MFTRY CHAISES R. DARLING C^arnell HtuuerBttg liihrary Jtljaca. JJcm $nrk BOUGHT WITH THE INCOME OF THE SAGE ENDOWMENT FUND THE GIFT OF HENRY W. SAGE 1891 arV17257 C ° rne " Universi, >' Librar y Pyrometr olin,anx 3 1924 031 238 615 The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31 924031 23861 5 PYROMETRY BY THE SAME AUTHOR HEAT FOR ENGINEERS A TREATISE ON HEAT WITH SPECIAL REGARD TO ITS PRACTICAL APPLICATIONS Third Edition, revised, with no illustrations, xiv+430pp. Demy Svo. Price 12s. 6d, net. LIQUID DROPS AND GLOBULES THEIR FORMATION AND MOVEMENT THREE LECTURES DELIVERED TO POPULAR AUDIENCES With 43 illustrations, x + 84 pp. Crown Svo. cloth. Price 3s. net. E. & F. N. Spon, Ltd. , Haymarket, London 1 , S.W. i PYROMETRY A PRACTICAL TREATISE ON THE MEASUREMENT OF HIGH TEMPERATURES BY CHAS. R. DARLING ASSOCIATE OF THE ROYAL COLLEGE OE SCIENCE, DUBLIN; WHITWORTH exhibitioner; FELLOW OF THE INSTITUTE of chemistry; FELLOW OF THE PHYSICAL SOCIETY, ETC. LECTURER IN PHYSICS AT THE CITY AND GUILDS TECHNICAL COLLEGE, FINSBUEY, E.C. AUTHOR OF HEAT l"i >Iv ENGINEERS SECOND EDITION, REVISED AND ENLARGED SIXTY-NINE ILLUSTRATIONS Xo 11 t>o n E. & F. N. SPON, Ltd., 57 HAYMARKET, S.W. SPON & CHAMBERLAIN, 120 LIBERTY STREET 1920 CjSo^o-Z Contents PAGE Preface ro Second Edition . . . . ix Preface to First Edition ..... xi CHAP. I. Introduction ...... i II. Standards of Temperature .... Absolute or Thermodynamic Scale — Constant Volume Gas Thermometer — Fixed Points for Calibration — National Physical Laboratory Scale — Temperatures above the Present Limit of the Gas Thermometer. III. Thermo-Electric Pyrometers ... 20 General Principles — Metals used for Thermal Junc- tions — Changes in Thermal Junctions when con- stantly used — Electromotive Force developed by Typical Junctions — Practical Forms of Thermocouples — Liquid Element Thermocouples — Indicators for Thermo-electric Pyrometers — Special Features of In- dicators — Standardizing of Indicators to read Tem- peratures directly — Standardization by Fixed Points — Standardization by Measurement of E.M.F. — Cold- Junction Compensators — Constant Temperature Cold Junctions— Special- Range Indicators — Potentiometer Indicators — Recorders for Thermo-electric Pyrometers vi PYROMETRY CHAP. —The Thread Recorder— The Siemens Recorder — Foster's Recorder — Paul's Recorder — The Leeds- Northrup Recorder — Control of Furnace Tem- peratures — Contact-Pen Recorders — Installations of Thermo-electric Pyrometers — Management of Thermo-electric Pyrometers — Laboratory Uses of Thermo-electric Pyrometers— Measurement of Lower Temperatures by the Thermo-electric Method — Measurement of Surface Temperatures — Measurement of Low Temperatures — Temperature of Steam, Ex- haust Gases — Measurement of Differences of Tem- perature — Advantages of the Thermo-electric Method of Measuring Temperatures. IV. Resistance Pyrometers. .... General Principles — Measurement of Resistance by the Differential Galvanometer — Measurement of Re- sistance 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' In- dicator — Whipple's Indicator — The Harris Indicator — The Leeds-Northrup Indicator — Siemens' Differen- tial Indicator — Recorders for Resistance Pyrometers — The Leeds-Northrup Recorder — Paul's Recorder — Installation of Resistance Pyrometers — Management of Resistance Pyrometers — Special Uses of Resistance Pyrometers. V. Radiation Pyrometers ..... General Principles — Practical Forms of Radiation Pyrometers — Fery's Mirror Pyrometer — Fery'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 Pyrometers. '34 CONTENTS CHAP. VI. Optical Pyrometers . . . .167 General Principles— Wien's Law— Practical Forms of Optical Pyrometers— Fe'ry's Optical Pyrometer— Le Chatelier's Optical Pyrometer— Warmer's Pyrometer — Cambridge Optical Pyrometer — Holhorn-Kurlbaum Pyrometer — Lovibond's Pyrometer — Mesure and Nortel's Pyrometer — Colour- extinction Pyrometers — Management of Optical Pyrometers — Special Uses of Optical Pyrometers. VII. Calorimetric Pyrometers .... 195 General Principles— Practical Forms— Siemens'Calori- metric or "Water" Pyrometer— Special Uses. VIII. Fusion Pyrometers . . . . . General Principles — Seger Pyramids or "Cones" — Watkin's Heat Recorder — "Sentinel" Pyrometers — Stone's Pyrometer — Fusible Metals — Fusible Pastes. IX. Miscellaneous Appliances .... 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 Thermo- phones — Joly's Meldometer — Brearley's Curve Tracer. Index Preface to the Second Edition Since the publication of the first edition in 191 1, a great extension has been witnessed in the use of pyro- meters in industrial processes and laboratory work, to which development the author hopes his book has contributed in some measure. During the stress occa- sioned by the war, pyrometers have proved invaluable in many processes, and British makers were fully able to meet the demands, owing to the status attained in pre-war days. 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 instruments, and it is hoped that the present treatise meets this need. In preparing the second edition, certain parts have been revised in conformity with modern practice, and the later developments included. The scope of the book remains as before. The author desires to acknowledge the assistance X PYROMETRY he has received from the British makers of pyrometers, all of whom have liberally provided him with informa- tion of a most useful kind, of which he has availed himself in the production of the present edition. CHAS. R. DARLING. Woolwich, 1920. Preface to the First Edition The present treatise has been founded on a course of Cantor Lectures on " Industrial Pyrometry," delivered by the author before the Royal Society of Arts in the autumn of 1910. The practice of pyrometry in recent years has progressed at a greater rate than the litera- ture bearing upon it ; 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 instruments. In the succeeding pages the exact measurement of temperature, as an end in itself, is made subordinate to the practical utility of pyrometers in controlling various operations ; and consequently descriptions of appliances of interest only theoretically have either been omitted, or at the most briefly described. Nevertheless, the fundamental principles are in all cases fully explained, as an under- standing of these is essential to the intelligent use of the appliances dealt with in the book. When neces- sary, numerical examples are given to illustrate the applications of the principles ; and the reader who Xll PYROMETRY finds any difficulty in following the various explana- tions — which of necessity involve an understanding of many portions of the subject of heat — is referred to the author's treatise on " Heat for Engineers," issued by the publishers of the present volume. With regard to temperature scales, the author has in the main employed Centigrade degrees, but has recognised that the Fahrenheit degree is still largely used, and has therefore frequently expressed tempera- tures in terms of both scales. The number of those who find it an advantage in their calling to measure and control high temperatures is constantly increasing ; and the manufacture of pyrometric appliances now gives employment to con- siderable numbers. The author trusts that the present treatise will prove of service to all thus concerned, and also to those who pursue the fascinating study of high temperature measurement from the purely scientific standpoint. In conclusion, the author expresses his thanks to the various firms, mentioned in the text, who have loaned blocks for the purpose of illustration, and who have furnished him with much valuable information. CHAS. R. DARLING. Woolwich, 1911. PYROMETRY CHAPTER I INTRODUCTION The term "pyrometer" — formerly applied to instru- ments designed to measure the expansion of solids — is now used to describe any device for determining temperatures beyond the upper limit of a mercury thermometer. This limit, in the common form, is the boiling point of mercury : 357 C. or 672 F. By leaving the bore of the tube full of nitrogen or carbon dioxide prior to sealing, the pressure exerted by the enclosed gas when the mercury expands prevents boiling ; and with a strong bulb of hard glass the readings may be extended to 550 C. or 1 020 F. Above this temperature the hardest glass is distorted by the high internal pressure, but, by substituting silica for glass, readings. as high as 700 C. or 1 290° F. may be secured. Whilst such thermometers are useful in laboratory processes they are too fragile for workshop use ; and if made of the length necessary in many cases in which the temperature of furnaces is sought, the cost would be as great as that of more durable and convenient appliances. No other instrument, 2 PVROMETRY however, is so simple to read as the thermometer ; and for this reason it is used whenever the conditions are favourable. The latest proposal in this direction is due to Northrup, who has constructed a thermometer containing tin enclosed in a graphite envelope, which is capable of reading up to I 500° C. or higher. This instrument is described on page 216. The origin and development of the science of pyro- metry furnish a notable example of the value of the application of scientific principles to industry. 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 fire ; but, although Newton's results were published in 1701, it was not until 1782 that a practical instrument for measuring high temperatures was designed. In that year Josiah Wedgwood, the famous potter, introduced an instrument based on the progressive contraction undergone by clay when baked at increasing tempera- tures, which he used in controlling his furnaces, finding it much more reliable than the eye of the most ex- perienced workman. This apparatus (described on page 211) remained without a serious rival for forty years, and its use has not yet been entirely abandoned. The next step in advance was the introduction of the expansion pyrometer by John Daniell in 1822. The elongation of a platinum rod, encased in plum- bago, was made to operate a magnifying device, which moved a pointer over a scale divided so as to read temperatures directly. Although inaccurate as com- INTRODUCTION 3 pared with modern instruments, this pyrometer was the first to give a continuous reading, and required no personal attention. The expansion pyrometer — with different expanding substances — is still used to a limited extent. The year 1822 was also marked by Seebeck's discovery of thermo-electricity. The generation of a current of electricity by a heated junction of two metals, increasing with the temperature, appeared to afford a simple and satisfactory basis for a pyrometer, and Becquerel constructed an instrument on these lines in 1826. Pouillet and others also endeavoured to measure temperatures by the thermo-electric method, but partly owing to the use of unsuitable junctions, and partly to the lack of reliable galvanometers, these workers failed to obtain concordant results. The method was for all practical purposes abandoned until 1886, when its revival in reliable form led to the enormous extension of the use of pyrometers witnessed during recent years. In 1828 Prinsep initiated the use of gas pyrometers, and enclosed the gas in a gold bulb. Later workers used porcelain bulbs, on account of greater infusibility, but modern research has shown that porcelain is quite unsuitable for accurate measurements, being porous to certain gases at high temperatures, even when glazed. Gas pyrometers are of little use industrially, but are now used as standards for the calibration of other pyrometers, the bulb being made of an alloy of platinum and rhodium. 4 PYROMETRY Calorimetric pyrometers, based on Regnault s " method of mixtures," were first made for industrial purposes by Bystrom, who patented an instrument of this type in 1862. This method has been widely applied, and a simplified form of " water " pyrometer, made by Siemens, is at present in daily use for indus- trial purposes. It is not capable, however, of giving results of the degree of accuracy demanded by many modern processes. The resistance pyrometer was first described by Sir W. Siemens in 1871, and was made by him for everyday use in furnaces. Many difficulties were en- countered before this method was placed on a satis- factory footing, but continuous investigation by the firm of Siemens & Co., and also the valuable re- searches of Callendar and Griffiths, have resulted in the production of reliable resistance pyrometers, which are extensively used at the present time. In 1872 Sir William Barrett made a discover)' which indirectly led to the present development of the science of pyrometry. Barrett observed that iron and steel, on cooling down from a white heat, suddenly became hotter at a definite point, owing to an internal mole- cular change ; and gave the name of " recalescence " to the phenomenon. Workers in steel subsequently discovered that this property was intimately connected with the hardening of the metal; thus Hadfield noticed that a sample of steel containing n6 per cent, of carbon, when quenched just below the change-point was not hardened, but when treated similarly at 15° C. INTRODUCTION 5 higher it became totally hard. The demand for accu- rate pyrometers in the steel industry followed imme- diately on these discoveries, for even the best-trained workman could not detect with the eye a difference in temperature so small, and yet productive of such pro- found modification of the properties of the finished steel. In this instance, as in many others, the instru- ments were forthcoming to meet the demand. The researches of Le Chatelier, published in 1886, marked a great advance in the progress of pyrometry. He discovered that a thermo-electric pyrometer, satis- factory in all respects, could be made by using a junc- tion of pure platinum with a rhodio-platinum alloy, containing 10 per cent, of rhodium ; a d'Arsonval moving-coil galvanometer being used as indicator. This type of galvanometer, which permits of an evenly- divided scale, is now universally employed for this purpose, and has made thermo-electric pyrometers not only practicable, but more convenient for general purposes than any other type. Continuous progress has since been made in connection with this method, which is now more extensively used than any other. Attempts to deduce temperature from the lumi- nosity of the heated body were first made by Ed. Hecquerel in 1863, but the method was not success- fully developed until 1892, when Le Chatelier intro- duced his optical pyrometer. This instrument, being entirely external to the hot source, enabled readings to be taken at temperatures far beyond the melting point of platinum, which would obviously be the 6 PYROMETRY extreme limit of a pyrometer in which platinum was used. The quantitative distribution of energy in the spectrum has since been worked out by Wien and Planck, who have furnished formula- based on thermo- dynamic reasoning, by the use of which optical pyro- meters may now be calibrated in terms of the thermo- dynamic scale of temperature. Other optical pyro- meters, referred to in the text, have been devised by Wanner, Holborn and Kurlbaum, Fery, and others ; and the highest attainable temperatures can now be measured satisfactorily by optical means. The invention of the total-radiation pyrometer by Fery in 1902 added another valuable instrument to those already available. Based on the fourth-power radiation law, discovered by Stefan and confirmed by the mathematical investigations of Boltzmann, this pyrometer is of great service in industrial operations at very high temperatures, being entirely external, and capable of giving permanent records. Modifica- tions have been introduced by Foster and others, and the method is now widely applied. Recorders, for obtaining permanent evidence of the temperature of a furnace at any time, were first made for thermo-electric pyrometers by Holden and Roberts-Austen, and for resistance pyrometers by Callendar. Numerous forms are now in use, and the value of the records obtained has been abundantly proved. For scientific purposes, all pyrometers are made to indicate Centigrade degrees, 100 of which represent INTRODUCTION 7 the temperature interval between the melting-point of ice and the boiling point of water at 760 mm. pres- sure, the ice-point being marked o° and the steam- point ioo°. In industrial life, however, the Fahrenheit scale is often used in English-speaking countries, the ice-point in this case being numbered 32 and the steam-point 21 2° ; the interval being 180°. A single degree on the Centigrade scale is therefore 1 - 8 times as large as a Fahrenheit degree, but in finding the numbers on each scale which designate a given tem- perature, the difference in the zero position on the two scales must be taken into account. When it is desired to translate readings on one scale into the corresponding numbers on the other, the following formula may be used : — C. reading F. reading— 32 ~T~ ~~9~ Thus by substituting in the above expression, 660° C. will be found to correspond to 1 220 F. and 1 530° F. to 832 C. It is greatly to be regretted that all pyrometers are not made to indicate in Centigrade degrees, as confusion often arises through the use of the two scales. An agreement on this point between instru- ment makers would overcome the difficulty at once, as the Centigrade scale is now so widely used that few purchasers would insist on Fahrenheit markings. It may be pointed out here that no single pyro- meter is suited to every purpose, and the choice of 8 PYROMETRY an instrument must be decided by the nature of the work in hand. A pyrometer requiring skilled atten- tion should not be entrusted to an untrained man ; and it may be taken for granted that to obtain the most useful results intelligent supervision is necessary. In the ensuing pages the advantages and drawbacks of each type will be considered ; but in all cases it is desirable, before making any large outlay on pyro- meters, to obtain a competent and impartial opinion as to the kind best suited to the processes to be con- trolled. Catalogue descriptions are not always trust- worthy, and instances are not wanting in which a large sum has been expended on instruments which, owing to wrong choice, have proved practically useless. An instrument suited to laboratory measurements is often a failure in the workshop, and all possibilities of this kind should be considered before deciding upon the type of pyrometer to be used. CHAPTER II STANDARDS OF TEMPERATURE The Absolute or Thermodynamic Scale of Tem- peratures. — All practical instruments for measur- ing temperatures are based on some progressive physical change on the part of a substance or sub- stances. In a mercury thermometer, the alteration in the volume of the liquid is used as a measure of hot- ness ; and similarly the change in volume or pressure on the part of a gas, or the variation in resistance to electricity shown by a metal, and many other physical changes, may be employed for this purpose. In con- nection with the measurement of high temperatures, many different physical principles are relied upon in the various instruments in use, and it is of the greatest importance that all should read alike under the same conditions. This result would not be attained if each instrument were judged by its own performances. In the case of a mercury thermometer, for example, we may indicate the amount of expansion between the temperatures of ice and steam at 76 centimetres pres- sure, representing 100° Centigrade, by a ; and then assume that an expansion of 2a will signify a tem- perature of 200 , and so on in proportion. Similarly, 9 IO PYROMETRY we may find the increase in resistance manifested by platinum between the same two fixed points, and indicate it by r, and then assume that an increase of 2/- will correspond to 200°. If now we compare the two instruments, we find that they do not agree, for on placing both in a space in which the platinum instrument registered 200 , the mercury thermometer would show 203 . A similar, or even greater, dis- crepancy would be observed if other physical changes were relied upon to furnish temperature scales on these lines, and it is therefore highly desirable that a standard independent of any physical property of matter should be used. Such a standard is to be found in the thermodynamic scale of temperatures, originally suggested by Lord Kelvin. This scale is based upon the conversion of heat into work in a heat engine, a process which is independent of the nature of the medium used. A temperature scale founded on this conversion is therefore not connected with any physical property of matter, and furnishes a standard of reference to which all practical appliances for measuring temperatures may be compared. 1 When readings are expressed in terms of this scale, it is customary to use the letter K in conjunction with the number : thus 850 K would mean 850 degrees on the thermodynamic scale. When existing instruments are compared with this standard, it is found that a scale based on the assump- 1 For a fuller account of the thermodynamic scale, see the author's treatise Heat for Engineers, pp. 391-3, STANDARDS OF TEMPERATURE II tion that the volume of a gas free to expand, or the pressure of a confined gas, increases directly as the temperature is in close agreement with the thermo- dynamic scale. It may be proved that if the gas employed were " perfect," a scale in exact conformity with the standard described would be secured ; and gases which approach nearest in properties to a perfect gas, such as hydrogen, nitrogen, and air, may therefore be used to produce a practical standard, the indica- tions of which are nearly identical with the thermo- dynamic scale. If any other physical change be chosen, such as the expansion of a solid, or the in- crease in resistance of a metal, and a temperature scale be based on the supposition that the change in question varies directly as the temperature, the results obtained would differ considerably from the absolute standard. For this reason the practical standard of temperature now universally adopted is an instrument based on the properties of a suitable gas. The Constant Volume Gas Thermometer.