ,the maximum error due to a possible incorrect reading of #a_/dj is therefore less than #a per cent4 ,example ,,ii4--,the loss of heat by radiation in transferring #ajj grams of nickel at #ibg ,c41 possessing a surface of #cj square centimetres1 and with radiating power #jg of a black body1 may be shown by the fourth-power law to be #ej calories per second 7see page #aci74 ,if two seconds were occupied in the transfer1 the error from this cause would be #a in #acj2 and adding this to the thermometric error1 the total is less than #b per cent4 ,practical ,forms of ,calorimetric ,pyrometers4--,when required to estimate the temperature of a muffle furnace or other laboratory appliance1 a sheet-copper vessel of about #aejj c4c4 capacity may be used4 ,this should rest on wooden supports in a second similar vessel1 about #b inches wider1 which acts as a shield against radiation4 ,a cylinder of nickel about #a inches long1 and #a inches in diameter1 with a hole of -inch diameter in the centre1 is suitable for test purposes4 ,this may conveniently be heated in a nickel crucible2 and when transferring to the water the crucible may be grasped with a pair of tongs1 and tilted so as to allow the cylinder to drop into the water4 ,when used in a tube furnace1 a length of thin nickel wire may be attached to the cylinder to enable withdrawal to be accomplished rapidly1 allowance being made for the weight of the heated wire4 ,the transfer should be accomplished as speedily as possible1 to avoid radiation errors4 ,the figure to be used to represent the specific heat of nickel may be obtained from the curve 7fig4 #fe71 when the range to be measured is approximately known4 ,the water equivalent of the vessel and thermometer should be determined as follows3--,place in the vessel one-half the quantity of cold water used in the experiment--say #gej c4c4--and note the temperature 7t#a7 after stirring with the thermometer4 ,then add an equal quantity of water at a temperature 7t#b7 about #aj higher than t#a ,mix thoroughly with the thermometer1 and note the temperature of the mixture 7t#c74 ,check results may be obtained by varying the proportions of cold and warm water1 the total quantity always being equal to that used for quenching the hot nickel4 ,if ,w#a the weight of cold water1 and ,w#b that of the warm1 the water equivalent 7x7 is obtained from the equation ,w#b 7t#b - t#c7 - ,w#a7t#c - t#a7 x t#c - t#a4 ,this figure represents the weight of water equal in thermal capacity to the vessel1 and in a pyrometric measurement is added to the weight of water taken4 ,in industrial practice1 it is desirable to dispense1 if possible1 with the necessity for calculations1 so that a reading may be taken by an unskilled observer4 ,the earliest form of calorimetric pyrometer1 patented by ,bystrm in #ahfb1 consisted of a lagged zinc vessel into which a piece of platinum was dropped1 and a table was provided from which the temperature of the furnace could be read by noting the rise in temperature of the water4 ,the modern industrial form1 made by ,messrs ,siemens1 will now be described4 ,illustration3 ,,fig4 #ff4--,,siemens' ,,calorimetric ,,or 8,,water0 ,,pyrometer4 ,siemens' ,calorimetric or 8,water0 ,pyrometer4--,fig4 #ff shows this instrument in longitudinal and transverse section4 ,it consists of a double copper vessel1 the inner containing water1 and the outer provided with a handle4 ,the space between is lagged with felt1 to prevent escape of heat from the water4 ,the thermometer1 b1 is protected by a perforated brass tube from damage that might be caused on dropping in the hollow nickel cylinder1 d4 ,opposite the stem of the thermometer is placed a sliding-piece c1 on which a temperature scale is marked4 ,in using the instrument1 the specified quantity of water is placed in the inner vessel1 and the pointer on c brought opposite to the top of the mercury column in the thermometer4 ,the nickel cylinder1 which has been heated in a crucible or muffle in the furnace1 is then dropped in1 and the vessel shaken to secure an equal temperature throughout the water4 ,when the thermometer is stationary1 the mark on c opposite the top of the mercury gives the temperature of the furnace1 the scale on c having previously been marked from calculations made for each #ej degrees4 ,the correctness of the reading evidently depends upon the accuracy with which c has been calibrated1 an operation which involves taking into account the water equivalent of the vessel and the variation of the specific heat of nickel at