— In applying the properties of a gas to practical tem- perature measurement, we may devise some means of determining the increase in volume when the gas is allowed to expand, or the increase in pressure of a confined gas may be observed. The latter procedure is more convenient in practice, and the instrument used for this purpose is known as the constant volume gas thermometer, one form of which is shown in fig. i. The gas is enclosed in a bulb B, connected to a tube 12 PYROMETRY bent into a parallel branch, into the bend of which is sealed a tap C, furnished with a drying cup. The Fig i. — Constant Volume Air Thermometer. extremity of the parallel branch is connected to a piece of flexible tubing T, which communicates with STANDARDS OK TEMPERATURE I 3 a mercury cistern which may be moved over a scale, the rod G serving as a guide. In using this instru- ment the bulb B is immersed in ice, and the tap C opened. When the temperature has fallen to o° C. , the mercury is brought to the mark A by adjusting the cistern, and the tap C then closed. The bulb B is now placed in the space or medium of which the temperature is to be determined, and expansion pre- vented by raising the cistern so as to keep the mercury at A. When steady, the height of the mercury in the cistern above the level of A is read off, and furnishes a clue to the temperature of B. If the coefficient of pressure of the gas used (in this case, air) be known, the temperature may be calculated from the equation P,= P ( 1 +6/), where P x is the pressure at f ; P the pressure at o° ; and b the coefficient of pressure ; that is, the increase in unit pressure at o° for a rise in temperature of i°. Thus if P = 76 cms. ; b = 0^003 67 ; height of mer- cury in cistern above A = 558 cms. ; then P 1 = (76+55-8) = 131 '3 cms., and by inserting these values in the above equation / is found to be 200°. In the instrument described, P is equal to the height of the barometer, since the tap C is open whilst the bulb is immersed in ice. The coefficient of pressure may be determined by placing the bulb in steam at a known temperature, and noting the increased pressure. In the equation 14 PYROMETRY given, Pj, P , and t are then known, and the value of b may be calculated. In using this instrument for exact determinations of temperature, allowance must be made for the ex- pansion of the bulb, which causes a lower pressure to be registered than would be noted if the bulb were non-expansive. Again, the gas in the connecting tube is not at the same temperature as that in the bulb ; an error which may be practically eliminated by making the bulb large and the bore of the tube small. The temperature of the mercury column must also be allowed for, as the density varies with the temperature. When the various corrections have been made, readings of great accuracy may be secured. When applied to the measurement of high tem- peratures, the bulb must be made of a more infusible material than glass. Gold, porcelain, platinum, and quartz have been used by different investigators, but the most reliable material for temperatures exceeding 900" C. has been found to be an alloy of platinum with 20 per cent, of rhodium. The most suitable gas to use inside the bulb is nitrogen, which is chemically inert towards the materials of the bulb, and is not absorbed by the metals mechanically. When measur- ing high temperatures with this instrument, a con- siderable pressure, amounting to 1 atmosphere for every increase of 273 degrees above the ice point, is requisite to prevent expansion of the nitrogen ; and this pressure tends to distort the bulb and so to falsify STANDARDS OF TEMPERATURE I 5 the indications. This trouble has been overcome by Day, who surrounded the bulb by a second larger bulb, and forced air or nitrogen into the inter- vening space until the pressure on the exterior of the thermometer bulb was equal to that prevailing in the interior. Even then it was not found possible to secure higher readings than i 550° C, as the bulb commenced to alter in shape owing to the softening of the material. This temperature represents the highest yet measured on the gas scale : but by using a more refractory material, such as fused zirconia, it may be found possible to extend this range to 2 ooo° C. or more. Experiments in this direction are very desirable, in order that high-reading pyrometers may be checked directly against the gas scale. Fixed Points for Calibration of Pyrometers. — It is evident that the gas thermometer is totally un- stated for use in workshops or laboratories when a rapid determination of a high temperature is required. Its function is to establish fixed points or temperature standards, by means of which other instruments, more convenient to use, may be graduated so as to agree with each other and with the gas scale itself. The temperature scales of all modern pyrometers are thus derived, directly or indirectly, from the gas thermo- meter. In the table on next page, a number of fixed points, determined by various observers, is given ; the error, even at the highest temperatures, probably not exceeding ±2° C. In preparing the temperature scale of a pyrometer i6 PYROMETRV for practical use, the instrument is subjected suc- cessively to a number of the temperatures indicated in the table, and in this manner several fixed points are established on its scale. The space between these points is then suitably subdivided to represent inter- mediate temperatures. Table of Fixed Points. Substance. Physical Coi dition Water (ice) Water . At Melting „ Boiling Point Aniline Naphthalene Tin . Lead ,, Melting » Zmc Sulphur Antimony ,, Boiling ,, Melting " Aluminium i > i > j , Common Sa t > ) Silver (in air) Silver (free fron ,, '•• oxygen) - Gold . ,. >' Copper (in a Copper (Or covered) Iron (pure) Palladium Platinum r) iphiu i " Deg. Deg. Cent. Fahr. 32 IOO 212 1 84 363 218 424 232 449 327 620 419 786 445 «33 631 1 167 657 1 214 800 1 472 955 I 751 962 I 763 1 064 1 947 1 064 1947 1 084 1983 1 520 2 768 « 549 2 820 1 755 3 190 It is necessary to point out that the figures given in the table refer only to pure substances, and that relatively small quantities of impurities may give rise to serious errors. The methods by which the physical STANDARDS OF TEMPERATURE '7 condition to which the temperatures refer may be realised in practice will be described in the succeeding chapter. National Physical Laboratory Scale.— Exact agreement with regard to fixed points has not yet been arrived at in different countries, and an effort to co-ordinate the work of the National Physical Laboratory, the United States Bureau of Standards, and the Reichsanstalt, with a view to the formation of an international scale, was interrupted by the war. In 19 16 the National Physical Laboratory adopted a set of fixed points on the Centigrade thermodynamic scale, in conformity with which all British pyrometers have since been standardised. It will be seen that the figures differ very slightly from those given in the previous table, which represent the average results of separate determinations in different countries. Nationai . Physical Laboratory Scale (191 5). Substance. Physical Condition. At Melting Point Deg. Cent. Deg. Fahr. Water (ice) . 32 Water . „ Boiling ,, (760 mm.) IOO 212 Naphthalene 1 ? .5 , J ) (1 217-9 424 Benzophenone ,, ,, }] 11 3059 582 Zinc ,, Melting ,, 419-4 787 Antimony . , ,, 630 I 166 Common Salt ,, ,, ,, bo I 1 474 Silver (in reducing atmosphere) ,, ,, „ 961 1 761 Gold . ,, , , , , 1063 1 945 Copper(in reducing atmosphere) " I 0S3 1 982 1 8 PYROMETRV For higher temperatures the melting points of nickel (1452 C.) and palladium (1 549° C.) are em- ployed, but the accuracy in these cases is not so certain as with the substances named in the table. A useful point, intermediate between copper and nickel, has been established by E. Griffiths, and is obtained by heating nickel with an excess of graphite, when a well-defined eutectic is formed which freezes at 1 330° C, or 2426 F. Temperatures above the Present Limit of the Gas Thermometer. — As it is not yet possible to compare an instrument directly with the gas thermo- meter above 1 550° C, all higher temperatures must be arrived at by a process of extrapolation. By care- ful observation of a physical change at temperatures up to the limit of 1 550° C, the law governing such change may be discovered ; and assuming the law to hold indefinite!)', higher temperatures may be deduced by calculation. An amount of uncertainty always attaches to this procedure, and in the past some ludicrous figures have been given as the result of indefinite extrapolation. Wedgwood, for example, by assuming the uniform contraction of clay, gave 12001 C, or 21 637° F., as the melting point of wrought iron, whereas the correct figure is 1 520° C, according to the gas scale. Even in recent times, the extrapolation of the law connecting the temperature of a thermal junction with the electromotive force developed, obtained by comparison with the gas scale up to 1 100° C, led Harker to the conclusion that the STANDARDS OF TEMPERATURE 19 melting point of platinum was 1 710° C, a figure 45 degrees lower than that now accepted. The laws governing the radiation of energy at different tem- peratures, however, appear to be capable of mathe- matical proof from thermodynamic principles, and temperatures derived from these laws are in reality expressed on the absolute or thermodynamic scale. Extrapolation of these laws, when used to deduce temperatures by means of radiation pyrometers, appears to be justified ; but it is still desirable to extend the gas scale as far as possible to check such instruments. Assuming the radiation laws to hold, it is possible to determine the highest temperatures procurable, such as that of the electric arc, with a reasonable degree of certainty. CHAPTER III THERMO-ELECTRIC PYROMETERS General Principles. — Seebeck, in 1822, made the discovery that when a junction of two dissimilar metals is heated an electromotive force is set up at the junction, which gives rise to a current of elec- tricity when the heated junction forms part of a closed circuit. Becquerel, in 1826, attempted to apply this discovery to the measurement of high temperatures, it having been observed that in general the E. M. F. increased as the temperature of the junction was raised. No concordant results were obtained, and the same fate befel the investigations of others who subsequently attempted to produce pyrometers based on the See- beck' effect. These failures were due to several causes, but chiefly to the non-existence of reliable galvano- meters, such as we now possess. It was not until 1886 that the problem was satisfactorily solved by Le Chatelier of Paris. Although an) - heated junction of metals will give rise to an electromotive force, it does not follow that any pair, taken at random, will be suited to the pur- poses of a pyrometer. A junction of iron and copper, for example, gives rise to an E.M.F. which increases THERMO-ELECTRIC PYROMETERS 21 with the temperature up to a certain point, beyond which the E.M. F. falls off although the temperature rises, and finally reverses in direction — a phenomenon to which the name of " thermo-electric inversion " has been applied. Evidently, it would be impossible to measure temperatures in this case from observations of the electromotive force produced, and any couple chosen must be free from this deterrent property. Moreover, the metals used must not undergo deteri- oration, or alteration in thermo-electric properties, when subjected for a prolonged period to the tem- perature it is desired to measure. These and other considerations greatly restrict the choice of a suitable pair of metals, which, to give satisfaction, should con- form to the following conditions : — i. The E.M.F. developed by the junction should increase uniformly as the temperature rises. 2. The melting point of either component should be well above the highest temperature to be measured. An exception to this rule occurs when the E.M.F. of fused materials is employed. 3. The thermo-electric value of the couple should not be altered by prolonged heating. 4. The metals should be capable of being drawn into homogeneous wires, so that a junction, wherever formed, may always give rise to the same E.M.F. under given conditions. It is a further advantage if the metals which fulfil the above conditions are cheap and durable. The exacting character of these requirements 22 PYROMETRY delayed the production of a reliable thermo-electric pyrometer until 1886, when Le Chatelier discovered that a junction formed of platinum as one metal, and an alloy of 90 per cent, of platinum and 10 per cent, of rhodium as the other, gave concordant results. In measuring the E. M. F. produced, Le Chatelier took advantage of the moving-coil galvanometer introduced by d' Arson val, which possessed the advantages of an evenly-divided scale and a dead-beat action. This happy combination of a suitable junction with a simple and satisfactory indicator immediately established the reliability of the thermo-electric method of measuring temperatures. As platinum melts at 1 755° C, and the rhodium alloy at a still higher temperature, a means was thus provided of controlling most of the industrial operations carried out in furnaces. So far, the effect of heating the junction has been considered without regard to the temperature of the remainder of the circuit, and it is necessary, before describing the construction of practical instruments, to consider the laws governing the thermo-electric circuit, the simplest form of which is represented in fig. 2. One of the wires is connected at both ends to separate pieces of the other wire, the free ends of which are taken to the galvanometer Two junctions, A and B, are thus formed, which evidently act in oppo- sition ; for if on heating A the direction of current be from A to B, then on heating B the direction will be from B to A. Hence if A and B were equally heated no current would flow in the circuit, the arrangement THERMO-ELECTRIC PYROMETERS 23 being equivalent to two cells of equal E.M.F. in oppo- sition. Thermal junctions are formed at each of the .galvanometer terminals, but the currents to which they give rise, when the temperature changes, are opposed and cancel each other. The law which holds for this circuit may be expressed thus: — " If in a thermo-electric circuit there be two junc- Fio. 2. —Two-junction thermo-electric Circuit. tions, A and B, the electromotive force developed is proportional to the difference in temperature between A and B." It is customary to refer to the two junctions as the " hot " and " cold " junctions ; but it is important to remember that fluctuations in the temperature of either will alter the reading on the galvanometer or indicator. 24 PYROMETRY A second law, which applies to all thermo-electric circuits, is that ' ' the E. M. F. developed is independent of the thickness of the wire. " This does not mean that the deflection of the galvanometer is the same whether thin or thick wires are used to form the junc- tion. The deflection depends upon the current flowing through the circuit, and this, according to Ohm's law, varies inversely as the total resistance of the circuit. Consequently, the use of thin wires of a given kind will tend to give a less deflection than in the case of thick wires, as the resistance of the former will be greater, and unless the resistance of the gal- vanometer be great compared with that of the junction, the difference in deflection will be conspicuous. The E. M. F. , however, is the same under given conditions, whatever thickness of wire be used. Reference to fig. 2 will show that in order to realise this circuit in practice, one of the wires form- ing the couple must be used in the form of leads to the galvanometer. This can readily be done if the material of the wire is cheap ; but if platinum or other expensive metal be used, and the galvanometer be some yards distant, the question of cost necessitates a compromise, and the circuit is then arranged as in fig. 3. The wires forming the hot junction are brought to brass terminals T T, from which copper wires lead to the galvanometer G. This arrangement results in three effective junctions, viz. the hot junction A to B ; the junction A to brass, and the junction B to brass. It will be seen that the two junctions of THERMO-ELECTRIC PYROMETERS *5 copper to brass are in opposition, and cancel each other for equal heating ; and the same applies to the galvanometer connections. A circuit thus composed of three separate junctions does not permit of a simple expression for the net E. M.F. under varying tempera- ture conditions, and to avoid errors in readings care must be taken to prevent any notable change of Fig. 3.— Three-junction Thermo-electric Circuit. temperature at the terminals T T in a practical in- strument arranged as in the diagram A point of practical utility in thermo-electric work is the fact that if a wire be interrupted by a length of other metal, as indicated at C in fig. 3, no current will be set up in a circuit if both joints are equally heated, as the electromotive forces generated at each junction are in opposition. It is thus possible to interrupt a circuit by a plug-key or switch, without 26 I'YROMETRY introducing an error ; always provided that an even temperature prevails over the region containing the joints. Another useful fact is that if two wires be brought into contact, the}' may be fastened over the joint by soldering or using- a third metal, without alteration of thermo-electric value, except in rare cases. Thus a copper-constantan or iron-constantan junction ma)' suitably be united by silver solder, using borax as a flux, thus avoiding the uncertainty of contact which must always occur when the wires are merely twisted together. Welding, however, is preferable to soldering. Metals used for Thermal Junctions. — Until recent years it was customary to employ a platinum- rhodioplatinum or platinum-iridioplatinum junction for all temperatures beyond the scope of the mer- cury thermometer. The almost prohibitive price of these metals has caused investigations to be made with a view to discovering cheaper substitutes, with successful results up to i ooo° C. or i 8oo° F., thus com- prehending the range of temperatures employed in many industrial processes. Above this temperature the platinum series of metals are still used for accurate working, but it will be of great advantage if the range measurable by cheap or "base" metals can be further extended. Promise in this direction is afforded by the properties of fused metals when used in thermal junctions. An investigation by the author has shown that in general the E. M.F. de- TIIKRMO-ELECTRIC PYROMETERS 27 veloped by a junction does not undergo any sudden change when one or both metals melt, but continues as if fusion had not occurred. By making arrange- ments to maintain the continuity of the circuit after fusion, it may be possible to read temperatures approximating to the boiling points of metals such as copper and tin, both of which are over 2 ooo° C. The base metals are not so durable as platinum and kindred metals, but as the cost of replacement is negligible, this drawback is of little importance. Moreover, base-metal junctions usually develop a much higher E.M.F. than the platinum metals, whicli enables stronger and cheaper galvanometers to be used as indicators. Thermal Junctions used in Pyrometers. Upper limit to which Junction may be used. Couple. Oeg. Cent. l)eg. Fahr. Platinum and rhodioplatinum (10 per cent. Rh) . I 400 2550 2 Rhodioplatinum alloys of different composition I 600 2 9OO Platinum and iridioplatinum (10 per cent. Ir) 1 IOO 2 000 Nickel and constantan ..... 900 I 650 Nickel and copper ...... Soo 1 475 Nickel and carbon ...... 1 000 I 850 Nickel and iron ....... 1 000 1850 Iron and constantan ...... 900 1650 Copper and constantan ..... 800 1475 Silver and constantan ..... Soo 1475 2 Nickel chrome alloys of different composilion ( Hoskin's alloys) ..... I IOO 2 OIO Nickel-chrome alloy and nickel-aluminium alloy . I IOO 2 OIO 2 Iron-nickel alloys of different composilion I 000 185O 2 8 PYROMETRY The electromotive force developed by a junction of an)' given pair of metals when heated to a given tem- perature varies according to the origin of the metals. It is not unusual, for example, for two samples of IO per cent, rhodioplatinum, obtained from different sources, to show a difference in this respect of 40 per cent, when coupled with the same piece of platinum. Equal or greater divergences may be noted with other metals ; and hence the replacement of a junction can only be effected, with accuracy, by wires from the same lengths of which the junction formed a part. As showing how platinum itself is not uniform, it may be mentioned that almost any two pieces of platinum wire, if not from the same length, will cause a deflec- tion on a sensitive galvanometer when made into a junction and heated. It is therefore customary for makers to obtain considerable quantities of wire of a given kind, homogeneous as far as possible, in order that a number of identical instruments may be made, and the junctions replaced, when necessary, without alteration of the scale of the indicator. The alloy known as " constantan," which figures largely in the foregoing table, is composed of nickel and copper, and is practically identical with the alloy sold as "Eureka" or "Advance." It has a high specific resistance, and a very small temperature coefficient, and is much used for winding resistances. Couples formed of constantan and other metals furnish on heating an E. M.F. several times greater than that yielded by couples of the platinum series, THERMO-ELECTRIC PYROMETERS 29 and show an equally steady rise of E.M.F. with temperature. This alloy has proved of great service in connection with the thermo-electric method of measuring temperatures. Couples formed of nickel- chrome alloys, known as " Hoskin's alloys," have been introduced into Britain by the Foster Instru- ment Company, which may be used continuously to 1 100° C. , and for occasional readings up to 1 300 C. Another couple, much used in America, consists of an alloy of 90 per cent, nickel and ro per cent, chromium, and an alloy of 98 per cent, nickel and 2 per cent, aluminium, which may be used up to 1 ioo° C. Other couples, formed of alloys of nickel, chromium, iron, aluminium, etc., have been intro- duced by different makers, but have not proved so satisfactory as those mentioned above. Changes in Thermal Junctions when con- stantly used. — No metal appears to be able to withstand a high temperature continuously without undergoing some physical alteration ; and for this reason the E.M.F. developed by a given junction is liable to change after a period of constant use. At temperatures above 1 ioo° C, platinum, for example, undergoes a notable change in a comparatively short period, but below 1 ooo° C, the change is very slight, and if this range be not exceeded, a platinum-rhodio- platinum or iridioplatinum junction may be used for years without serious error arising from this cause. This liability to change is one of the factors which restricts the range of thermal junctions, which should 30 PYROMETRY never be used continuously beyond the temperature at which the alteration commences to become large. A second cause of discrepancy is the possible altera- tion in the composition of an alloy, due to one of the constituents leaving in the form of vapour, as is noted with iridioplatinum alloys, from which the iridium volatilises in tangible quantities above i ioo° C, causing a fall of 10 per cent, or more in the thermo- electric value of the junction of these alloys with platinum. Constantan appears to be very stable in its thermo-electric properties, and the various junctions in which it plays a part show a high degree of stability if not overheated. Rhodioplatinum alloys are very stable, and for temperatures exceeding I ioo° C. a junction of two of these alloys, of different com- position, is more durable than one in which pure platinum is used. 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 E. M. F. considerably, the change in some cases representing over ioo° C. on the indicator. A stable condition, due to the relief of strains or other change, was finally reached, and the conclusion drawn that the materials should be thoroughly annealed before calibration. It is desir- able in all cases periodically to test the junctions at some standard temperature, and if an)- conspicuous error be noted, to replace the old junction by a new one. In addition to the errors due to slow physical THERMO-ELECTRIC PYROMETERS 31 changes, a junction may be altered considerably, if imperfectly protected, owing to the chemical action of furnace gases, or of solids with which the junction may come into contact. The vapours of metals such as lead or antimony are very injurious ; and platinum in particular is seriously affected by vapours contain- ing phosphorus, if in a reducing atmosphere. So searching is the corrosive action of furnace gases that adequate protection of the junction is essential if errors and damage are to be avoided. When a wire has once been corroded, a junction made with it will not develop the same E.M. F. as before. Electromotive Force developed by Typical Junctions. — The following table exhibits the E.M.F. generated by several junctions for a range of ioo° C. , taken at the middle part of the working range in each case. These figures are subject to considerable varia- tion, according to the origin of the metals. I; Couple. E.M.F. in millivolts for a rise of 100° at middle of working range. Platinum-rhodioplatinum (10 per cent. Kb) i'i Platinum-iridiopiatirmm (ioper cent. Ir) I '2 Nickel-constantan ..... 2'3 Copper-constantan ..... 5-8 Nickel-copper ..... 6-1 Iron-constantan ..... 67 Hoskin's alloys ..... 7 '4 It will be noted that the ba St ;-metal junctions /e much higher values than the platinum series, and 32 PYROMETRY hence can be used with a less sensitive, and therefore cheaper, indicator. Base-metal junctions are also, in consequence of the greater E. M.F. furnished, capable of yielding more sensitive readings over a selected range of temperature. Practical Forms of Thermocouples. — When ex- pensive junctions are employed, wires of the minimum thickness consistent with strength and convenience of construction are used, a diameter of No. 25 standard wire gauge being suitable. A common arrangement is shown in fig. 4, in which J is the hot junction, the Fig. 4. — Practical Form of Thermo-electric Pyrometer. wires from which are passed through thin fireclay tubes which serve as insulators (or through twin-bore fireclay) to the reels R R, in the head of the pyro- meter, upon which a quantity of spare wire is wound to enable new junctions to be made when required. Two brass strips, S, are screwed down on to the wires at one end, and are furnished with screw terminals at the other end, from which wires are taken to the galvanometer or indicator. A protecting-tube, T, surrounds the wires and hot junction. The head, H, may be constructed of wood, fibre, or porcelain, and should be an insulator for electricity and heat. THERMO-ELECTRIC PYROMETERS 33 There are various modifications in use, but the general method described is adopted by most makers. In order to guard against errors arising from alterations in the temperature of the cold junctions in the end of the pyrometer, some firms construct the head so as to leave a hollow space, through which cold water is constantly circulated (fig. 5), the arrangement being known as a " water-cooled head." In some forms Fio. 5. — Pyrometer with Water-cooled Head. the supply of spare wires is made to take the form of two spiral springs in a hollow head, the upper ends of the springs being taken to terminals. The choice of a protecting-tube is a matter of considerable importance. Obviously, such a tube should not soften at the highest temperature attained, and when expensive metals are used to form the junction the sheath should not be permeable to gases or vapours. It should also, if possible, be a good conductor of heat, so that the junction may respond 3 34 PYROMETRV quickly to a change of temperature in its surroundings, and should be mechanically strong. It is difficult to secure all these properties in any single material, and the choice of a sheath is decided by the conditions under which the couple is to be used. The substances employed, and their properties and special uses, may be enumerated as follows : — I. Iron or Mild Steel. — For temperatures not ex- ceeding i ioo° C. iron or mild steel covers are cheap and efficient from the standpoint of conductivity, although liable to deteriorate owing to oxidation. The tendency to oxidise is greatly diminished by " calorising " the exterior by Ruder's process, in which the iron is heated in a mixture of metallic aluminium and oxide of aluminium, a surface alloy being formed which resists oxidation. A result nearly as good may be obtained by smearing the surface with fine aluminium powder, and bringing to a white heat. This treatment greatly prolongs the life of an iron sheath. Some makers employ an inner steel tube round the wires, and an outer tube which comes into contact with the furnace gases, corrosion of the latter being detected before the inner tube has given way and exposed the junction. Some makers do not consider it safe to expose heated platinum to an iron surface, with only air intervening, and hence use an inner cover of silica or porcelain, which the outer iron or steel tube protects from mechanical damage. For ordinary work seamless steam or hydraulic steel tubing, with a welded end, is satisfactory ; but for THERMO-ELECTRIC PYROMETERS 35 dipping into molten lead or other metals the tube should be bored from the solid. The great advantage of an iron or steel sheath is its mechanical strength, which protects the couple from damage in case of rough usage. 2. Niclirom. — Certain alloys of nickel and chro- mium, and especially that known as Niclirom II, may be kept at 1 ioo° C. without oxidising to any appreciable extent ; and hence sheaths of this material may be used up to the temperature named. In addition to being more durable than iron, nichrom possesses the same advantages of strength and good conductivity ; on the other hand, it is more costly. 3. Molybdenum. — This metal, which possesses a melting point of about 2 500° C, may be dipped in molten brass, bronze, copper, etc., without being attacked, and has been used to form the tip of a protecting-tube designed to measure the temperature of molten alloys. A junction covered only by a thin tube of molybdenum quickly attains the temperature of its surroundings. 4. Graphite and Graphite Compositions. — Carbon has the highest melting point of all known substances, and in the form of artificial or Acheson graphite may be easily machined to any desired shape. Graphite sheaths are sometimes used for immersion in molten metals, but at 1 000" C. and higher Acheson graphite oxidises easily and becomes friable. It is a good conductor of heat, but is easily broken. Compositions 36 PYROMETRV of natural graphite and refractory earths, such as Morgan's ' ' Salamander, " are inferior to pure graphite in conductivity, but are stronger and not readily oxidised, and may be used to form sheaths for tem- peratures up to i 400 C. or possibly higher, when penetration of furnace gases to the junction is not of moment. 5. Porcelain. — This material, in its best forms, may be used up to 1 400 C. , but must be efficiently glazed to prevent the ingress of furnace gases to the junction. It is easily broken by a blow, and when circumstances permit should be protected by an iron covering-sheath. The variety known as " Marquardt " has been found very satisfactory for high-reading thermal couples. Porcelain is not a good conductor of heat, and a junction encased in it does not respond quickly to external changes in temperature. 6. Vitrified Silica. — This substance, which may be worked in the oxy-hydrogen blowpipe, is largely used as a protecting-tube. It is not advisable, however, to use it for continuous work above I 100 C, as beyond this temperature devitrification occurs, and the tube becomes porous. It is a fairly good conductor of heat, and withstands rapid changes in temperature without cracking. It is very brittle, and for this reason is generally encased in iron. 7. Alundum. — This material is made from" fused bauxite, and has a melting point of 2 050 C. A special form of alundum, used for protecting-tubes, is non-porous up to 1 300 C. , and forms a satisfactory THERMO-ELECTRIC PYROMETERS 37 covering. Alundum is a moderately good conductor of heat, but is easily broken. 8. Carborundum. — This is an electric furnace pro- duct, which may be heated above 2 ooo° C. without damage. For making into pyrometer tubes, it is bonded with a suitable material, and baked after shaping. Carborundum, and the amorphous variety known as "silfrax," have proved useful for protecting junctions at temperatures as high as 1 6oo° C. The thermal conductivity is relatively good, but the tubes are easily broken. 9. Magnesia. — Tubes of this material, which melts at a temperature considerably above 2000° C, have been used for special work. Magnesia is a poor conductor of heat, and has little mechanical strength. 10. Zirconia. — This is a very refractory material, its melting point exceeding 2 500 C. It may be made into a vitreous variety, which is non-porous and proof against sudden temperature changes. At present, only a moulded form of pyrometer tube, made from zirconia powder, is available, the material worked in this manner being termed " zirkite. " Although zirconia is a bad conductor of heat, its other qualities are such that it forms an excellent material for work at the highest temperatures possible for thermal junctions ; and when the vitreous variety is available, may come into extended use. It will be seen from the foregoing that the ideal protecting-tube has yet to be found, and the user must choose the one which comes nearest to his 3» PYROMETRY requirements. Special consideration must be given in cases when chemical fumes are present, and a sheath selected which is not attacked or penetrated by them. Returning to the junction, it is advisable always to weld the wires, and not to rely upon the contact resulting from twisting them together. Platinum and the platinum alloys ma)' be welded readily by placing the junction in a coal-gas blowpipe fed with oxygen instead of air. For work at lower temperatures the platinum metals may be soldered by means of a small quantity of gold, in the flame of a Bunsen burner. When cheap metals are used for the junc- tion the construction may be considerably modified, and often with advantage. In fig. 6, for example, which represents a thermocouple made by A. Gallenkamp & Co., the metals used are copper and con- stantan, and the hot junction, fastened by silver solder, is supplemented by a cold junction of the same metals located in the head. The copper wire from the hot junction passes directly to a copper terminal, from whence a copper wire lead is carried to the galvanometer ; and the same procedure is carried out with the copper wire from the cold junction, thus realising the circuit shown in fig. 2. The cold junction is kept in oil, Fig. 6. Pyrometfk WITH Special Coi.d Junction in Head. THERMO-ELECTRIC PYROMETERS 39 the temperature of which is registered by a short thermometer, thus enabling (as will be explained later) the correct temperature of the hot junction to be deduced under any circumstances. In this instru- ment twin-bore fireclay is used to insulate the wires, and the protecting-tube is of iron — which suffices for the upper limit (8oo° C.) to which the junction may be used. Iron and constantan could be used in this manner by employing iron leads to the galvanometer. Another type of instrument, rendered practicable by the use of cheap metals, and which may be termed the "heavy type," is constructed of thick pieces of the metals welded together instead of wires, thus ensuring greater strength and longer life. Messrs Crompton & Co. were the first to introduce thermo- couples of this type, consisting of a heavy steel tube, to one end of which a nickel rod is welded, the other end being free, and the length of the rod suitably- insulated from the steel tube ; leads for the rod and tube being taken to the galvanometer. Fig. 7 shows a couple of this kind, made by Paul, consisting of an iron tube down the middle of which a constantan rod is passed, insulated from the tube by magnesia. At the tapered end the two metals are welded together, and at the free end a special cap, fitted over the tube and rod, the contact parts being insulated from one another, serves to enable leads to be taken to the galvanometer. Similar thermocouples are made by the Foster Instrument Company (fig. 8), and are simple, cheap, and reliable up to 900° C. with an iron- 4Q PYROMETRY constantan couple, and to i rco° C. with nichrom couples. When worn out they may be replaced, at a trifling cost, by others made from the same batch of metal. 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 coupled. In nickel-carbon junctions the contact is sometimes ensured by the aid of a spring, which Heavy Type, Cheap-metal Pyrometer. presses the two substances together. Such an arrange- ment is evidently not so reliable as one in which the materials are welded, and a defective contact, arising from any cause, would lead to serious error. A preferable plan is to screw both the nickel and carbon rods into a cross-piece of either element. When applying a thermal junction to the measure- ment of surface temperatures, such as steam-pipes or the exterior of furnaces, the wires ma)' be passed through a thin disc of metal, about \ in. in diameter, THERMO-ELECTRIC PYROMETERS 41 and soldered at the back. Suitable materials are copper and constantan, soldered to a thin copper disc with silver solder, and brought to a cold junction in the head of the instrument as shown in fig. 6. The terminal piece of the insulation ma be made Si Fig. 8. — Foster's Cheap-metal Pyrometer. of hard wood, with the holes countersunk so as to cover the solder and enable the wood to touch the disc, which, when pressed on the hot surface, will then rapidly acquire the temperature. The author has found, by trials under varying circumstances, that this method of measuring surface temperatures gives reliable and concordant results. For very high surface temperatures a platinum disc, with one of the 42 PYROMETRY usual platinum metal couples -soldered to the disc with pure silver, and a piece of twin-bore fireclay brought to the back of the disc, will be found to suffice for most cases arising in practice. A small blowpipe flame is best for soldering the wires to the disc, borax being used as flux in the first case ; but no flux is necessary in soldering the platinum metals with pure silver. 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 junction. When the temperature of a furnace is uniform it is sufficient to allow the end of the thermocouple to protrude about 12 inches into the interior, but when following the change of temperature undergone by objects in a furnace the end must be located near the objects. If the distance from the exterior of the furnace to the objects exceed 2 feet, the thermo- couple should be inserted through the roof so as to hang vertically, as if placed through the side it would droop by its own weight at high temperatures. The distance between the exterior of the furnace and the cold junctions should be at least 15 inches in all cases in which the heating of the cold junction is not automatically compensated. After inserting the couple the opening through the furnace wall should be closed by means of suitable luting-clay. In certain instances, such as flues, it is necessary to use a long instrument in a horizontal position. A rail may then be placed across the flue, at a suitable place, to serve as a support and so to prevent drooping. THERMO-ELECTRIC PYROMETERS 43 Liquid Element Thermocouples.— An investigation by the author and A. W. Grace has shown that the continuity of the E.M.F. produced by a rising tem- perature is not interrupted by fusion, except in the cases of bismuth and antimony, which both show an abrupt change in thermo-electric properties at the melting point. It would therefore appear feasible to measure temperatures by constructing a thermo- couple so as to retain the circuit after fusion, 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 possible. The boiling points of some of the common metals are appended : — Aluminium Silver Tin Copper . Nickel . Iron Metal ; Point. I)e s . C. Deg. F. I 800 3270 1 955 3550 2 270 4 120 2310 4 190 2330 4225 2450 4440 From inspection of these figures, it will be seen that if a suitable couple could be obtained, 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 series. Instead of using two metals, graphite might 44 PYROMETRY form one member of the couple, provided that no objection to its use existed on other grounds. The form of thermocouple designed by the author to permit of the use of molten elements is shown in fig. 9. A rod of refractory material, R, is perforated longitudinally by two holes, down which are passed rods of the thermo-elements, A and B. The lower ends of A and B are inserted in a graphite block G, which is jointed on its upper face to R ; the whole ^\\v;;;;;;;;; /;/;;;;;;// ;;;;;;;;V;77P ww/;;/;;;////;;;///;//;;/;/;///; a \\v;;;;;;;;;;;;;/;;;;;;;;;//;;/;a B Fig. q. — Liquid-element Thermocouple. being surrounded by the refractory cover C. On either or both of the elements melting, the circuit is maintained through G, which serves also to prevent the mixing of A and B when molten, whilst not affecting the E.M.F. developed. In order to allow for the expansion of the metals on melting, A and B are made to fit loosely in R. When inserted in a furnace to a depth represented by EF, only the portion of the metals adjacent to the closed end will melt, the outer parts remaining solid. At present it has not been found possible to procure the refractory THERMO-ELECTRIC PYROMETERS 45 parts in a form suited to commercial use, but when this obstacle is overcome this type of thermocouple should prove of service for measuring temperatures beyond the scope of ordinary base-metal junctions. Indicators for Thermo-electric Pyrometers. — As the electromotive force developed by a single junction when heated is small, a sensitive galvanometer is required to indicate the minute current flowing through the circuit. Delicate millivoltmeters, of the moving-coil type, are universally employed, as they possess the advantage of an evenly-divided scale combined with the requisite degree of sensitiveness. The original d'Arsonval galvanometer, consisting of a coil suspended by a metallic strip between the poles of a horse-shoe magnet, was used by Le Chatelier, who, by its aid, was enabled to lay the foundations of this branch of pyrometry. Three forms of this instrument are now in use, viz. (a) the suspended coil "mirror" type; (b) the suspended coil "pointer" type; and (c) the pivoted type. Examples of each will now be described. Fig. 10 represents a mirror galvanometer working on the d'Arsonval principle, designed by Gen. Holden, F. R.S. The horse-shoe magnet is laminated, and an iron core, supported by a pillar, is placed between the poles. The coil, which moves in the space between the core and the poles of the magnet, is suspended by a thin, flat strip of phosphor-bronze, which carries a small circular mirror. A similar phosphor-bronze strip is fastened to the lower part of 4 6 PYROMETRY the coil, and is continued to an adjusting-screw in the base. The ends of the suspension strips com- municate with the terminals of the galvanometer, and a current entering at one terminal passes through the metallic suspensions and the coil to the other. The effect of passing a current through the coil, which is located in a powerful magnetic field is to Fir,, io. — Hoi.den-d'Aesonval Mirror Galvanometer. produce an axial movement tending to twist the suspension strips, which movement is greatly magni- fied by a spot of light reflected from the mirror on to a distant scale. When the current ceases, the untwisting of the strip restores the coil to its former position. Galvanometers of this type are remarkably "dead-beat" in action, that is, the movement and restoration of the coil are accomplished without vibra- tion. A semi-transparent scale, placed at i metre THERMO-ELECTRIC PYROMETERS 47 distance, and 50 centimetres long, is suitable for use with this galvanometer. When used in workshops, it is necessary to protect a mirror galvanometer from the vibrations produced by machinery, which would cause the spot of light to become unsteady. The Fig. ii.