different temperatures4 ,allowing for the sources of error attaching to the method1 results by this pyrometer cannot be guaranteed to better than #b or #c per cent1 at #ijj or #ajjj ,c41 but in cases where this degree of inaccuracy is not of importance1 the instrument may be used with advantage4 ,as no calculation is necessary1 the determination may be made in the workshop by any workman who exercises care in conducting the operation4 ,copper and iron cylinders are sometimes supplied instead of nickel1 but are not to be recommended1 as they decrease in weight with each test1 and necessitate the use of a multiplying factor to convert the reading on c into the true temperature4 ,special ,uses of ,calorimetric ,pyrometers4--,the great drawback to the calorimetric method is that each observation necessitates a separate experiment1 involving time and labour4 ,the accuracy1 moreover1 is not comparable with that obtainable by the use of a thermo-electric or resistance pyrometer2 and practically the only recommendation is the low initial cost of the outfit4 ,when an occasional reading of temperature1 true to #c per cent41 suffices1 the calorimetric pyrometer may be used2 and in special laboratory determinations the method will frequently be found of value4 ,considering the low cost of thermo-electric pyrometers at the present time1 it is probable that the calorimetric method will be entirely superseded in industrial practice1 as the former method gives a continuous1 automatic reading1 and is capable of furnishing records4 ,many firms have already replaced their 8water0 pyrometers by the more accurate and useful appliances now available4 ,,chapter ,,viii ,,fusion ,,pyrometers ,general ,principles4--,if a number of solids1 possessing progressive melting points1 be placed in a furnace and afterwards withdrawn1 some may be observed to have undergone fusion whilst others would be unaffected4 ,the temperature of the furnace would then be known to be higher than that of the melting point of the last solid melted1 and lower than that of the first which remained intact4 ,taking1 for example1 a series of salts1 the following might be used3- ,melting ,point4 ,salt4 ,deg4 ,cent4,deg4 ,fahr4 #a molecule common salt #fej #abjb #a molecule potassium chloride ,common salt #hjj #adgb ,anhydrous sodium carbonate #hej #aefb 0 0 sulphate #ijj #afeb ,sodium plumbate #ajjj #ahcb ,anhydrous potassium sulphate #ajgj #aieh 0 magnesium sulphate #aaej #bajb ,if1 on inspection1 it were found that the sodium sulphate had melted1 whilst the sodium plumbate had survived1 the temperature of the furnace would be known to lie between #ijj ,c4 and #ajjj ,c4 ,if a number of salts or other solids could be found with melting points ranging between #ijj and #ajjj1 it would be possible to obtain a reading within narrower limits4 ,the accuracy of the method in all cases is decided by the interval between the melting points of successive test materials4 ,wedgwood1 the famous potter1 appears to have been the first to apply this method of determining the condition of a furnace1 his test-pieces consisting of special clay compositions4 ,the effect of the furnace on these was noted1 and the suitability of the temperature for the work in hand deduced from the observations4 ,wedgwood in this manner investigated the variations in temperature at different levels in his firing-kilns1 and was thus enabled to place the various wares at the positions best suited for their successful firing4 ,modern potters still use such test-pieces1 as the information gained is not merely the degree of heat1 but the effect of such heat on the articles undergoing firing4 ,the fusion method1 however1 is now used to determine the temperature of all kinds of furnaces1 and the chief modifications will now be described4 ,illustration3 ,,fig4 #fg4--,,seger ,,pyramids ,,or 8,,cones40 ,seger ,pyramids or 8,cones40--,seger1 of ,berlin1 published in #ahhf an investigation dealing with the production of silicates of progressive melting points4 ,by varying the composition1 he was able to produce a series of materials with melting points ranging from #ahij ,c4 to #eij ,c41 the interval between successive compositions being #bj between #ahij and #iej1 and #cj from the latter temperature to #eij4 ,the highest member of the series has the composition ,al#b,o#c1 ,si,o#b2 and the lowest member #b,si,o#b1 ,b#b,o#c4 ,for convenience in use the materials are made in the form of triangular pyramids1 #e cms4 in height1 and each side of the base #ae cms4 long4 ,each