— Lambert's Anti-vibration Stand for Galvanometers. best method of effecting this is shown in fig. 11, which represents the mode of suspension devised by W. J. Lambert for use in the Royal Gun Factory, Woolwich Arsenal. The usual supports of the galva- nometer are abolished, and the instrument suspended from the ring of a brass tripod, so as to keep three 4 8 PYROMETKV springs partly in compression. When suspended in this manner, a mirror galvanometer is quite suited to commercial use ; in the quiet of the laboratory the ordinary supports may be employed. The advantage gained by using the mirror type is that a much longer scale is possible than with instruments furnished Fig. 12.— Siemens' Thermo-electric Indicator. with a pointer, and hence greater accuracy in deter- mining temperature readings may be secured. In suspended coil instruments furnished with a pointer, the construction differs only in detail from the foregoing. In place of the mirror, a light pointer is attached to the suspension so as to rest on the coil and a scale is furnished over which the pointer moves. Fig. 12 is an example of this type, made by Messrs Siemens, the suspension being contained in the tube which rises from the body of the instrument. The THERMO-ELECTRIC PYROMETERS 49 maximum length of scale moved over by the extremity of the pointer is about 6 inches, as a longer and there- fore heavier pointer would reduce the sensitiveness below the point requisite for thermo-electric work. In the double-pivoted type, the suspension is elim- inated, and pivots are fastened to each end of the moving coil which rest in bearings. The turning of the coil is made to compress a hair spring, made of phosphor-bronze ; and when the current ceases the unwinding of this spring restores the coil to its former position. The coil carries a pointer which moves over a scale. These instruments are not so sensitive as those in which the coil is suspended, but can be made sufficiently sensitive to work with any kind of junction in practical use. The pivoted form is cheaper and stronger than the suspended type, and is used whenever sufficiently sensitive. The " Uni-pivot " galvanometer, made by R. W. Paul, is shown in figs. 13 and 17. The coil, which carries the pointer, is circular, and moves round a spherical core of iron placed between the poles of the magnet. A hole is drilled in the iron core, and the coil rests on a single bearing at the bottom of this hole. A phosphor-bronze control-spring serves to restore the coil to the zero position. The lessened friction due to the use of a single pivot enables this instrument to be made very sensitive when needed, so that a relatively small rise in the temperature of a junction may cause the pointer to traverse the whole length of the scale. 4 5o I'YROMETRY Special Features of Indicators. — All moving- coil instruments, whether suspended or pivoted, are liable to alteration of the zero point owing to what is termed "creep." The suspension strip, when first fixed in position, generally possesses a certain amount of initial torsion, which comes into operation gradually and causes a slight movement of the coil. Similarly, Fie -Principle of Uni-pivot Galvanometer. in a pivoted instrument, the strength or shape of the control-spring undergoes a gradual alteration at first, causing the pointer to move away from the zero position. For this reason adjusting arrangements are fitted by means of which the spot of light or pointer may be brought back to the zero. This creeping ceases after a time — often requiring twelve months — and if not subjected to any strain, error from this cause does not recur to any notable extent. With a mirror THERMO-ELECTRIC PYROMETERS 51 galvanometer it is better to move the scale, or turn the galvanometer round on its axis to restore the correct zero, rather than to twist the coil back ; but with a fixed scale and pointer the only remedy is to turn the coil bodily round. In a single-pivot indicator constantly used in the author's laboratory, the creep amounted to a movement of the end of the pointer through an angle of 2 degrees in the first few months, since when, after the lapse of several years, no further alteration has occured. It is advisable to test the zero point of an indicator from time to time by breaking the circuit, and if an error be discovered the pointer should be re-set, or an allowance made in taking a reading. 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 ma)' arise in practice. If leads of considerable length were used to connect the pyrometer with the indicator, and were subject to fairly large alterations of tem- perature, the consequent changes in the resistance of such leads would be noticeable on a low-resistance indicator; and similarly, if a pyrometer were inserted at different depths in a furnace at separate times, thus heating up varying lengths of the junction wires, a discrepancy would arise for the same reason. The resistance of an indicator, however, cannot be raised beyond a certain point without reducing the sensitive- ness below the required limit. A mirror galvanometer of the type described may have a resistance — partly 52 PYROMETRY in the coil and partly in an added series resistance — of i ooo ohms or more, and still be sufficiently sensitive ; and in the latest types of instruments provided with pointers the resistance may be made as high as I ooo ohms, although it is more usually 400 to 500 ohms. Many indicators are in use, however, in which the resistance is 100 ohms or less. As, from Ohm's law, the current varies inversely as the total resistance in the circuit, any alteration in resistance should be small relatively to the total to render the error negligible. This point is made clear in the following example : — Example. — A thermocouple and leads have a resist- ance of 5 ohms and are subject to alterations amounting to 1 ohm. To find the errors re- sulting when indicators of resistances 800, 400, and 50 ohms respectively are used. From Ohm's law, C = — , the variation in R C, with E constant, will be 1 in 805, 1 in 405, and 1 in 55 respectively. As the indications are proportional to the current, the alterations caused will be approximately \ per cent., \ per cent., and 2 per cent. The first two may be ignored ; the last may be quite serious and lead to the failure of an operation. It will be seen from the foregoing that low-resist- ance indicators should only be used for fixed thermo- couples and short leads not subject to temperature changes, or, in other words, in a circuit of fixed resistance. THERMO-ELECTRIC PYROMETERS 53 The resistance of an indicator, when unknown, may be found by the following method, suggested by the author : — A resistance box is joined at one end to one terminal of the indicator. To the other terminal a fairly stout iron wire, 18 inches long, is connected, and a similar length of constantan wire is coupled to the other end of the resistance box. The free ends of the wires are twisted into a junction which is dipped into boiling water. The deflection obtained with no resist- ance in the box (Dj) is noted, and resistances (R) are then unplugged until trie deflection (D 2 ) is approxi- mately one-half of D r The resistance (G) of the indicator, ignoring that of the wires, is then given by the formula r D.,R G= D ] --D 2 as may readily be proved from Ohm's law, E being constant. This method is extremely simple and reasonably accurate. Reliable indicators are now procurable from many instrument-makers at a comparatively small cost, pro- gress in this direction having been most marked in recent years, particularly in the case of pivoted instru- ments. The most convenient form for workshop use is made with an edgewise scale (fig. 14) and may be placed in a suitable position fixed to a bracket. The flat-scale pattern is preferable for use on a laboratory table, or for a portable pyrometer. The sector pattern is also good for workshop use, the dial being visible from a distance. 54 PYROMETRV Standardizing' of Indicators to read Tempera- tures directly. — The temperature scale of an indicator, for use with a given thermal couple, is always marked by the maker in the case of instruments furnished with a pointer, and, generally speaking, is correct within reasonable limits. It is customary and necessary to send with the instrument a statement of the cold-junc- 14. — Indicator with Edgewise Scale. tion temperature for which the markings are correct ; say 2o" C. or 6o° F. The user should then endeavour to maintain the cold junction at this specified tem- perature when taking a reading, or otherwise a con- siderable error may be introduced. It is highly desirable, however, that the user should be able to perform the standardizing himself, if only for checking purposes ; and when using a mirror galvanometer as indicator it is necessary to standardize on the spot at THERMO-ELECTRIC PYROMETERS 55 which the instrument is fixed. Ability to prepare a temperature scale is further useful, inasmuch as any good millivoltmeter, of range o to 20 millivolts, may be used for thermo-electric work of all kinds, and may be calibrated for different junctions, a suitable series resistance being added to enable E.M.F. 's higher than 20 millivolts to be measured. Such an instru- ment may thus be made extremely useful, both in the workshop and laboratory. Standardization may be effected either by subject- ing the hot junction to several known temperatures, and noticing the deflections corresponding thereto ; or by measuring the electromotive force developed by the junction, and calculating the corresponding tem- perature from a formula which is known to hold for the range comprehended by the instrument. The former method is simpler ; and if carefully conducted is quite accurate. The latter method possesses the advantage that readings in millivolts may be trans- lated directly into temperatures when the constants of a given thermal couple are known. It is now usual to mark indicators with a double scale, one reading millivolts and the other temperatures. Standardization by Fixed Points. — Taking any millivoltmeter which, with a maximum of 20 millivolts at the terminals, will give a full scale deflection, the first step is to arrange that the pointer (or spot of light) shall just remain on the scale at the highest temperature to be attained by the junction. This may be done by placing the hot junction in boiling water 56 PYROMETRV and noting the deflection obtained, either in millivolts or equal arbitrary divisions, and also the temperature of the cold junction. The deflection observed is due to a difference of temperature (ioo— t) deg. C, where ^is the temperature of the cold junction. If the highest temperature to be measured is 10 times (ioo — t), the deflection should be rather less than -^ of the scale, and similarly for any other required temperature limit. If the observed deflection exceed this proportion, a series resistance should be added until the correct value is obtained. This resistance is then perma- nently installed in the circuit for use with the junction under trial. Before proceeding further it is necessary to con- sider whether the pyrometer is to possess a single cold junction of ascertainable temperature (as in fig. 6), or whether it will be arranged with two cold junctions in the head, as in fig. 4. In the former case it is simpler to prepare a " difference " scale ; that is, one which reads differences of temperature between the hot and cold junctions, from which the tempera- ture of the hot end may be obtained by adding to the difference that of the cold junction. In the latter case the cold end should be kept by artificial means at the temperature likely to be attained in practice — say 25" C. — a water-bath being suitable for this pur- pose. It is advisable to remove the shield of the pyrometer when standardizing, so as to expose the hot junction, as closer readings can then be taken. A number of materials — preferably cheap — of THERMO-ELECTRIC PYROMETERS 57 known boiling points or melting points are then selected from a table of fixed points (page 16) so as to give about six points, distributed fairly evenly over the scale. As an example, if it were desired to prepare a temperature scale from o° to i 100° C, the following might be chosen : — Substance and Condition. T mpt raiure. Water at boiling point ..... 100° c 212° F. Tin at melting point ..... 232 449 Zinc at melting point ..... 419 7b6 Antimony at melting point .... 63' 1 167 Common salt at melting point 800 I 472 Copper at melting point (covered with graphite) 1 0S4 I9S3 The hot junction is allowed to attain these tem- peratures successively, and the corresponding deflec- tion in each case is noted. It is then possible to divide up the whole of the scale to read temperatures directly. The first reading is taken by placing the junction in a vessel of boiling water, 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 pressure. To ensure that the other readings are taken when the substances are exactly at the melting point, the procedure is as follows : about 2-3 lb. of the substance are melted in a salamander crucible, and a small fireclay tube, closed at one end, is inserted in the molten mass. The hot junction is placed in the fireclay tube, and 5- c PVROMETRY the intervening space filled with asbestos fibre. Great care must be taken not to let the junction touch the [used substance The crucible is now allowed to cool, and a reading of the deflection taken every half- minute. When the substance is exactly at its solidi- fying point — identical in general with the melting point — the deflection remains stationary for several consecutive readings, owing to the liberation of latent heat of fusion in sufficient quantity to balance the loss by radiation. This stationary reading is noted for each substance, and represents the deflection given when the hot junction is at the temperature corresponding to the melting point, and the cold junction or junctions at the temperature existing when the observation is made. For melting the materials, a Davies furnace with a large Teclu or Meker burner is convenient up to 850' C. ; but to melt the copper a blast lamp is requisite. The molten mass ma)' be allowed to cool in the furnace. From these observations a calibration curve may be drawn either for differences between hot and cold Pyrometer 1. Iron-constantan. Pyrometer 2. Temperature (Series resistance in galvanometer circuit.) Platinum -iriclioplatinum. Junction. Deflection. Cold Junction. Difference. Deflection. Cold Junction. ioo° C. 8-9 15° C. ss°c. 5'5 232 2I'S 40'6 17 215 15-6 419 19 400 29-4 631 6 3 -8 19 t>[2 45'5 25° c. Soo 83O 20 7S0 59-0 1 084 820 THERMO-ELECTRIC PYROMETERS 59 junctions, or for a steady temperature of the cold junctions. Two sets of data are appended to illustrate the procedure. Pig. 15, A, is a calibration curve for thermocouple I, i?uo 1100 1,000 900 U 800 \ V- rHI* .1 .11 =-B P h Fig. 30. — Differential Galvanometer Method of Measuring Resistance. coil of the galvanometer being equal, it follows from Ohm's law that P is equal to R when no deflection is obtained. The accuracy of this method depends upon the sensitiveness of the galvanometer, and also upon the extent to which the two coils may be regarded as truly differential, as the measurement evidently 104 PVROMKTRY assumes complete equality in resistance and effect on the moving part. With modern galvanometers of this pattern, it is possible to secure readings of suffi- cient accuracy for the purposes of pyrometry. The method, however, is less sensitive than the Wheat- stone bridge, now to be described. Fig. 31. —Principle of Wheatstone Bridge. Measurement of Resistance by the Wheatstone Bridge. — The principle of this method is shown in n g- 3 1 , where a and b are two fixed resistances of known value ; d is an adjustable resistance ; x the resistance to be measured ; B a battery ; and G a sensitive galvanometer. If, in this circuit, d be adjusted until no deflection is shown on the galvano- RESISTANCE PYROMETERS 105 meter, then , = — ; or 1= — ; — . Hence, if a = b, b a b then x will be equal to d. It is not difficult to construct a portable apparatus, suitable for workshop use, by means of which the value of x may be determined to O'Oi ohm ; and in the laboratory, with a very delicate galvanometer, O'OOl ohm may readily be detected. The Wheatstone bridge method is the best for the accurate measure- ment of resistance ; but in resistance pyrometers it is sometimes advisable to sacrifice extreme accuracy in order to gain advantages in other directions, as will be shown subsequently. Relation between the Resistance of Platinum and Temperature. — As platinum is the only feasible metal to use in the construction of resistance pyro- meters, it is essential that the effect of temperature on the resistance of this metal should be known. Difficulties were experienced, in the early days of resistance pyrometers, from the fact that different samples of platinum wire, of varying degrees of purity, gave widely differing results in this connec- tion ; and no certainty was attained until 1886, when Professor Callendar thoroughly investigated the sub- ject, and evolved a formula from which the tempera- ture of a given kind of platinum could be deduced with great accuracy from the resistance. In order to understand this formula and its application, it will be necessary to consider the underlying principles upon which it is founded. 106 PYROMETRY If the resistance of a platinum wire be measured at a number of standard gas-scale temperatures, and the results depicted graphically by plotting resist- ances against corresponding temperatures, the curve obtained is part of a parabola, exhibiting a decrease in the rate at which the resistance increases at the higher temperatures. A second platinum wire, of different origin and purity, and of the same initial resistance as the foregoing, would furnish a curve which, although parabolic, would not overlap that obtained with the first wire. 