pyramid is stamped with a distinguishing number1 and altogether #fj are made to cover the range #ahij to #eij4 ,when conducting a test1 several pyramids are selected with melting points known to be near the temperature of the furnace1 as discovered by previous trials4 ,these are inserted in the furnace standing on a slab of refractory material1 as in fig4 #fg1 and may be watched through a sight-hole or withdrawn from the furnace for examination after attaining the existing temperature4 ,if the right pyramids have been chosen1 the appearance presented will be as in fig4 #fg1 in which ,d is seen to have collapsed completely1 ,c has bent over1 ,b has been rounded at the top1 whilst ,a is intact4 ,the temperature of the furnace is then taken to correspond to the melting point of ,c1 which is found by reference to a table in which the melting points corresponding to the different distinguishing numbers are given4 ,the pyramids are extremely cheap1 and only those with melting points near to the working temperature need be purchased4 ,in cases where it is desired to increase the heat to a specified point1 and then to allow the furnace to cool1 these pyramids fulfil all requirements2 an examination through a sight-hole closed with darkened glass enabling the furnace attendant to discover when the requisite temperature has been attained4 ,the procedure is more difficult when it is desired to maintain a steady temperature1 as this involves frequent renewal of pyramids already melted4 ,these appliances are sold under the name of ,seger 8cones10 the latter word being evidently a misnomer4 ,watkin's ,heat ,recorder4--,this arrangement consists of a small block of fireclay1 having a number of cylindrical holes in its upper face4 ,pellets of materials of progressive melting points are placed in the holes1 in which they fit loosely4 ,the block is placed in the furnace1 and afterwards withdrawn and examined1 when those which have completely melted will be seen to have sunk1 and to possess a concave surface2 others which have been superficially fused1 will show rounded edges1 whilst others will be intact4 ,the melting point of the highest member of the series which is observed to have rounded edges is taken as the temperature of the furnace4 ,the materials used in the manufacture of the pellets are approximately the same as those employed by ,seger1 being the same in number 7#fj71 and differing progressively by similar intervals4 ,it is not evident that the method of observation is superior to the use of pyramids1 although some workers may prefer it1 and the arrangement is merely an alternative plan of using the ,seger compositions4 ,watkin has also introduced a modification in which straight bars of clay compositions are supported at the edges1 the temperature being deduced by observing which numbers melt1 droop1 or remain intact4 8,sentinel0 ,pyrometers4--,under this name1 ,brearley1 of ,sheffield1 has introduced a number of compositions1 chiefly of salts1 which possess definite melting points4 ,these are made in the shape of cylinders1 about #a inch long and inch in diameter1 which collapse completely when the melting point is attained4 ,compositions have been found which melt at certain temperatures known to give the best results in the treatment of different kinds of steel1 and a cylinder of correct melting point1 placed in the furnace on a small dish near to the steel1 furnishes a simple and correct clue to the attainment of the desired temperature4 ,the existing condition of a furnace may be discovered by taking a number of cylinders1 having progressive melting points1 and making observations after the manner described under the heading of ,seger pyramids4 ,a few 8,sentinel0 cylinders are frequently of use in the workshop or laboratory for other purposes1 such as a rapid check of a given temperature in confirmation of the reading of an indicating pyrometer1 or in discovering whether a certain temperature has been exceeded in a given case4 8,sentinel0 cylinders have been used in such a manner as to give audible warning of the attainment of a given temperature by means of a metal rod1 which is made to rest on the cylinder1 and which1 when the cylinder melts1 falls and completes the circuit of an electric bell4 ,the upper range attainable by the use of ordinary metallic salts is not so great as in the case of silicates1 but up to #aajj ,c4 metallic sulphates1 chlorides1 etc41 or mixtures of these1 give results quite as good as those obtained with ,seger pyramids4 ,stone's ,pyrometer4--,this