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 resistance, after three measurements at known gas-scale temperatures had been determined. The calibration of a resistance pyrometer was thereby reduced to three exact observations, instead of a large number distributed over the scale ; and, moreover, the formula in question was found to give results of great accuracy over a wide range of temperature for any kind of platinum wire. Before dealing with Callendar's formula, the term "degrees on the platinum scale" will be explained. Such degrees are obtained by assuming that the increase of resistance of platinum is uniform at all temperatures ; that is, that the temperature-resistance curve is a straight line, and not a parabola. For example, a piece of platinum wire of 2 - 6 ohms resist- ance at o° C. will show an increase to 3 '6 ohms at 100 C. — an addition of 1 ohm for ioo°. We now RESISTANCE PYROMETERS IO7 assume that a further augmentation of 1 ohm, bringing the total to 4 - 6 ohms, will represent an increase of ioo°, or a temperature of 200°. Similarly, a total resistance of 5-6 ohms would indicate 300 , and 1 2 '6 ohms 1 000°. The temperature scale ob- tained by this process of extrapolation is called the " platinum scale, " and differs considerably from the true or gas scale, the difference becoming greater as the temperature rises. This is indicated in fig. 32, in which A represents the true parabolic relation between resistance and temperature, and B the as- sumed straight-line relation. Reading from curve A, the temperature corresponding to 8 ohms resist- ance is 6oo° C. ; but from B the same resistance is seen to represent only 545 C, which is the "temperature on the platinum scale" to which this resistance refers. An inspection of fig. 32 shows that at all temperatures, except between o° and ioo°, the platinum-scale readings for given resistances are less than those indicated on the gas scale. Callendar's formula is expressed in terms of the difference between the gas-scale and platinum-scale readings, and takes the form 100/ Vioo/J' -p = l\\~ where t = temperature on the gas scale, p = temperature on the platinum scale. (5 = a constant, depending upon the purity of the wire. ioS PYROMETRY In order to determine the value of S, it is neces- sary to measure the resistance of the wire at o°, ioo°, and a third temperature, which should be considerably 12 // 10 3 a e c o c CO ■5 «j / / / / y > / / '/ \ A /< '/ // '/ y // 2 / 100 200 ZOO 4-00 S00 BOO 700 800 300 IJOO0 1)00 Temperature , Degrees C. Fig. 32. — Connection between Resistance of Platinum and Temperature: A, on Gas Scale; B, on Platinum Scale. above ioo°. The readings at o° and ioo° are requisite to establish the platinum scale of temperatures ; the third reading is required to calculate the value of 0, as p and t are equal at 0° and 100 , these points RESISTANCE PYROMETERS 109 forming the basis of both scales. An example is appended to make this matter clear. Example. — A platinum wire has a resistance in ice of 2'6 ohms ; in steam, y6 ohms ; in boiling sulphur, 6'8i 5 ohms. To find the value of 8, the boiling point of sulphur being 444'5 on the gas scale. Since an increase of (3'6— 2'6) = i ohm is produced by 100°, the increase observed in boiling sulphur, (68 1 5 — 2 '6) = 4 '2 I 5 ohms. will represent a temperature, on the platinum , c 4'2i; x 100 H o . scale, of ^ - J = 42 15 p. Applying Callendar's formula, (44«-4,,5)^{(^)'-(^)) the value of $ is found to be P5. Callendar, in his experiments, employed the boiling point of sulphur for the third point, and determined this temperature on the gas scale with great accuracy, The necessity for extreme precision in applying this formula is made clear by noting the effects on the value of S resulting from small differences in the figures chosen in the above example. If, for instance, the boiling point of sulphur on the gas scale were taken at 2° lower, or 442 "5, the value of S would work out to 1 ^7 ; and the error at 1 200' C. thus caused would amount to 17'. The same discrepancy would IIO PYROMETRY be observed if the resistance in boiling sulphur were taken as 6 '835 ohms, an error of C02 ohm ; and a still greater error would result if the difference in re- sistance at o° and ioo° were measured as C99 ohm instead of 1 ohm. From an extensive experience of the difficulties attendant on correctly determining the value of <5, the author has found that no reliable result can be obtained unless measuring instruments of the highest precision are used, and elaborate precautions taken to ensure the exact correction for alterations in the boiling points of water and sulphur occasioned by changes in atmospheric pressure. Unless the neces- sary facilities are at hand, an operator would be well advised to standardize a resistance pyrometer by taking several fixed points and drawing a calibration curve, after the manner recommended for a thermo- electric pyrometer. If a resistance pyrometer be calibrated so as to read in platinum-scale degrees, and the value of <5 be known for the wire, the correct gas-scale temperatures may be calculated from Callendar's formula. The table on next page gives the results of a number of calculations made in this manner. Changes in Resistance of Platinum when constantly Heated. — The resistance of platinum undergoes a gradual change when the wire is kept continuously above a red heat ; and if the tempera- ture exceed 1 000° C. the change becomes very marked after a time, leading to serious errors in temperature indications when used in a pyrometer. The altera- RESISTANCE PYROMETERS III tion under notice is due, as shown by Sir William Crookes, to the fact that platinum is distinctly volatile above i ooo° C, and hence the diameter of the wire diminishes. This variation constitutes a serious Comparison of Gas and Platinum Scales. 8 1-5. Platinum Thermometer Air Thermometer Reading Difference Reading (Pt.). t (deg. C). (/-Pt.). - IOO 97-1 -1- 2'9 O 5° 49'6 0'04 IOO IOO O 200 203' I 3' 300 309-8 9-8 400 4 20 '2 20'2 500 534'9 34'9 60O 654-4 54'4 700 7 79 '4 79 '4 Soo 910-7 110-7 900 1 049-4 1494 1 000 1 i97 - o 197-0 I IOO ' 355'° 255-0 I 200 i 526-7 3267 1 300 1 716-0 4l6'o drawback to the use of resistance pyrometers for temperatures exceeding 1 ooo° C. Terms used in Resistance Pyrometry. — Follow- ing on the researches of Callendar and others, certain terms relating to resistance pyrometers have come into use, and will now be defined. I 12 PYROMETRY (1) The Fundamental Interval is the increase in resistance between o° C. and too c C, or R 10O — R . It should be remembered that the increases between 200° and 300°, or 8oo c ' and 900 , all temperatures being taken on the gas scale, differ from the funda- mental increase. (2) The Fundamental Coefficient is that fraction of the resistance at o° C. by which it increases per degree between o~ and ioo°, on the average, or Tl p JN -lfK) 1X R x 100' This figure is in reality the average temperature co- efficient between o J and 100". For pure platinum the value is 0-^5-, or 0'003 846. ( 3 ) The Fundamental Zero is the temperature, on the platinum scale, at which the resistance would vanish ; it is evidently the reciprocal of (2), prefaced by a minus sign, or _ R x 1 00 t> rj ' JV 100 tx o For pure platinum this temperature would be — 260/, since it is assumed that the average increase or de- crease per degree holds throughout ; that is, for even- degree the metal is cooled the loss of resistance is taken to be 075-75 of the resistance at o : . Hence at — 260/ the resistance, on this assumption, would vanish. (4) The Difference Formula is the expression which RESISTANCE PYROMETERS "3 OXWOOO HEAP gives the relation between gas-scale and platinum scale temperatures, or (V 100/ Vioo/J This formula has already been fully dealt with. (5) The Platinum Constant _g_ is S in the above expression. The value for pure platinum is about i'S, but small quantities of impurities may alter the figure considerably. The truth of the formula (4), however, is unaffected by changes in S, ' as p would be correspondingly altered. Practical Forms of Resist- ance Pyrometers. — A typical form of resistance pyrometer, made by the Cambridge and Paul Instrument Company, is " 5 °",. illustrated in fig. 33. The coil of platinum wire is wound round the edges of a mica framework, made of two strips of mica fastened at right angles so as to form a + in section. This method of winding is due to Callendar, who discovered that mica was chemically inert to- wards platinum, even at high ^%1^ p ^vrc,metek. e ' MICA WASHERS PORCELAIN TUBE rHnOUCHOUT WHOLE LEMCTH I 14 PYROMETRY temperatures. The leads, also of platinum wire, pass from the coil through mica washers to terminals fastened to the boxwood head. A second wire, not connected with the coil, but identical in length and diameter with the ordinary leads, is bent into two parallel branches, which are passed through the mica washers side by side with the leads, and are brought to a second pair of terminals in the head. The function of this wire is to compensate for changes in the resistance of the leads when heated, by opposing the compensating wire to the pyrometer in the measuring arrangement, when the resistance of the leads and wire, being equal, will cancel, the resistance actually measured being in consequence that of the coil only. Fig. 34 shows the connections for a Wheatstone bridge when this method of compensation is employed, a and b representing two equal fixed resistances, P the pyrometer coil, x the leads, L the compensating wire, and d the adjustable resistance. When no deflection is observed , , a x-\- P . . , . on the galvanometer, 7- = -s r, and since a = /; and & b L + d x = L, it follows that P = d. The protecting tube used by the Cambridge and Paul Instrument Company is made of porcelain, which is found to shield the platinum completely from the furnace gases, but is extremely fragile, and for work- shop use should be protected by an outer iron sheath. Resistance pyrometers made by other firms differ in detail from the foregoing. In the Siemens pyro- meter the coil is wound on special fireclay, and pro- RESISTANCE PYROMETERS "5 tected by an iron sheath, the space between the coil and the sheath being filled with magnesia, which effectively prevents the corrosion of the platinum ; and compensation is effected by means of a single wire passing down the centre and connected to one end of the coil, a special form of Wheatstone bridge Fig. 34.— Wheatstome Bridge as used with a Resistance Pyrometer. being used to take the measurement. In the instru- ments made by R. W. Paul the coil is made of flat strip rolled out from wire, wound on mica, and pro- tected by a silica tube and outer iron sheath. The Leeds-Northrup Company of Philadelphia employ a rod of obsidian on which to wind the coil, and also make a form in which the coil is wound so as to be 116 PYROMETRV self-sustaining, thus dispensing with the support. In all cases the coil is wound non-inductively, i.e. the wire is doubled before making into a spiral. The zero resistance of a given instrument depends upon the accuracy of the measuring appliances used, and upon the degree of precision it is desired to attain. If, for example, it is intended to read to i° C, with appliances capable of measuring to T ^y of an ohm, a convenient zero resistance is 2"6 ohms ; it being found that with pure platinum the resistance rises from 2 '6 ohms at o° to 3'6 ohms at 100 C, an increase of 1 ^ JJ of an ohm for i° C. With coarser measuring arrangements, for the same degree of precision, a correspondingly higher zero resistance will be required ; thus if -%-% ohm be the least amount detectable by the measuring device, a zero resist- ance of io'4 ohms would enable i° C. to be observed. It is evident that a suitable zero resistance may be calculated similarly in all cases when the limit of the measuring appliance is known, and the minimum temperature interval specified. For work above a red heat, the leads from the coil should always be made of platinum. Copper, leads, when heated, give off vapour in sufficient quantity to attack the platinum ; and the same applies to a greater degree to all kinds of solder. For low temperature work, however, copper leads may be used, thus reducing the cost of the instru- ment. Mica, above 1 ooo° C. , tends to crumble ; and most forms melt at 1 300 C. or lower ; hence a mica- RESISTANCE PYROMETERS W] wound instrument should not be used continuously above [ ooo° C. The fireclay winding used by Siemens permits of occasional readings being taken up to i 400 " C, and the same applies to wires wound on obsidian (melting point = 1 550 C), or those in which the. coil is self-sustaining. As previously mentioned, however, alterations in the platinum itself render continuous readings above 1 ooo° C. inaccurate after a short time. It has been pointed out that with accurate measuring devices, a resistance corresponding to a change of 1° (.'. can be measured ; and it might appear at first sight that the resistance method is considerably more accurate in practice than the thermo-electric. If a perfectly constant temperature were to be measured, a resistance pyrometer would undoubtedly give a closer indication ; but constancy to 10' C. is seldom possible with gas-fired or coal furnaces or other hot spaces in which pyrometers are used. The accuracy of a pyrometer under workshop conditions therefore depends upon the rapidity with which it responds to temperature fluctuations, which condition will evidently be influenced by the thermal conductivity of the sheath. As it is necessary to protect a resistance pyrometer with a porcelain or silica sheath, both of which are poor conductors of heat, this instrument is in consequence not capable of following a rapidly changing temperature. The same applies to the magnesia packing used in the Siemens form ; whereas a thermo-electric pyrometer I I 8 PYROMETRY is often sufficiently shielded by an iron tube, which transmits heat with a fair degree of freedom. The superior delicacy of the resistance method is there- fore nullified by the sluggishness of its indications ; and for reading changing temperatures the thermo- electric pyrometer is at least equally accurate. If, however, a constant temperature can be obtained, as in the determination of melting points, or when using experimental furnaces capable of exact regulation, the stead)' temperature reading may be secured with greater precision by using the resistance pyrometer. Indicators for Resistance Pyrometers. — All existing indicators for resistance pyrometers are in reality outfits for measuring resistance, either by the Wheatstone bridge, differential galvanometer, or other method, the resistance being translated on the dial into corresponding temperatures. Typical examples will now be described. Siemens' Indicator.— This instrument is based upon the Wheatstone bridge principle, and is shown m fig- 35- The galvanometer is mounted in the centre of the dial, round the edge of which is fixed a ring on which the adjustable resistance is wound in spiral form. Suitable terminals are provided, duly labelled, to which the battery, pyrometer leads, and compensator are attached. A brass arm. movable about the centre of the dial, terminates in a tapping- key which moves over the adjustable resistance ; the key being placed in the batter)' circuit. The fixed known resistances are located in the interior of the RESISTANCE PYROMETERS I IO, indicator. The adjustment consists in moving the key round the circumference until, on tapping, no deflection is obtained on the galvanometer. The pointed end of the movable arm then indicates the temperature of the pyrometer on the dial, which is marked in temperatures corresponding to the resist- Fig. 35. — Siemens' Dial Indicator. ance opposed to the pyrometer for different positions of the key. In taking a reading, the operator is guided by the fact that when the temperature indicated is too high, the movement of the galvanometer needle will be in one direction ; whereas if too low an opposite deflection will be given. The intermediate position of no deflection must then be found by trial ; and the 120 PYROMETRY procedure should not occupy more than two minutes if the observer possess an approximate notion of the temperature to be measured. ""■ -"in Fig. 36 — Whipple's Indicator. Whipple's Indicator.— This instrument (fig. 36) is employed by the Cambridge and Paul Instrument Company, and is also a form of VVheatstone bridge. The pyrometer leads and compensator are connected RESISTANCE PYROMETERS 121 to properly labelled terminals T, and the battery to other terminals at the opposite side of the box. The pointer of the galvanometer is visible through the small window B, and a battery of two dry cells is placed at the side of the box. The fixed resistances are contained in the interior, and the adjustable resistance consists of a continuous wire wound on a drum, which may be rotated by the handle H. The shaft connecting H with the drum is screwed, and works in a nut, so that the turning of H produces a spiral movement of the drum. The adjustment con- sists in rotating H until, on tapping the key F, no deflection of the galvanometer pointer is observed. The temperature of the pyrometer is then read off directly from a paper scale wound round the drum and rotating with it, visible through the window A, the reading being indicated by a fixed pointer. This arrangement forms a compact and convenient indicator. The Harris Indicator. — In the Siemens and Whipple indicators it is necessary, before a reading can be taken, to adjust a resistance until the galvano- meter shows no deflection — an operation which takes up time and requires a fair amount of skill. This is obviated in the Harris indicator, made by R. W. Paul, and shown in fig. 37. This instrument is a special form of ohmmeter, which automatically indicates the resistance of the pyrometer by the movement of the pointer ; the scale, however, being divided so as to read corresponding temperatures. In this indicator 122 PYROMETRY the scale may be made to notify an excess tempera- ture — say ioo° — above a given fixed number, and hence is capable of yielding an exact reading over the working range for which it is used. It may also be connected so that the whole scale represents the Fig. 37. — The Harris Indicator. complete range — say 0° to 1 ooo° C. — or other specified interval. The advantage possessed by this instru- ment is that the manipulation is much simpler than in the indicators previously described. The Leeds-Northrup Indicator. — In this apparatus the Wheatstone bridge principle is employed, but the galvanometer is provided with a scale divided or RESISTANCE PYROMETERS I 23 temperatures. Coils are provided which correspond to an increase of resistance due to a rise of ioo" C. on the part of the pyrometer, and by inserting these coils in the circuit the temperature is obtained to the nearest ioo°. If the temperature were exactly at an even hundred — say 700' — the pointer of the galvano- meter would be at zero on its scale ; but if now the temperature rose, the system would no longer be balanced, and the galvanometer pointer would move over its scale by an amount depending upon the potential difference at its terminals. A very sensitive galvanometer would give a movement to the end of its scale with a slight alteration from the conect balance of the system ; but by using a coarser instru- ment the pointer would remain within bounds ; and the greater the increase of resistance, the larger would be the deflection. It is possible, in such a case, to divide the galvanometer scale to read temperatures corresponding to a given increase above that of the coils placed in the circuit. In one form of the Leeds- Northrup indicator, the whole scale is thus divided to read 100°, and the reading is obtained by adding the figure shown on the galvanometer to the hundreds represented by the coils inserted. In another form the galvanometer has a central zero, and its scale is divided both right and left, one side giving the number of degrees above, and the other below, the nearest hundred. The observations are thus much simpler than in the case where adjustment to the condition of no deflection is requisite. 124 PYROMETRY Siemens' Differential Indicator. — This form of indicator is still in use, and consists of a differential galvanometer and box of resistance coils, connected as shown in fig. 30. By adjusting the coils until no deflection is produced, the resistance of the pyrometer is obtained, and the corresponding temperature read off from tables provided. This form of indicator is preferred by some users, but it is less sensitive than the more recent YVheatstone bridge indicator made by this firm (fig. 35), and equally difficult to manipulate. Recorders for Resistance Pyrometers. — The value of records in high-temperature work has led to the invention of recording mechanisms for use with resistance pyrometers. The form in common use in Britain is that devised by Calleudar, shown in fig. 38, and consists of a mechanism for restoring auto- matically the balance of the resistances in a Wheat- stone bridge circuit, in such a manner as to indicate the existing resistance on a chart. To this end the moving coil of the galvanometer carries a. boom, or contact-arm, which, on swinging to the right or left, completes one of two electric circuits. The closing of either circuit brings into action a clockwork mechanism, which causes a slider carrying a pen to move over the bridge wire until the balance is restored, and incidentally to produce a mark in ink on a paper wound on a drum, which rotates at a known speed. When the resistance of the pyrometer is balanced, the galvanometer boom will be in a central position, and the slider at rest ; whereas a rise in temperature RESISTANCE PYROMETERS 125 causing an increase in the resistance of the pyrometer, will result in the boom swinging over and completing the circuit, which introduces more resistance in opposi- Eio. 3S. — Callendar's Recorder. tion to the pyrometer. A fall in temperature will similarly result in the liberation of the second mechan- ism, owing to the boom swinging in the opposite direction, with the result that the slider moves so as 126 PYROMETRY - „ j p ! --»■.= M , .^-^ E / _ -v ':.£:< : : _ ^ . . \. . M - i ' r - -5, -<.-'- ^ ^j, _, _^. - = :~h , ,- H <=,, ___ ::::.!!:: : " -~" _j 5 "^Z'~ fc' --- = ■ _, t~ _*=. <: :z** i _ " -~~^ 6C z — »■ c /■ = — 2 r _ ::«§'=£:: : \ c~ < L - i --~-^.