instrument is intended to indicate the correct temperature at which a metal or alloy should be poured1 and consists of a silica tube at the bottom of which is placed an alloy melting at the temperature at which the material operated on should be poured4 ,a silica rod rests on this alloy1 and is connected at its upper end to an iron extension1 the extremity of which engages a pointer moving over a scale4 ,when the alloy in the silica tube melts1 the rod falls through the molten mass and moves the pointer over the scale1 thus giving a certain indication that the desired temperature has been attained4 ,arrangements exist for adjusting the pointer to zero at the commencement of a test4 ,fusible ,metals4--,instead of clays or salts1 a number of metals and alloys are sometimes used4 ,these are placed in the form of short rods in numbered holes in a piece of firebrick and inserted in the furnace1 and on withdrawal those which have undergone fusion will be seen to have taken the form of the holes in which they were placed4 ,the temperature of the furnace is considered to lie between the melting points of the last of the series to undergo fusion and the first which remains unchanged4 ,a series of metals of this description is more costly than clays or salts1 but is more rapid in action1 owing to the superior conductivity of metals4 ,fusible ,pastes4--,these consist of salts incorporated with vaseline or other suitable fat1 and are used to detect the attainment of a specified temperature by a piece of metal4 ,if1 for example1 it were desired to heat a piece of steel to #hjj ,c4 for a given purpose1 a paste containing common salt might be smeared on its surface before placing in the furnace4 ,on heating1 the vaseline burns away1 leaving a white mark due to the salt1 and this white mark will be visible till the salt fuses4 ,the disappearance of the white mark therefore indicates that the required temperature has been reached2 and the method is simple and useful in cases where a number of articles are to be worked at a uniform temperature4 ,,chapter ,,ix ,,miscellaneous ,,appliances ,expansion and ,contraction ,pyrometers4--,most substances1 on heating1 exhibit an increase in size1 and on cooling return to the original dimensions4 ,if1 however1 a chemical alteration occur during the heating1 the resultant material may be permanently altered in size1 so that on cooling the substance may be of less or greater dimensions than before4 ,both these phenomena have been applied to the measurement of high temperatures2 the permanent shrinkage undergone by clay being utilised by ,wedgwood in the instrument which was the first practical pyrometer2 the expansion of a solid by ,daniell1 and of liquid by ,northrup4 ,both forms are still in use to a limited extent1 and will now be described4 ,illustration3 ,,fig4 #fh4--,,wedgwood',s ,,pyrometer4 ,wedgwood's ,pyrometer4--,in #aghb ,wedgwood introduced a method of determining the condition of a furnace by observing the contraction shown by cylinders prepared from a special clay4 ,the measuring device took the form of a tapered groove 7fig4 #fh7 made in two parts1 each #f inches long1 and one a continuation of the other4 ,each inch of the groove was divided into #bj equal parts1 making #bdj divisions in all1 and each division was called #a degree4 ,the width of the groove opposite the zero mark was #je inch1 and opposite #bdj1 #jc inch4 ,before firing1 the cylinders entered the groove until the lower end was opposite or near the zero mark2 and after being inserted in the furnace and allowed to cool on removal1 the cylinders were pushed as far as possible down the groove1 when the mark opposite the lower end indicated the condition of the furnace in terms of ,wedgwood's scale4 ,the degrees were1 of course1 arbitrary2 but with cylinders of uniform make a given position in the groove after heating always represented the same furnace temperature1 and thus furnished an indication more reliable than the judgment of a workman's eye4 ,wedgwood attempted to express the divisions on his scale in terms of ,fahrenheit degrees1 and by extrapolation of results obtained at the highest limits of the mercury thermometer1 where #a degree of contraction was caused by a rise of #acj ,f41 arrived at figures which now appear ludicrous1 but which were accepted for forty years4 ,as examples1 the melting point of silver was given as #dgag ,f42 of cast iron1 #agigg ,f42 and of wrought iron1 #bafcg ,f4--the last figure being nearly #aijjj higher than the present accepted value of #bggj ,f4 ,the error arose from the assumption of uniform contraction with increase of temperature1 and furnishes a striking example of the danger of indefinite extrapolation from meagre data4 ,but although the expression of the result in ,fahrenheit degrees was so erroneous1 the observed contraction always corresponded to a given condition of the furnace1 and the firing was continued until that known to be the best for the work in hand was attained4 ,the permanent shrinkage referred to is caused by dehydration of the clay1 and it therefore follows that this method can only give uniform results when exactly the same kind of clay is used for the test-pieces4 ,a given manufacturer might secure consistent indications by making a quantity of clay1 to be kept specially for this purpose2 but the same contraction at a given temperature would not be obtained by a second observer who also had prepared a quantity of clay1 as slight differences in composition cause large variations in the observed contraction4 ,in practice1 therefore1 pyrometers of this type are not interchangeable1 and each user must standardize for his own special conditions4 ,wedgwood's pyrometer is still used to a small extent2 its replacement1 however1 by the more convenient and accurate instruments now available is only a question of time4 ,daniell's ,pyrometer4--,in #ahbb ,daniell published an account of a pyrometer based on the expansion of a platinum rod enclosed in a plumbago tube4 ,one end of the rod pressed against the end of the tube1 whilst the other end was free to move1 and was connected to a multiplying device which magnified the expansion1 the increased movement being indicated by a pointer1 moving over a dial4 ,the scale on the dial was divided evenly into a suitable number of parts1 it being assumed that the difference between the expansions of graphite and platinum was uniform at all temperatures4 ,the scale was calibrated as far as possible by comparison with a mercury thermometer1 the remainder being extrapolated4 ,with this pyrometer ,daniell obtained a value of #bbcc ,f4 for the melting point of silver1 and #cdgi ,f4 for that of cast iron--results considerably higher than those now accepted1 but much nearer than those obtained by ,wedgwood4 ,daniell's pyrometer was widely used1 and its modern representatives are fairly common4 ,platinum1 owing to its cost1 is no longer used in these instruments1 which are now generally made with a graphite rod encased in an iron tube1 on the end of which the graduated dial is placed1 as shown in fig4 #fi4 ,another form1 commonly used in baker's ovens1 is constructed with an iron rod surrounded by a porcelain or fireclay tube4 ,illustration3 ,,fig4 #fi4--;,,expansion ,,pyrometer4 ,the defect of pyrometers of this type is that the coefficient of expansion of the materials alters with prolonged heating1 causing the readings to become erroneous4 ,re-adjustment in boiling water or other substance does not compensate properly for this alteration1 as both materials are not equally affected4 ,again1 the readings will be too low unless the whole of the expanding parts are in the interior of the furnace1 in which respect this pyrometer is inferior to a thermo-electric instrument1 which may be inserted at any convenient depth1 and may therefore be used for a greater variety of purposes4 ,the chief recommendation is cheapness2 but an expansion pyrometer should never be used for work of precision4 ,a graphite rod in an iron enclosure gives more consistent results than other materials4 ,northrup's ,molten ,tin ,pyrometer4--,tin melts at #bcb ,c41 and boils at #bbgj ,c4 ,it does not give off vapour sensibly up to #agjj ,c41 and expands with great uniformity4 ,it is therefore suitable for measuring high temperatures on the same principle as an ordinary thermometer1 and ,dufour1 in #aijj1 attempted to make a high-reading thermometer by enclosing tin in a silica bulb4 ,northrup has constructed an instrument in which the bulb and stem are of graphite1 and the height of the molten tin is determined by lowering a nickel wire through a gland until it touches the tin1 thereby completing an electric circuit and causing a bell to ring or producing a deflection on a galvanometer4 ,the upper end of the nickel wire moves over a scale1 which may be marked at two suitable fixed points1 and the scale divided up as in the case of an ordinary thermometer4 ,the durability of the graphite cover will determine the utility of this pyrometer1 and protection by some good refractory