- ---;>■ = ~ := " 4= izC" u RESISTANCE PYROMETERS 1 27 to oppose a less resistance to the pyrometer, If the chart be divided horizontally into equal spaces, repre senting equal increments or decrements of resistance, they may be marked to represent degrees on the platinum scale, which may be translated into ordinary degrees by reference to a conversion table. In careful and skilled hands this recorder gives excellent results, and the value of the records obtained is clearly shown by an inspection of the example shown in fig. 39, which represents the fluctuations of an annealing furnace during a period of nine hours. It will be noted that during the period covered by workman A the furnace has received constant and careful atten- tion ; but workman B has evidently neglected his duty conspicuously at two separate times. The Leeds-Northrup Recorder. — In the Calen- dar 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 gal- vanometer coil, which necessitates the use of delicate mechanism if certainty of action is to be secured. A surer contact is secured in the instrument made by the Leeds-Northrup Company of Philadelphia, by means of an intermittent action which will be understood from the annexed drawing (fig. 40). The boom from the galvanometer terminates in a platinum tip, P, which moves between two blocks, the upper of which con- sists of two pieces of silver, A and B, separated by a strip of ivory, I, whilst the lower block, C, is another piece of silver, which is moved periodically up and 128 PVKOMETRV down by an electro-magnetic contrivance not shown in the drawing. When the galvanometer is at the position of balance, the tip of the boom is beneath the ivory piece I ; and when C ascends the tip P is then squeezed on to the ivory, and no current will then pass from the battery through either of the cir- cuits E or F. If, however, the point of the boom be beneath A, owing to an alteration in the temperature Fig. 40. — Principle or Leeds-Nokthrup Recorder. of the pyrometer, then on C rising the circuit through E will be completed ; and, similarly, if beneath K the circuit through F will be established. The result in either case is to bring into action a mechanism which moves a slider, carrying a pen, over a resistance wire opposed to the pyrometer in such a manner as to restore the balance. Certainty of contact is thus secured, which enables all the parts to be strongly made. The actual recorder is shown in fig. 41, in which it will be seen that the slider carries an ordinary RESISTANCE PYROMETERS 129 stylographic pen in contact with the chart. This recorder is worked on the differential galvanometer method ; and the adjusting resistance, over which the slider moves, consists of a manganin wire wound on a tapered core, such that horizontal movements repre- I li' 1 i'l ::,:l :■'. J I I 1 I Fig. 41. — Leeds-Northrup Recorder. sent equal changes of temperature, and not of resist- ance, thus obviating the necessity of translating platinum-scale readings into ordinary degrees. Con- cordant and accurate results, coupled with robust construction, are claimed for this instrument by the makers. The other type of recorder made by this 9 130 PVROMETRV firm (fig. 26) may also be used in conjunction with a resistance pyrometer. In this case the movements described introduce or cut out resistance opposed to the pyrometer in a Wheatstone bridge circuit, until the balance is restored. Paul's Recorder. — This instrument, as used for thermo-electric pyrometers, has already been described. By replacing the galvanometer by a Harris indicator, and using a suitable chart, the same mechanism serves to record the indications of a resistance pyro- meter. Installations of Resistance Pyrometers. — The resistance method cannot be so readily applied to the purpose of a centrally controlled installation as the thermo-electric, owing to the difficulty of producing a set of pyrometers exactly equal in resistance. The introduction of the ohmmeter method of measuring resistances, as in the Harris indicator (page 122), has, however, rendered this project feasible, 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 coefficient. Several instruments, brought thus to a zero resistance of 3 ohms, for example, may then be wired up to a Harris recorder, and will give closely identical results. For various reasons, however, a thermo-electric instal- lation is preferable. Management of Resistance Pyrometers. — It is not advisable to use resistance pyrometers continuously above 900 C. ( 1 650° F.), although an occasional read- RESISTANCE PYROMETERS T 3 I ing may be taken up to 1 200 C. (2 190 F.). Great care must be taken that metallic vapours or furnace gases do not find access to the interior, and for this reason a cracked or defective sheath should imme- diately be replaced. As the resistance gradually changes, even when 900° C. is not exceeded, a reading should be checked at a fixed point in the neighbour- hood of the working temperature, and allowance made for the observed error. Another method of cor- rection recommended by some makers is to measure the resistance in ice, and to note how much this differs from the zero resistance noted when the indicator was marked, and to correct by simple pro- portion. Thus, if the observed resistance in ice were iO'2 ohms, the original having been IO'O the reading on the indicator would be multiplied by — = O'oS IO-2 a correction which assumes a linear relation between resistance and temperature, and is therefore only- approximate. Generally speaking, any serious defect entails the sending of the instrument to the maker, as a special degree of skill is required to execute the necessary repairs. As the indicators are usually not automatic in action, care should be taken in the manipulation not to damage any part, particularly the galvanometer ; and it is advisable not to trust the instruments to un- skilled observers. The remarks applying to recorders and protecting sheaths in relation to thermo-electric pyrometers (page 92) apply equally in this case. 132 PYROMETRY Special Uses of Resistance Pyrometers. — In all cases in which an exact reading is required, and a steady temperature can be secured, the resistance pyrometer can be used to advantage. Thus for accurate determinations of melting points and boiling points, or for exact readings of temperatures in ex- perimental furnaces, a resistance pyrometer is superior to appliances of other kinds. On the other hand, it is not capable of responding to changes with the same rapidity as a thermal junction, and is therefore inferior for such purposes as the determination of recalescence points, or the temperature of exhaust gases from an internal combustion engine. The resistance method may be applied to atmospheric and very low tem- peratures (liquefied gases, etc.), to measure steady conditions with accuracy, nickel wire being some- times used instead of platinum below 400" C. Many cold stores are fitted with resistance thermometers, the temperature being read directly on the galvano- meter, which is placed across a Wheatstone bridge, and shows a deflection which depends upon the amount by which the bridge is thrown out of balance. Changes in the temperature of the resist- ance element may thus be read accurately. Whether the resistance method is suitable to a given purpose must be decided by the three factors : (1) tempera- ture to be measured, which must not exceed 1 ooo° C. continuously ; (2) degree of accuracy required (a thermo-electric pyrometer giving results to io° C.) ; (3) stability of the temperature measured, rapid RESISTANCE PYROMETERS I 33 changes not being readily shown by resistance pyrometers. One advantage of resistance pyrometers is that the readings are independent of the resistance of the wires used to connect the pyrometer with the indicator, as such wires are duplicated and opposed to each other in the measuring device, their resistance being thereby cancelled. Hence the same reading is obtained at any distance, and, in addition, the head of the pyro- meter may vary in temperature to any extent without- altering the reading. These are points of superiority over the thermo-electric method ; but, on the other hand, resistance pyrometers and indicators are more costly, more fragile, more difficult to repair, require more skilled attention, and are more liable to get out of order when used for industrial purposes. These drawbacks have resulted in restricting the use of resistance pyrometers to special purposes, the general run of observations being conducted by means of thermo-electric pyrometers. CHAPTER V RADIATION PYROMETERS General Principles. — It is a common experience that the heat radiated by a substance increases as its temperature rises ; and it would obviously be an advantage if the temperature of. a hot body could be deduced from the intensity of its radiations, as the measurement could then be made from a distance, without the necessity of placing a pyrometer in contact with the heated substance. At temperatures above i ooo" C, when difficulties are experienced either with the metals or protecting sheaths of thermo- electric or resistance pyrometers, the advantage gained would become more conspicuous as the temperature increased. A brief survey of our knowledge of the relations between radiant energy and temperature will indicate how this desired end may be achieved. Any substance at a temperature above absolute zero ( — 273° C.) radiates energy to its surroundings by means of ether waves. Below 400° C. these waves produce no impression on the retina of the eye, and the radiating body is therefore invisible in a dark room. Above 400° C, however, a proportion of visible waves are emitted ; and as the temperature 134 RADIATION PYROMETERS I 35 rises the effect on the retina is enhanced, and the body increases in brightness. The difference between the non-luminous and luminous waves is merely one of wave-length, the shorter wave-lengths being visible to the eye ; and both represent radiant energy. In addition to giving out radiant energy, a substance receives waves from its surroundings, which it absorbs in greater or less degree, and which when absorbed tend to raise the temperature of the receiving sub- stance A number of objects in a room, all at the same temperature, are therefore radiating energy to one another, and equality of temperature is established when each object receives from its surroundings an amount of energy equal to that which it radiates. A hot substance radiates more energy than a cold one ; thus if a hot iron ball be hung in a room it will radiate more energy to its surroundings than it receives from them, and will therefore cool until the outgoing energy is balanced by the incoming, when its temperature will be equal to that of the other objects in the room. The rate at which a substance emits or takes up radiant energy depends upon the nature of its surface. A rough, black surface, such as may be obtained by holding an object in the smoke from burning camphor, radiates and absorbs heat with greater freedom than an)' other ; whilst a polished, metallic surface, which acts as a reflector, is worst of all in these respects. Even a surface of finely divided soot, however, does not completely absorb all the radiations which fall upon it, but exhibits a small degree of reflection. An 136 T'YROMETRV "absolute black surface," if such could be found, would be totally devoid of reflecting power, and would absorb all the radiant energy incident upon it ; and conversely would radiate all energy reaching it from its under side, without reflecting any back, or allowing any to pass through in the manner that light waves are transmitted through a transparent substance. No such perfect surface is known ; but, as Kirchoff showed, 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 temperature. Such an arrange- ment is termed a "black body," and radiations from it are designated "black-body radiations." Any enclosure, if opaque to radiant energy, and kept at a constant temperature, constitutes a black body, and radiations received from the interior through a small opening in the side are black-body radiations. Fig. 42 represents such an enclosure ; in which, to show the application to pyrometry, a body A is indi- cated opposite to an opening in the side, through which radiations escape from the surface of A. If this surface were "perfect," all the waves falling upon it would be completely absorbed and completely radiated ; but to prevent change of temperature the energy radiated must balance the energy received. If, on the other hand, the surface of A were a polished metal, the waves falling upon it from the sides of the enclosure would in the main be reflected ; but here again the energy leaving the surface must equal the amount RADIATION PYROMETERS 137 received if the temperature be constant. It follows, therefore, that if no alteration in temperature occur, the energy leaving the surface of A is independent of the nature of that surface ; and the amount escaping through the opening will therefore be the same, what- ever be the character of the surface opposite th opening. With a good radiating surface the rays from the enclosure will first be absorbed and then Y^^^^^^^^ m \\\\\\\\\\^\\\\\\\\\^ Fio. 42. — Black-Body Radiations. radiated through the opening ; in the case of a poor radiating surface, the rays will be directly reflected through the opening; the total energy escaping being the same in either case. It will be seen later that radiation pyrometers are based upon black-body radiations ; and it is important to note that the arrangement under discussion is realised in a furnace at a constant temperature, in which A might represent an object such as a block of steel. It happens, therefore, that the condition of perfect radiation is I38 PVROMETKV attained by the appliances in everyday use ; and, moreover, black-body radiations can always be secured by placing a tube, closed at one end, in the heated space, and receiving the radiations through the open end ; for this again represents an enclosure at a constant temperature. Similarly, radiations from a solid in the interior of the tube of the electric furnace shown in fig. 29 will be of the same description, and we can therefore apply with accuracy any instrument based upon black-body radiations, knowing that the same may be readily realised in practice. The law connecting the energy radiated by a sub- stance, under given conditions, with its temperature, was variously stated by different observers until Stefan, in 1S79, deduced the true relation from certain experimental data obtained by Tyndall. 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 tempera- ture. Numerous experiments, under different con- ditions, showed that the fourth-power law did not apply to all kinds of surfaces or circumstances ; but a strong confirmation of its truth when applied to black-body radiations was forthcoming in I 084, when Bolt/.mann showed, from thermodynamic considera- tions, that the quantity of energy radiated in a given time from a perfect radiator must vary as the fourth power of its absolute thermodynamic temperature. Certain assumptions made by Boltzmann in this in- vestigation were subsequently justified by experiment ; RADIATION PYROMETERS I 39 and numerous tests under black-body conditions have since amply verified the law. It is upon the Stefan- Boltzmann law that radiation pyrometers are based ; the energy received by radiation from the heated substance, under black - body conditions, being measured by the instrument, and translated into corresponding temperatures on its scale. Expressed in symbols, the fourth-power law takes the form — E = K(T, 4 -T 2 4 ), where E is the total energy radiated ; Tj the absolute temperature of the black body ; T 2 the absolute tem- perature of the receiving substance, and K a constant depending upon the units chosen. If E be expressed as watts per square centimetre, the value of K is 56X io - ' 2 ; if in calories per square centimetre per second, the value is i'34X icr 12 . The introduction of the temperature of the receiving substance, T 2 , is rendered necessary by the fact, previously cited, that energy will be radiated back to the hot body, 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 substance. If T 2 were absolute zero, the energy leaving the black body would be KT X 4 ; whereas if T 2 were equal to Tj, the loss of energy would be nil, as a substance cannot cool by radiation to a lower temperature than its sur- roundings. The temperatures Tj and T 2 refer to the thermodynamic scale (page 9), but as the gas scale is practically identical, Centigrade degrees may be 14-0 PYROMETRY used, measured from absolute zero, or — 273 . An example is appended to illustrate the application of the law : — Example. — To compare the energy radiated through an opening in the side of a furnace at tempera- tures of 527 , 727 , and 927 C. respectively, to surroundings at 27 C. The quantities will be as K(8oo 4 - 300 4 ) : K(i ooo 4 - 300 4 ) : K(i 200 4 - 300 4 ). since 273 must be added to each temperature to convert into absolute degrees. Dividing each by K, and expanding in each case, the ratio becomes (4096-81) 10" : (10000-S1) 10 s : (20736-81) > IO 8 . Dividing each by io 8 and subtracting, the result is 4015 : 9919 : 20655, or ' : 2 '47 : 5' 12 - It will be noted in the above example that the effect of the surrounding temperature, taken as 27" C, is small in quantity, and becomes proportionately less as the temperature of the furnace increases. If T 2 had been ignored in the calculation, the amounts of energy radiated would have appeared as 1 : 2 -44 : 5-06. It will be seen later, that in calculating the tempera- ture scale of a radiation pyrometer, the temperature of the surroundings is for this reason not taken into account. Fig. 43 is a graphic illustration of the fourth-power law. RADIATION PYROMETERS 141 When the relation between temperature and quantity of energy radiated is known, any instrument which will indicate the amount of the radiations it JO 28 26 O 22 Q- °3 C 20 Q. 18 ~Q oj 16 m -5 1* ) 20 feet. Since — = — ; then, from the results of Ex- d x v 12 1:0 ample I, — = at 10 feet distance, and "1 J t s = ^— at 20 feet. Hence the linear dimensions, i.e. the dia- meters of the circular images, will be C308 148 PYROMETRY and 0-152 inch respectively; and the areas 0*074 and C0182 square inch. These areas are to each other practically as 4 : 1. That is, the area of the image decreases in size directly as the square of the distance of the object ; the squares of the distances being 100 and 400, or as 1:4; whereas the areas of the images are as 4 : 1. Example III. — To find, for a 6-inch mirror, and a junction of ^th of an inch in diameter, the greatest distance at which the mirror may be placed from an opening 1 foot in diameter, so as to give an image not less than the junction. From Example I it is evident that at any distance exceeding 20 feet the position of the image will only be a minute and negligible fraction over 3 inches ; hence v may be taken as 3. Applying 1 values in the formula — =- and 1 F 3 s d 1 v taking d 1 as equal to the diameter of the junction, = ci inch, — = - , and u = 360 inches, or 30 feet, o-i 3 Beyond this distance the image would be less than the junction. The conclusions to be drawn from the foregoing examples are : ( r ) that the amount of energy received by the junction does not vary, provided the image overlaps it ; and (2) that the limiting distance at which a correct reading can be secured is that at RADIATION PYROMETERS 1 49 which the size of the image is equal to that of the junction. Thus, taking distances of 10 and 20 feet, as in Example II ; at the former distance the energy striking the mirror is four times as great as with the latter ; but, on the other hand, the area of the image at 10 feet distance is four times as great as that obtained at 20 feet. Hence, at the greater distance, the proportion of the image impinging on the junction is four times as great, and the fact that only \ the amount of energy strikes the mirror is thus counter- 's^- Fig. 46.— Fkry's Spiral, balanced. All the reflected rays which fail to strike the junction are ineffective, and pass out through the entrance of the tube. The two-scale form of instrument described above is extremely useful for general purposes, but when all the temperatures to be controlled fall within the limit of one of the scales, it is simpler and cheaper to dis- pense with the diaphragm, and to use an indicator furnished with one scale only. The single- scale mirror pyrometer is for this reason more generally employed for industrial purposes ; and the Cambridge and Paul Instrument Company now make a pivoted ISO PYROMETRY indicator for use with full aperture, which is less liable to damage than one which possesses a suspended coil. Fury's "Spiral" Radiation Pyrometer. — This instrument differs from the preceding merely in the fact that the rays are focused on a small spiral, formed of a compound strip of two metals, fixed at one end and furnished with a pointer at the free-moving ^rwiA w it=t J=-- ^ Fig. 47.— Fery's Spiral Pyrometer. Section. end (fig. 46). The effect of alterations of temperature on this spiral are to cause it to coil up or uncoil, according to whether the temperature rises or falls. This movement is magnified by the pointer, the end of which moves over a dial graduated to read tempera- tures directly. This arrangement is shown in section in fig. 47, where C is the mirror, E the eye-piece, S the spiral, P the pointer, and D the dial, viewed through the window W. The appearance of the RADIATION PYROMETERS 151 apparatus when viewed from the front is shown in fig. 48. The advantage gained by the use of the spiral is that the instrument is self-contained, no galvanometer being necessary ; but, on the other hand, the indications are not so exact, an error of Eig. 48. — P'kry's Spiral Pyrometer. Front View. 20 C. being probable at temperatures over 1 ooo° C. In using this pyrometer, it is observed that after focusing the hot substance, the pointer moves rapidly for a time and then pauses, after which it again com- mences to creep along the scale. The temperature indicated at the moment the pause occurs is generally taken as the reading, but this is not always correct. 152 PYROMETRY The creeping movement is probably due to the whole instrument, and the air in the interior, becoming heated by the entering rays, and by proximity to the hot source. In a number of trials made by the author, it was noticed that when the instrument was allowed to stand near the furnace for some time before using, thereby attaining the temperature existing in the vicinity, the "creep" almost entirely vanished. All things considered, the spiral form of Fery's pyrometer must be regarded as more portable but less accurate than that in which the rays are received on a thermal junction. Foster's Fixed-Focus Radiation Pyrometer. — The necessity for focusing, common to all Fery's radiation pyrometers, is obviated in Foster's pyro- meter, which, however, cannot be used from so great a distance. 