will be essential to prevent oxidation4 ,such a pyrometer will not respond quickly to changes in temperature1 but may prove useful in reading temperatures at ranges beyond the scope of present thermo-electric pyrometers4 ,northrup anticipates that this instrument may be used up to #ahjj ,c4 ,vapour-,pressure ,pyrometers4--,in these instruments mercury is placed in a stout steel tube1 to which a pressure-gauge is attached1 which registers the vapour-pressure of the mercury4 ,readings of pressure may be translated into temperatures by calibration with a standard pyrometer4 ,the range of these instruments is limited--#fjj or #gjj ,c4--and they are seldom used at present1 having been superseded by more modern types4 ,water-,jet ,pyrometers4--,in these instruments water is passed through a pipe placed in the furnace or hot space at a definite rate1 and from the rise in temperature produced in the water that of the furnace may be obtained4 ,an outfit of this kind entails the provision of a steady source of water-pressure1 and the indications can only remain accurate so long as the bore of the pipe remains uniform4 ,the calibration is made by comparison with a standard pyrometer4 ,the drawbacks to the method are its inconvenience1 and the necessity for continuous skilled supervision2 and in consequence of these the arrangement is seldom used4 ,pneumatic ,pyrometers4--,attempts have been made to deduce furnace temperatures by blowing air at uniform pressure through a pipe located in the hot space1 and noticing the increase in the temperature of the air4 ,in the ,uehling pyrometer1 air from the hot space is drawn through an opening of fixed size by means of a steam-jet1 which acts as an aspirator4 ,the opening is placed at one end of a chamber1 and the steam-jet aspirator at the other end2 and a diaphragm with a central hole divides the chamber into two parts4 ,the pressures existing in the two portions of the chamber vary according to the temperature of the air drawn in1 and are measured by water-gauges1 the readings of which may be translated into temperatures by calibration against a thermo-electric or other pyrometer4 ,the method is ingenious1 but is elaborate and costly2 and is therefore little used4 ,conduction ,pyrometers4--,if one end of a rod of metal be inserted in a furnace1 heat will be conducted along it to the portion external to the furnace1 and a steady condition will be obtained when the heat escaping from the external part of the rod1 by convection and radiation1 is equal to the quantity conducted along the rod4 ,the hotter the portion in the furnace1 the higher will be the temperature of all parts of the external length4 ,a series of thermometers placed at intervals in the exterior portion would show a progressive fall in temperature along the rod2 and the hotter the furnace the higher would be the reading on each thermometer4 ,in applying this principle to the measurement of high temperatures1 a bar of copper or iron is passed through the wall of the furnace1 so that a length of #b feet or more protrudes on the outside4 ,near the end of the external portion a hole is drilled to a sufficient depth to cover the bulb of a thermometer1 which is inserted in the hole1 into which a quantity of mercury is poured to make a metallic contact between the bulb and the bar4 ,the reading of the thermometer furnishes an approximate clue to the temperature of the furnace1 rising or falling with corresponding changes in the hot space4 ,a calibration might be effected by comparison with a standard2 but the method is only applied to the production of a prescribed condition1 known by experience to be attained when the thermometer reading has a certain value--say #abj ,c4 ,changes in atmospheric temperature1 or currents of air1 seriously affect the readings1 and the method at best is only approximate4 ,gas ,pyrometers4--,wiborgh1 ,bristol1 and others have constructed pyrometers in which the pressure of an enclosed gas is recorded by a ,bourdon pressure-gauge1 the scale of which is calibrated so as to read temperatures4 ,a porcelain bulb1 terminating in a capillary tube which is connected to the gauge1 is used to contain the air or other gas2 but at temperatures above a red heat the readings become uncertain1 owing either to leakages or the distortion of the bulb4 ,the most suitable material for the bulb 7alloy of platinum1 #hj per cent41 and rhodium1 #bj per cent47 is too costly to use industrially1 and would deteriorate under the influence of furnace gases4 ,in the ,bristol