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 send- ing rays to the mirror, through a fixed opening, increases as the square of the distance. This will be understood from fig. 49, in which C is the mirror, D a thermal junction fixed so as to be in the focus of the opening E F, and A B the heated surface. The lines joining E and F to the edge of the mirror intersect in a point G, and provided the lines G E and G F, if produced, fall within the heated surface A B, the quantity of energy falling on D will always be the same. A cross section of the cone G A B is a circle ■ RADIATION PYROMETERS I 53 and if A B be twice as far away from G as E F, the areas of the circles of which A B and E F are diameters will be in the ratio 4 : 1. But as A B is twice as far from G as E F, the intensity of its radiations will be as 1 : 4 ; and hence loss of radiating power is exactly balanced by increase in area. In the actual instrument the tube in which the mirror is placed is blackened internally, so that no rays reach the mirror by reflection from it. The diameters of the opening E F and the mirror C are such that the perpendicular from G on to A B is ten A "- -. E ---:;"p<'c) p Fig. 49. — Principle of Foster's Fixed-Focus Pyrometer. times the length of A B. Hence, if the heated object be 6 inches in diameter, the limiting distance of G is 10x6 = 60 inches. The position of the point G is indicated by a ring on the outside of the tube, and in taking a measurement the tube is brought well within the distance prescribed, which is in all cases ten times the diameter of the heated object. Temperatures are read from a galvanometer connected to the thermal junction, the whole arrangement being portable, as shown in figs. 50 and 5 1 , which represent the in- strument in use. The advantages derived from the use of a fixed- 1 54 PYROMETRY focus instrument are simplicity and cheapness ; but, as many occasions arise in practice in which focusing on an object is a necessity, Foster's pyrometer must be regarded as a simplified apparatus not capable of Fie. ,o. — Foster's Pyrometer, mounted on Stand. the wider applications of Fery's instruments, but of great service in many cases. Whipple has recently adapted the Fery spiral pyrometer to produce an instrument with a fixed focus, by fastening the instru- ment to a fireclay tube, on the closed end of which RADIATION PYROMETERS I 55 the pyrometer is permanently focused. This form is specially useful for determining the temperature of molten metals, into which the end of the fireclay tube is plunged, thus giving true black-body conditions. Paul's Radiation Pyrometer. — Thwing, in America, has introduced a radiation pyrometer in which the Fig. 51. — Foster's Pyrometer, in use. rays from the furnace enter the wide end of a cone, and by internal reflection are brought to the apex, at which a thermal junction is located. Paul, in this country, has marketed a similar instrument, the action of which is shown in fig. 52, where E is a tube containing a polished cone, C, at the apex of which is fixed a thermal junction, T. Rays from the hot 1 56 PYROMETRY source A A' enter the tube at D, and pass into the cone, being finally reflected on to T, which is connected to the indicator. So long as the lines joining the outside of the cone with the extremities of the entrance D, crossing at O, fall within the hot source, A A', the reading will be the same at all distances. Fig. 53 shows the actual pyrometer, mounted on a tripod. Indicators for Radiation Pyrometers. — When the radiations are focused on a thermal junction, the temperature of which is raised in consequence, the E. M.F. developed is in accordance with the laws discussed in Chapter II, and any thermo- (2 electric indicator, if sufficiently sensitive, will serve for the purposes of a radiation pyrometer. The effect on the galvano- meter is influenced by : (1) the nature of the junction ; (2) the size of the mirror or cone ; and (3) the highest temperature attained by the junction. The indicators used in connection with radiation pyro- meters are of the pivoted type, which can now be made sufficiently sensitive to give full-scale deflection for a rise of ioo° C. in the temperature of the junc- tion. For the junction itself, Heil's alloy (zinc and antimony in atomic proportions) partnered with constantan has been used, < RADIATION PYROMETERS 157 owing to the high E.M.F. developed ; but cases of deterioration of this alloy have been noted, causing it to be replaced by some makers by iron. Two iron or copper constantan junctions in series give an E.M.F. for a rise of 100° C, sufficient to work a Fig. 53. — Paul's Radiation Pyrometer. pivoted indicator, and are preferable to Heil's couple for a radiation pyrometer. Calibration of Indicators for Radiation Pyro- meters. — The deflections on the indicators are due to the E.M.F. generated, which is proportional to the difference in temperature between the hot and cold junctions. If both these are at the same temperature — say, 20 C. — the deflection is zero ; and on allowing the radiations to fall on the hot junction its tempera- 158 PYROMETRY ture is raised by an amount depending upon the intensity of the radiations — say, to 90 C. The deflec- tion produced is then due to a difference of (90—20) = 70°, the radiations having raised the temperature of the hot junction yo° above its surroundings. If the surroundings (including the cold junction or junctions) had been at 15 to commence with, the hot junction under the same conditions would have risen to 85 , giving again a difference of 70 , and thus causing the same deflection as before. Provided both hot and cold junctions are located so as to attain the same atmospheric temperature in the absence of radiations, a given quantity of energy impinging on the hot junction will always produce in it the same excess temperature, and will therefore give rise to the same deflection at all ordinary atmospheric temperatures. As the junctions are so arranged in radiation pyro- meters as to fulfil this condition, no correction for fluctuations in the cold junctions is necessary. The deflections, therefore, correspond to excess temperatures of the hot junction, which in turn are directly proportional to the energy received by the junction. Readings in millivolts on the indicator thus represent directly the proportions of energy received by the hot junction, 4 millivolts correspond- ing to twice the energy, which produces 2 millivolts, and so on ; and hence the millivolt scale becomes an energy scale. In order to translate energy into corresponding- temperatures, the fourth-power law must be applied. RADIATION PYROMETERS I 59 If Ej correspond to an absolute temperature T, on the part of the black body from which radiations are received, and E 2 correspond to another temperature T 2 , the following relations will hold good ; Ej, = K(T 1 *-.r*), and E, = K(T 2 4 -.r 4 ), where x is the temperature of the surroundings re- ceiving the radiations. As previously pointed out (see Example on page 140), the term x i may be ignored for the range of high temperatures measured by a radiation pyrometer, hence Ej = KT/, and E T E., = KT., 4 and therefore ~ = ,=!■=. But, as shown ''2 L t above, readings in millivolts on the indicator are directly proportional to the energy received, and if Rj and R 2 = millivolts due to E, and E 2 , the relation R, _ T x 4 R, T « is then obtained. L 2 In order to prepare a temperature scale from this relation, it is necessary to take one correct reading at a known temperature, after which the remainder of the scale may be marked by calculation, as shown in the example appended : — Example. — A tube closed at one end is at 927 C. (1 200 abs.), and gives a deflection corresponding to 2 millivolts on the in- dicator. To find the temperatures which would yield deflections due to 1, 3, 4, and 5 millivolts. l6o PYROMETRY Taking the case of i millivolt and applying in the formula R x _ Tj 4 2_ = 1 200 4 R 2 ~T 2 " f = T 2 4 ' 1 200 4 from which T 2 4 = — and T 2 = 1 009 abs. = 736° C. Similarly, 3 millivolts represent 1 055° C. ; 4 millivolts = 1 154 C. ; and 5 milli- volts = 1 236 C. These values are readily obtained by the use of four-figure logarithms. Having calculated the temperature corresponding to each whole millivolt, a curve may be plotted to represent millivolts against corresponding tempera- tures, and intermediate values deduced from it. Evidently, the standard reading must be taken with great accuracy, as the whole scale hinges upon it ; and for this purpose an accurate resistance or thermo- electric pyrometer may be used, placed inside the tube of an electric furnace, and the radiation pyro- meter sighted on a thin sheet of iron placed just in front of the naked junction. A check at the higher readings of the scale is necessary, as an exact realisa- tion of the fourth-power law is seldom obtained in practice. This may be taken in the same manner, as thermo-couples may now be calibrated directly against the gas scale up to 1 550 C, thus enabling the gas-scale reading to be transferred to the radiation pyrometer. For delicate readings over a given range, the scale of a mirror galvanometer may be calibrated RADIATION PYROMETERS 161 in this manner, sufficient resistance having first been added in series to ensure that at the highest tempera- ture employed the spot of light will remain on the scale. Recorders for Radiation Pyrometers.— Any of the thermo-electric recorders described in Chapter II may be applied to radiation pyrometers, the chart being suitably divided according to the fourth-power law. When taking a record, the pyrometer is fixed on a stand or bracket and focused on the desired IOSO C k QHO V VUrf y^ ^ w S-W^ w^ H\V »v*V *vv S-^M. jv/V A^, U/ A r !••/ V" *S ^y ^ ^ 1 »so BOO Fig. 54. — Record obtained with Radiation Pyrometer. spot. Fig. 54 is an example of a record taken with a Thread recorder and Fery pyrometer, in which the division of the temperature scale according to the fourth-power law will be noticed. It is possible to arrange that the working temperature shall lie on the open part of the scale, by adjusting the sensitiveness of the galvanometer accordingly before calibrating. Management of Radiation Pyrometers. — It is not advisable to place a radiation pyrometer in the hands of an unskilled observer, as intelligent oversight is required if good results are to be secured. Care must l62 PVROMKTRY be taken to adjust the galvanometer needle to zero before taking a reading, and the needle should always be locked during transit. When focusing on an object in a furnace it is necessary to make certain that the red image seen is actually that of the object, which may be done by moving the pyrometer until the side of the object, or some special feature, is visible in the eye-piece, when the pyrometer may be moved until the image surrounds the junction. Occasions may arise, as in taking the temperatures of various zones of a rotary cement-kiln or other furnace, in which it is required to focus the mirror for a specified distance ; in which case the author has adopted the plan of placing a fixed pointer opposite the milled head which controls the mirror (P, fig. 44) and focusing the bars of a w.ndow at measured distances, marking the same on the milled head opposite the pointer ; and it would be a convenience if all radiation pyrometers were thus marked initially. A good check to correct focusing in the case of a heated object is to alter the focus in both directions, and finally to adjust to the maximum reading, which should correspond to the true focus. Great care should be taken not to damage the mirror. If, in a workshop, the surface become covered with dirt, this should be removed by gentle brushing with a camel-hair brush or by blowing air over the mirror. The focusing device should never be strained beyond its working limits ; when these are reached, the pyrometer should be moved bodily until the object can be correctly sighted within the ordinary limits of RADIATION PYROMETERS 163 the movement of the milled head. If metallic fumes or dense smoke intervene between the furnace and the pyrometer, the radiations will be impeded and the temperature recorded will be too low ; and in such cases the pyrometer should be placed at the open end of a tube and sighted upon the closed end, which should terminate at the spot under observation. In all cases it must be borne in mind that the indications only apply to black-body conditions If a block of steel be sighted inside a furnace, and then be removed to the exterior and again sighted, the external reading will be much less than the internal, owing to the inferior radiating power of the surface, which now derives no assistance from the furnace. All readings should therefore be taken whilst the object is still in the furnace, or (as in taking the temperature of molten metal in a ladle) a fireclay tube with a closed end inserted in the mass ma)' be used, and readings taken through the open end. Statements are some- times made that the difference between external read- ings and black-body readings is constant for a given surface, and that the one may be translated into the other ; but this is true only for unchanging surfaces, such as platinum, and seldom applies to ordinary working surfaces. As black-body conditions are so easy to ensure, it is simpler and safer always to arrange to take observations under such conditions, rather than to trust a relation seldom constant in practice. When using a radiation pyrometer for a number of furnaces, fireclay tubes, closed at one end, may be 164 PYROMETRY inserted in each, so that the closed end terminates at the working spot, the open end being left flush with the exterior of the furnace. The diameter of such tubes will depend upon the length and also upon the make of the pyrometer ; in all cases the image of the closed end must be large enough to overlap the receiving junction or spiral. Information on this point can always be obtained from the makers, or can be discovered by trial with openings of known diameter. When using the pyrometer to obtain temperatures in the interior of the tube of an electric furnace, such as that illustrated in fig. 29, a solid object, such as a short fireclay cylinder, or a piece of graphite, should be placed in the middle of the tube, and focused on the junction. Special Uses of Radiation Pyrometers. — For regular use at temperatures above 1 ooo° C. or 1 850 F. the radiation pyrometer will be found to be more useful than instruments of the thermo-electric or re- sistance type as the latter undergo deterioration owing to the continuous action of the furnace gases, which becomes more marked as the temperature increases. Examples of industrial processes in which 1 ooo° C. is considerably exceeded are the manufacture of glass, pottery, and cement, the treatment of special steels, and the casting of metals and alloys. Even for temperatures between 750 and 1 ooo° C. a radiation pyrometer may be used, but is not so convenient for this range as a thermo-electric instrument. There is no upper limit to the instrument, which may be RADIATION PYROMETERS 1 65 calibrated by the fourth-power law to the highest temperature attainable, that of the electric arc, which has been found to be 3 720 C. by the use of a Fery radiation pyrometer. Measurements may therefore be made beyond the limits of thermal junctions, such as the temperature of electric furnaces and of thermit in the mould, and of molten steel before pouring, thus opening out the possibility of accurate control at ex- tremely high temperatures. There is always a danger, however, of the cold junction becoming unduly heated when near to large masses at very high temperatures, and serious errors may arise from this cause. Two examples may be cited to illustrate the usefulness of the radiation pyrometer in practice : (I) the hardening of steel projectiles ; and (2) the determination of the temperature of the clinkering zone in a rotary cement kiln. In (1) the projectile is brought to a given spot near the brink of the furnace, where it is in the focus of a radiation pyrometer, and when at the specified temperature is raked out of the furnace and drops into an oil-trough. It has been found that a difference of io° C. from the standard temperature at which the projectiles should be quenched may cause a serious lowering of the penetrative power of the finished pro- jectile ; and hence a radiation pyrometer, which may readily be sighted on each individual shell, is the best to use for this purpose. In (2) the hottest spot may be found by focusing the pyrometer to different dis- tances up the kiln, and, by taking a record, any fall in temperature due to defect of coal or air supplies, or 1 66 PYROMETRY to excessive feed of raw material, may be detected, thus furnishing information from which the process may be regulated to the best advantage. At the tem- peratures prevailing in such kilns — I 300° to I 450° C, or 2 370 to 2 640 ° F., according to the nature of the kiln — a Fery radiation pyrometer is quite sensitive to changes of io° C. or 18° F., and the author has found it to be entirely satisfactory in this connection. The adaptability of radiation pyrometers to all tempera- tures above a red heat, combined with the absence of deterioration, renders these instruments of great value, and the possibility of obtaining records is a further recommendation. The radiation method, however, is not suited to the purposes of an installation, as even if mirrors and junctions could be constructed so as to be identical, the arrangement would be very costly. A cheap adaptation of the radiation principle, by means of which a number of furnaces, such as a set of cement-kilns, could be controlled from a centre, would be of great advantage, and would add further to the general utility of this class of pyrometer. CHAPTER VI OPTICAL PYROMETERS General Principles. — When a solid is heated to 450" C, it commences to send out luminous radiations and appears a dull-red colour in a darkened room. As the temperature rises, the luminous radiations become more intense ; the colour changes to a lighter red, then to orange, yellow, white, and finally to a dazzling white. Attempts have been made to assign tempera- tures to specified colours, and Fouillet, in 1836, intro- duced a table which purported to give the relation between colour and temperature The following table, published by Howe in 1900, differs considerably from that of Fouillet, who had no accurate means of measuring the temperatures he assigned to the colours : — Howe's Table. Description. Temp. Dec;. C. Temp. l>eg. F. Lowest red vis hie in darkness 470 S7S , , , , ,. daylight 475 SS7 Dull red 550 to 625 I 022 to I 157 Full cherry . 700 I 292 Light red S50 1 562 Full yellow . 950 to I OOO i 742 to i S32 Light yellow I 050 I 922 White . I 150 2 IOS 167 I 68 PYROMETRY If it were possible for all observers to detect exactly the colours to which these temperatures refer, the table would be of great utility ; but in practice an}' two persons might differ in judgment to the extent of 50 C. below a yellow ; and when the white is reached, and becomes dazzling, accurate discrimina- tion is impossible. At the same time, a trained workman, used to quenching steel at a fixed tempera- ture, say 850 C. , acquires a high degree of judgment with constant practice, and may not vary by more than 20" C. at temperatures below a light yellow. The personal equation, however, is too great for colour judgment by the unaided eye to be taken as an accurate guide to temperature. A fairly close approximation, however, may be obtained by match- ing the colours against prepared standards, as will be referred to later. The determination of the intrinsic brightness of the heated substance by a photometric method natur- ally suggests itself as a possible means of ascertaining temperatures by optical means, and it will be found that all the optical pyrometers used for industrial purposes are based on this procedure. The law connecting the intensity of the whole of the light waves emitted with temperature, for a given solid, is approximately given by Rasch's formula : — where I x and L are the intensities corresponding to OPTICAL PYROMETERS 1 69 absolute temperatures T 1 and T 2 ; and the exponent 25000 X = — - _ Hence at 1 250 abs. the brightness increases as the 20th power, and at 2 500 abs. as the 10th power of the temperature. This rapid increase in brightness for a small rise in temperature enables small incre- ments to be readily observed ; but a difficulty arises in practice owing to vast differences in brightness displayed by different substances at the same tem- perature. For example, the light emitted by an incandescent gas-mantle, which consists of thorium oxide, is vastly greater than that given out by a metal, such as platinum, at the same temperature ; and it is therefore evident that the luminosity of a substance depends not merely upon its temperature, but also upon its nature. It is possible, however, to obtain indications for any substance in terms of a black body ; thus if a heated solid possessed the same intrinsic brightness as a black body at a temperature of T, the "apparent" or "black-body" temperature of the solid would also be called T. 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 T ; and to obtain the true temperature of the solid, T must be multiplied by a factor which expresses the ratio of its emissive power to that of a black body. In all photometric methods a standard light is 170 PYROMETRY employed, which should not vary in brightness, and with which the light from the source is compared. In optical pyrometers no attempt is made to measure the illumination in terms of candle-power ; all that is necessary is to bring" the standard and the source to the same degree of brightness by suitable adjust- ments. Amongst the standards employed are carbon- filament electric lamps, amyl-acetate lamps, and for higher temperatures the centre of an acetylene gas- flame ; each of which is capable of producing a fixed degree of brightness when used under specified con- ditions. A black body, at known temperatures, is compared with the standard used, thus furnishing a scale of "black-body" temperatures to which the indications of a given source may be referred, as ex- plained in the previous paragraph. Above I 000° C, however, the light becomes too dazzling to enable a proper comparison of the standard and source to be made, and absorbing glasses must then be used to reduce the brightness. Any coloured glass, taken at random, might not reduce the standard and source equally ; but if a monochromatic glass be used — that is, 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 tempera- ture of the source As optical pyrometers are used for temperatures above 1 ooo C. in most cases, in- volving the use of such glass, it will be necessary briefly to consider the relations between the wave- lengths of light and the temperature of the radiating OPTICAL PYROMETERS 171 substance, which in all cases will be assumed to be a black body. Wien's Law. — When the temperature of a sub- stance increases, the enhanced brightness which results is shared by all parts of its spectrum ; and if the substance were viewed through a glass prism, it would be noticed that every portion was brighter than before. Taking a ray of wave-length A, the relation between its intensity and the temperature of the (black-body) source is given by Wien's formula : — J = qX-s x *-<•/*''' . . . (1) where J = energy corresponding to wave-length A ; e = the base of the natural system of logarithms ; T = absolute (thermodynamic) temperature of the black-body source, and c x and c 2 are constants, the values of which may be found by measuring J at two known temperatures for light of a known wave- length. Experiment has shown that this formula is correct for wave-lengths which lie in the visible spectrum, but does not hold for longer waves ; and modifications of Wien's equation have been given by Planck and others which are of more extended application. For the purposes of optical pyrometry, however, using red light of wave-length about 65 millionths of a centimetre, Wien's law may be applied with great accuracy : and a calibration based upon this law agrees closely with the values obtained by other pyrometric methods. 