recording instrument the moving index of the pressure-gauge terminates in a pen1 which touches a chart-paper revolving by clockwork4 ,good results are obtained up to #djj ,c41 but beyond this the indications are uncertain1 and the instrument is more correctly described as a recording thermometer4 ,wiborgh's ,thermophones4--,these consist of infusible clay cylinders1 #be cms4 long and #b cms4 in diameter1 which contain an explosive4 ,when placed in a hot space1 the explosion occurs after a definite time1 the interval being less at high temperatures than at lower1 as the rate at which heat is conducted through a solid varies directly as the difference between the external and internal temperatures4 ,the interval elapsing between placing in the furnace and the subsequent explosion is noted on a stop-watch to the nearest #a_/e second1 and from the observed time the temperature is obtained from a table1 drawn up from the results of experiments under known conditions4 ,if the cylinders be kept dry1 an observer experienced in the use of thermophones may secure a reading to within #dj ,c4 ,joly's ,meldometer4--,this device1 due to ,dr ,joly1 is intended for laboratory determinations of melting points4 ,it consists of a strip of platinum1 heated by electricity1 upon which a tiny fragment of the material is placed1 which is viewed through a microscope4 ,the temperature of the platinum is regulated by means of a rheostat in the circuit1 and in making a test the temperature is gradually raised until the material is observed to become globular1 or to flow over the platinum strip4 ,the temperature at which this occurs is deduced from the linear expansion of the platinum strip1 which is measured by a micrometer attached to the instrument4 ,when carefully used1 very accurate determinations may be made by the meldometer1 the results1 moreover1 being obtained rapidly1 and with the use of the minimum of material4 ,brearley's ,curve ,tracer4--,this apparatus made by the ,cambridge and ,paul ,instrument ,company1 is designed to take a large-scale record of an operation which only occupies a short period of time4 ,it consists of a drum1 round which the record paper is wound1 and capable of rotating on its axis once in ten or thirty minutes by the aid of clockwork4 ,attached to the arm of the pen is a pointer1 which moves along the scale of a sensitive mirror galvanometer to which a thermocouple is connected4 ,the operator1 by turning a handle1 moves the drum longitudinally so as to keep the pointer opposite the centre of the spot of light1 and this movement is traced on the chart1 combined with the rotary movement1 by the pen4 ,in this manner the large change in deflection1 due to a few degrees increase or decrease in temperature1 can be recorded in ink4 ,this instrument is of special service in recording the critical points of steel1 or any operation which involves delicate readings over a limited range of temperature4 ,index ,air pyrometers1 #bai4 ,anti-vibration stand for galvanometers1 #dg4 ,atmospheric temperatures1 measurement of1 #if1 #acb4 ,automatic compensators1 #ff4 ,barrett1 ,sir ,w41 discovery of recalescence1 #d4 ,becquerel1 ,ed41 optical pyrometer1 #e4 ,black-body radiations1 #acf4 ,brearley1 sentinel pyrometers1 #bjh4 -- curve-tracer1 #bba4 ,bristol1 air-recording pyrometer1 #bai4 -- compensator1 #ff4 ,bystrm1 calorimetric pyrometer1 #bja4 ,calorimetric or 8water0 pyrometers1 #aie4 -- -- -- special uses of1 #bjc4 ,centigrade scale of temperatures1 #f4 ,clay-contraction pyrometers1 #baa4 ,colour-extinction pyrometers1 #ahi4 ,colour1 in relation to temperature1 #afg4 ,comparison of gas and platinum scales1 #aaa4 ,compensators for cold junctions1 #ff4 ,conduction pyrometers1 #bah4 ,constant temperature cold junction1 #ga4 ,contact-pen recorders1 #hh4 8,critical0 points of steel1 #id4 ,daniell1 expansion pyrometers1 #b1 #bad4 ,darling1 automatic compensator1 #fh4 -- and ,grace1 liquid element thermocouples1 #dc4 ,day1 extension of gas scale1 #ae4 ,electromotive force 7,e4,m4,f47 developed by junctions1 #ca4 -- -- measurement of1 #fb4 ,expansion pyrometers1 #bad4 ,fahrenheit1 temperature scale1 #g4 ,fry1 lens pyrometer1 #adb4 -- mirror pyrometer1 #adc4 -- optical pyrometer1 #agd4 -- spiral pyrometer1 #aej4 ,fixed points for calibration of pyrometers1 #af1 #ag4 ,foster1 base-metal pyrometer1 #ci4 -- fixed-focus pyrometer1 #aeb4 -- recorder1 #hb4 ,furnace1 electric tube1 #ie4 -- temperatures1 control of1 by pyrometers1 #hg4 ,fusible metals1 #bji4 -- pastes1 #baj4 ,fusion pyrometers1 #bjd4 ,gas pyrometers1 #bai4 -- scale of temperatures1 #aa4 -- thermometer1 constant volume1 #aa4 ,hadfield1 effect of temperature on hardness of steel1 #d4 ,harris1 indicator1 #aba4 ,holborn-,kurlbaum1 optical pyrometer1 #aha4 ,holden-d',arsonval1 galvanometer1 #de4 ,holman1 formula for thermal junctions1 #fb4 ,howe1 colour-temperature table1 #afg4 ,indicators1 for radiation pyrometers1 #aef4 -- for resistance pyrometers1 #aah-abd4 -- special range1 #ga4 -- standardizing of1 #ed1 #ajh1 #aeg4 -- for thermo-electric pyrometers1 #de-ec4 ,installations of resistance pyrometers1 #acj4 -- of thermo-electric pyrometers1 #hi4 ,joly1 meldometer1 #bbj4 ,kelvin1 thermodynamic scale of temperature1 #i4 ,kowalke1 base-metal couples1 #cj4 ,lambert1 anti-vibration stand for galvanometers1 #dg4 ,le ,chatelier1 optical pyrometer1 #agg4 -- thermo-electric pyrometer1 #e1 #bb4 ,leeds-,northrup1 indicator1 #abb4 -- recorders1 #he1 #abg4 -- resistance pyrometers1 #aae4 ,liquid element thermocouples1 #dc4 ,lovibond1 optical pyrometer1 #ahf4 ,low temperatures1 measurement of1 #ig1 #acb4 ,meldometer1 #bbj4 ,mercury thermometer1 limits of1 #a4 ,mesur and ,nouel1 optical pyrometer1 #ahg4 ,morse1 optical pyrometer1 #ahb4 ,national ,physical ,laboratory1 scale of temperatures1 #ag4 ,newton1 researches on high temperatures1 #b4 ,northrup1 molten tin pyrometer1 #baf4 -- pyrovolter1 #gd4 ,optical pyrometers1 #afg4 -- -- management of1 #aib4 -- -- special uses of1 #aic4 ,paul1 base-metal pyrometer1 #ci4 -- compensator1 #gj4 -- radiation pyrometer1 #aee4 -- recorder1 #hc4 -- uni-pivot indicator1 #di4 ,peake1 compensated leads1 #fg4 ,pivoted galvanometers1 #di4 ,planck1 modifications of ,wien's formula1 #aga4 ,platinum scale of temperatures1 #ajf4 ,pneumatic pyrometers1 #bag4 ,potentiometer indicators1 #gc4 -- method for measurement of ,e4,m4,f41 #fc4 ,prinsep1 gas pyrometer1 #c4 ,protecting sheaths for pyrometers1 #cd4 ,pyrometer1 definition of1 #a4 ,pyromike1 #ahi4 ,radiation pyrometers1 #acd4 -- -- calibration of1 #aeg4 -- -- indicators for1 #aef4 -- -- management of1 #afa4 -- -- recorders for1 #afa4 -- -- special uses of1 #afd4 ,rasch1 luminosity formula1 #afh4 ,recalescence of steel1 #d1 #id4 ,recorders for radiation pyrometers1 #afa4 -- -- resistance pyrometers1 #abd4 -- -- thermo-electric pyrometers1 #ge4 ,resistance1 measurement of1 #ajb4 -- of platinum1 #aje4 ,resistance pyrometers1 #aja4 -- -- indicators for1 #aah4 -- -- management of1 #acj4 -- -- recorders for1 #abd4 -- -- special uses of1 #acb4 -- pyrometry1 terms used in1 #aaa4 ,roberts-,austen1 recorder1 #gf4 ,salts1 melting points of1 #bjd4 ,seebeck1 discovery of thermo-electricity1 #c1 #bj4 ,seger1 pyramids or 8cones10 #bje4 ,siemens1 calorimetric or 8water0 pyrometer1 #bja4 -- indicator for resistance pyrometer1 #aah4 -- -- -- thermo-electric pyrometer1 #dh4 -- optical pyrometer1 #ahb4 -- recorder1 #ha4 -- resistance pyrometer1 #aad4 ,specific heat of nickel1 #aig4 ,standardizing of indicators1 #ed1 #ajh1 #aeg4 ,standards of temperature1 #i4 ,steam1 measurement of temperature of1 #ih4 ,stefan-,boltzmann1 fourth-power law1 #aci4 ,stone1 pyrometer1 #bji4 ,surface temperatures1 measurement of1 #ig4 ,suspended-coil galvanometers1 #de-dh4 ,temperature differences1 measurement of1 #ii4 -- scales1 #g-i4 -- fixed points of1 #af-ag4 ,thermal junctions1 changes in1 #bi4 -- -- choice of metals for1 #ba4 -- -- ,e4,m4,f41 developed by1 #ca4 -- -- methods of joining1 #bf4 -- -- used in pyrometers1 #bg4 ,thermo-electric circuits1 #bb4 -- pyrometers1 #bj4 -- -- calibration curves for1 #ei4 -- -- for surface temperatures1 #ig4 ,thermo-electric circuits1 indicators for1 #de-ec4 -- -- management of1 #ia4 -- -- practical forms of1 #cb4 -- -- recorders for1 #ge-hg4 -- -- standardization of1 #ed4 ,thermodynamic scale of temperatures1 #i4 ,thermometer1 constant volume gas1 #aa4 -- mercury1 #a4 ,thermophones1 #bbj4 ,thread recorder1 #gh4 ,uehling1 pyrometer1 #bah4 ,uni-pivot galvanometer1 #di4 ,vapour-pressure pyrometers1 #bag4 ,wanner1 optical pyrometer1 #agh4 ,water-cooled cold junction1 #cc4 ,water equivalent of calorimeter1 #bjj4 8,water0 pyrometer1 #bja4 ,water-jet pyrometer1 #bag4 ,watkin1 heat recorder1 #bjg4 ,wedge pyrometer1 #aij4 ,wedgwood1 pyrometer1 #baa4 -- test-pieces1 #bje4 ,wheatstone bridge for measuring resistance1 #ajd-aad4 ,whipple1 indicator1 #abj4 ,whipple-,fry1 pyrometer1 #aed4 ,wiborgh1 gas pyrometer1 #bai4 -- thermophones1 #bbj4 ,wien1 luminosity law1 #aga4 ,,printed ,,in ,,great ,,britain ,,by ,,neill ,,and ,,co41 ,,ltd41 ,,edinburgh4