172 PYROMETRY log 10 J=K 1 + K 3 I . . (2) Wien's formula may be written in the form lo£f € where K x = log c t — 5 log A and K 2 = c 2 — ^ — . A This simplified expression shows a linear relation between log J and „ ; and hence if the temperatures corresponding to two intensities be known, the results may be plotted on squared paper in the form of a straight line connecting T and J, from which line intermediate or extraneous readings of temperatures may be obtained for any given intensity. Another useful form of Wien's equation, referring to the ratio of two intensities Jj and J 2 , is as under : — l og Jl - C * lQ g ? (1 l _\ (,) b J 2 " A vT 2 Tj ■ ' l3) where T 2 and T a are the absolute temperatures corre- sponding to J 2 and J r The value of c 2 is 1 450000, when A is expressed in millionths of a centimetre. M J.' be known, Tj may be calculated. When A is not known, as in the case of a piece of red glass for which its value has not been determined, two readings at known temperatures will establish the value of 2 fa , and all other results may then be calculated. A Examples illustrating the application of the formula will now be crjven. Evidently, if the ratio ~, and the value of c 2 , A, and T 2 OPTICAL PYROMETERS 173 Example I. — A black body at an absolute tem- perature Tj is found to give twice the intensity observed at 1 200 abs., the comparison being made with red glass transmitting wave-length 65 x io~' ; cms. To find the value of T r Applying values to formula (3) , 1 4 so 000 . / 1 1 \ >°g 2 =^^pr, log 27183 x( 4A 65 & \i 200 T,/ and 1450000x0-4343 / 1 T, 0'30io = --- J x '- - 65 \i 200 1 from which T l = I 237 abs. Example II. — The intensity of the radiations from a black body at 2 000° abs. are found to be equal to those from a given standard, taken as unity. To find the intensity at 3000 abs., compared with the same standard. X = 65 x io~ u cms. Applying in (3) as before, loo- li = I450 000XO-434 3 x / 1 i_ \ & 1 65 \2000 3000/ from which log Jj = ['615, and Jj = i4'5. 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 standard, an electric furnace (fig. 29) may be used, with a piece of iron in the centre, coated with oxide, which gives black-body radiations. A thermo - electric pyrometer in contact with the oxide may be used to measure the standard temperatures, and bright- 174 rVROMETRV nesses may then be compared with that of an amyl- acetate or other lamp giving a flame of constant luminosity. Temperatures corresponding to other intensities may then be deduced by calculation, as previously shown. Practical Forms of Optical Pyrometers. — The instruments used in practice fall under the following heads : — i. The standard light is constant, and the intensity of the light from the source varied in the instrument until equal to the standard. (Fery, Le Chatelier, Wanner, and Cambridge.) 2 The standard is varied until equal to that of the source, which may be reduced in intensity if this exceed that of the standard. (Holborn-Kurlbaum, made in commercial form by Siemens.) 3. The colour of the source is matched against a standard colour, made to agree with that obtained in a given operation (Lovibond) ; or the source may be made to produce a standard colour by a polarising device (Mesure and Nouel) ; or the colour of the source is extinguished by suitable absorbents (various forms). Examples of each type will now be described. F6ry's Optical Pyrometer. — This instrument (shown in figs 55 and 56) consists of a telescope furnished with a side-branch, in which a standard lamp E is placed. Light from E is focused upon a piece of transparent glass F, inclined at an angle of 45" to the axis of the telescope, from whence it is reflected into the eye-piece. To render the light OPTICAL PYROMETERS 175 O 1" jggjjgiiij Er q 1 176 PVROMETRV received from the lamp monochromatic, a piece of red glass is interposed between E and the mirror. The telescope is sighted on the hot substance, rays from which pass through a piece of red glass D, and thence through two wedges of darkened glass, which diminish the intensity to a greater or less degree Fig. 56. — Fery's Optical Pyrometer. External View. according to the thickness of absorbent glass inter- posed, which is reduced by sliding the wedges apart, and increased by the contrary movement. After passing through the wedges, the light proceeds through the inclined mirror to the eye-piece ; consequently, the appearance presented to the eye is that of a field illuminated one-half by the standard lamp, and the OPTICAL PYROMETERS 177 other by the hot source. The adjustment consists in sliding the wedges, by a screw movement, until both portions of the field are equally illuminated. A tem- perature scale is provided on the moving piece which actuates the wedges, and is derived by YVien's equa- tion from the thickness of the wedges interposed when equality is obtained. Calibration is effected by noting the thickness of the wedges corresponding to two known temperatures, from which a straight line con- necting thickness with the reciprocal of the absolute temperatures may be drawn, and a table formed giving values of T in terms of the thickness of the wedges. The calibration may be extended indefinitely, the accuracy of the readings depending upon the truth of Wien's law. Fery's optical pyrometer is a convenient instrument for occasional readings of high tempera- tures, combining simplicity with portability. Le Chatelier's Optical Pyrometer. — This pyro- meter was the original form of instrument in which the temperature of a luminous source was deduced by photometric comparison with a standard light ; and Fery's apparatus, described above, is merely a con- venient modification of the original. Instead of the absorbent glass wedges, Le Chatelier employed an iris diaphragm to reduce the quantity of light entering the telescope ; 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 stan- dard. The intensity of the light received in the telescope will vary as the square of the diameter of i 7 3 PYROMETRY 5 feH=i^ the opening ; and calibration at two known tempera- tureswith a given monochromaticglass enables a temperature scale corre- sponding to diameter of opening to be computed by Wien ' s law. Le Chatelier's pyrometer is a valuable implement for research work in the laboratory, but is not so convenient for workshop pur- o poses as Fery's modification. - Wanner's Pyrometer. — The prin- J ' ciple of this pyrometer is the com- * parison of the brightness of a red ray 5 from the standard with that of the ray g of some wave-length obtained from fc the source, both rays being produced "a spectroscopically and therefore being | truly monochromatic. The brightness > is compared by the aid of a polarising [ device, resulting in a somewhat com- "■ plicated optical arrangement, which ." is shown in fig. 57. Light from a standard electric lamp passes through the slit S r and from the hot source through So. Both beams are rendered parallel by means of an achromatic lens O p which is placed- at a distance equal to its focal length from the slits. The parallel beams are dispersed by the direct- vision spectroscope V ; and then pass through the polarising prism R, which separates each beam into ! Si ,QL OPTICAL PYROMETERS 1 79 two beams, polarised in planes at right angles. A bi- prism, B, placed in contact with a second achromatic lens, 0. 2 , is made of such an angle that two fields of red light, polarised in planes at right angles, one from the source and the other from the standard, are focused on the slit D. These fields are viewed through an analyser A, and are brought to equal brightness by rotating the analyser, to which a graduated scale is attached, the temperature being deduced from the angle through which the analyser is turned. The calibration is effected by Wien's law (equation (3) page 173), the intensities of standard and source being related to the angle of rotation as indicated by the equation. .'• = tan -6 where J 2 J i and J t represent the intensities of source and stan- dard respectively, and 6 = angle of rotation. Intro- ducing this value into Wien's equation (page 172), the relation between 6 and T may be shown to take the form log tan # = °-2 500° C, and 1 400°-4000°C. The Cambridge optical pyrometer has proved a useful instrument in skilled hands, and has been found of great service in the steel, glass, and pottery industries. Trained observers have found it possible to detect a difference of 10° C. at the region of Fig. 58. — Cambridge Optical Pyrometer. i 900 C. The adjustment of the two fields to equality, however, involves a judgment which varies with different observers, and in practice it is advisable for one individual to be entrusted to take all readings. Holborn-Kurlbaum Pyrometer. — In the optical pyrometers previously described a constant standard is used, and the brightness of the light from the source varied until equality is obtained. The idea of varying the brightness of the filament of an electric lamp until its colour matched that of the source, and deducing the l82 PYROMETRV temperature from the current taken by the lamp, was due to Morse, who used a filament in the form of a flat spiral, heated by a battery of E.M.F. 40 volts. This spiral was placed in a metal tube and interposed between the eye and the heated object. The Holborn- Kurlbaum pyrometer, as made by Siemens, is a refine- ment of that of Morse, and capable of reading over a more extended range. In fig. 59, L is a small electric lamp with a hairpin filament, as shown at A. This Fig. 59. — Holborn-Kurlbaum Pyrometer. Section. lamp is placed in a telescope, so that the filament is in the focus of the eye-piece and is lighted by a 4-volt accumulator, in series with which is a rheostat, R, and a milliammeter, M. The heated source is focused by moving the object-glass of the telescope, and both lamp and source are viewed through red glass placed in front of the eye-piece, D. The rheostat, R, is then adjusted until the tip of the filament is indis- tinguishable from the background, which is illuminated by the source. If the lamp be too bright, the filament will appear as a bright line; if duller than the source, OPTICAL PYROMETERS 1 83 as a dark line ; and when equal to the source it will merge into the background. When equality is obtained, the milliammeter is read, and the temperature deduced from the current taken by the lamp. The relation between current and the temperature of the filament varies with each lamp, but is in all cases represented by a formula of the type C = a + b/ + cl 2 where C = current, t = temperature in degrees C, and a, b, and c are constants depending upon the lamp used, and which can be determined by making a number of observations at known temperatures. The instrument is calibrated in this manner by the makers, and a scale affixed from which temperatures may be read corresponding to observed currents. When the temperature of the source exceeds that of the standard at maximum current, an absorbing device, E, consisting of two prisms of darkened glass, with their reflecting faces parallel, is placed over the end of the telescope, so as to reduce the intensity of the source below that of the lamp. A separate cali- bration is performed with the absorber in position, and a second temperature scale provided, from which readings are taken when the absorbing device is used. Fig. 60 represents the instrument as made by Messrs Siemens, for use in a fixed position, the telescope, milliammeter, and rheostat being mounted on an upright supported by a tripod, and the current obtained from a portable accumulator. A second 1 84 PYROMETRY form (fig. 61) is designed for use in cases when observations at a number of different places are required, the rheostat being mounted on the telescope, and the milliammeter contained in a leather case pro- vided with shoulder-straps. Fig. 60. — Siemens' Optical Pyrometer, on Stand. The adjustment in this pyrometer is simple, and the condition of equality sharply defined. Whereas, in matching the colours of two contiguous fields, separate observers may disagree to an extent repre- senting 40° C. or more, a divergence of io° C. is OPTICAL PYROMETERS I8 5 seldom exceeded when different operators adjust the tip of the filament to extinction. In a special test to decide this point, the author compared the observa- FlG, 61. — Siemens' Optical Pyrometer, Portable Form. tions of five persons, some trained and others un- trained, with the result that all agreed to within 10° at a steady temperature in the vicinity of 1 200 ° C. ; 1 86 PYROMETRY and in this respect the Holborn-Kurlbaum pyrometer is superior to other forms of optical pyrometer. The continuous accuracy of the readings depends upon the permanence of the standard lamp, which is en- sured by over-burning for 20 hours, after which the lamp may be used at its proper voltage for a long period without further change. As used for occasional readings in the workshop, such a lamp will last for a year or more without varying in brightness by an amount representing io c C. at a temperature of 1 800 ° C. When a new lamp is used, a fresh calibration is necessary ; the makers, however, in such case send out a new temperature scale with the lamp. . Lovibond's Pyrometer. — It is possible, by the use of coloured glasses superposed, to match closely any given colour ; and Lovibond, whose tintometer for this purpose is well known, has applied this method to temperature measurement. Taking" the case of a block of steel in a furnace, it is possible to arrange combinations of glasses which, when illuminated by a standard light, will give the same tint as the steel at any specified temperature. If it be desired to work the steel at 850 C, for example, glasses are provided which, when viewed by the light transmitted from a 4-volt glow-lamp, using a constant current, represent the tint of steel at 840°, 850°, and 86o° respectively. The image of the steel is reflected by a mirror through one hole in a brass plate, which forms the end of a wooden box, at the opposite end of which an eye- OPTICAL PYROMETERS 1 87 piece is placed. A second hole in the brass plate receives light from the standard lamp, after passing through the glasses ; and the appearances of the two lights are then compared. A skilled eye can readily detect a disagreement in the two fields corresponding to io° C. ; and by introducing the glasses in turn it can be observed whether the steel is within io° C. of the temperature required. This in- strument is cheap and simple, but is obviously only useful in deciding a pre-arranged temperature, as to take a measurement at an undefined temperature would involve an unwieldy number of glasses, and absorb a considerable time. The correct glasses to use for a given operation are decided under working conditions at temperatures measured by a standard pyrometer ; after which any number of instruments may be made from glasses of the same colour and absorptive power as those used in the calibration. Correct matching is difficult below 700° C. Mesure and Nouel's Pyrometer. — This instru- ment, shown in fig. 62, consists of two Nicol prisms, between which is placed a piece of quartz cut perpen- dicularly to its axis. Light from the source, in passing through the first Nicol prism, is all polarised in the same plane ; but on passing through the quartz is polarised in various planes, according to the wave- length. The colour seen after passing through the second prism, used as analyser, will depend upon the angle between this and the first or polarising prism. PYROMETRY The analyser is connected to a rotating disc, divided into angular degrees ; and on viewing the heated source the colour will appear red if the analyser be turned in one direction, and green if rotated in the opposite. The intermediate colour is a lemon-yellow ; and the adjustment consists in rotating the analyser until this tint is obtained. The angular reading is then taken, and the temperature read off from a table prepared by making observations at known tempera- tures. Observers may disagree by as much as 100° C. Fig. 62. — Mbsure and Nouel's Pyrometer. in using this pyrometer, owing to differences in eye- sight and judgment of the lemon-yellow tint ; but a given operator, who has trained himself to the use of the instrument, may obtain much closer results with practice. 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 temperature, within limits of 25°C. on either side at the best ; and hence it is convenient for a foreman or metallurgist to carry about for this purpose when other pyrometers are not in use. A great advantage is that the instrument is always ready for use, and has no accessories. OPTICAL PYROMETERS 1 89 Colour - extinction Pyrometers. — Various at- tempts have been made to produce superposed glasses, or cells of coloured fluids, which will have the effect of extinguishing the colour of a heated source. As an example, three cells containing various dyes in solution may be prepared which, when looked through, will extinguish the colour at 840°, 850°, and 86o° C. respectively. If it be desired to work at 850 , a difference of ro° on either side may be detected by a trained eye ; but to follow a changing temperature a large number of cells would evidently be necessary. Heathcote's extinction pyrometer, in its early form, consisted of an eye-shade in front of which two pairs of cells containing coloured fluid were mounted. In bringing a furnace to an assigned temperature, observa- tion was made from time to time until a faint red. image was perceived through one pair of cells, when the heat supply was regulated so as to maintain the existing temperature. When viewed through the second pair of cells, which contained a slightly darker fluid, no red image was to be seen at the correct temperature. With training, a workman could con- trol a furnace to a fair degree of accuracy by this means, but the operation was tedious, and useful only for the attainment of a single temperature. In a later instrument, known as the " Pyromike " (fig. 63), Heathcote employs a single cell with flexible walls, so that by turning the screw-end, the length of the column of fluid interposed between the eye and the furnace can be altered. In taking a reading, the furnace is 190 PYROMETRY sighted and the screw turned so as to increase the length of the column of coloured fluid, until the image is no longer visible. A direct reading of the temperature is then obtained on a spiral scale marked on the cylindrical body of the instrument, over which the screwed portion rotates. This forms a simple and convenient temperature gauge for workshop use. The " Wedge " Pyrometer, designed by Alder and Fn -Heathcotk's Extinction Pyrometer or ' I'YROMIKIC. Cochrane (fig. 64). consists of a small telescope through which a prism of darkened glass may be moved, and which is focused on the heated object. By turning a head the wedge may be moved so as to interpose a thicker laver of dark glass between the eye and the furnace, and the same operation causes a temperature scale to pass in front of a fixed pointer. When the image of the hot source is just extinguished, the temperature is read from the mark opposite the fixed point. Training is needed to enable an observer OITICAL PYROMETERS 191 to judge the exact point of extinction, when it becomes possible to obtain results of 20 C. in the region of MM IB 1 vfi mBm ■!*?* Fig. 64. — "Wedge" Pyrometek. i 300° C. On the other hand, when used by one un- accustomed to the instrument, the reading may be wrong by 50° C. or more. As an aid to the judgment 192 PYROMETRV near the extinction point, the hand may be interposed between the telescope and furnace, when, if extinction be complete, no alteration should be observed in the field of view. The simple construction of this pyro- meter is an advantage, no accessories being needed ; and when used with the precautions stated above, readings sufficiently close for many processes can easily be obtained. Management of Optical Pyrometers. — Careful usage is essential with optical pyrometers, which are liable to get out of adjustment with rough handling ; and for this reason a trained observer should be in charge of such instruments. Skilled attention is equally requisite in taking readings, as the matching of tints correctly is an operation which demands a high degree of judgment. Careful attention must be paid to the standard lights ; if flames, regulation to the standard height is essential ; if electric lamps, care must be taken not to use them for a longer period than necessary, in order to increase the useful life. Accumulators should be recharged regularly — say once in two weeks — to keep in good order. Separate parts, such as absorption glasses, should be kept in a place of safety, as their destruction may involve a new calibration. It should be kept in mind that the temperatures indicated by optical pyrometers are "black" temperatures; that is, they correspond to the readings that would be given by a black-body of the same degree of brightness. In consequence, readings should always be taken under black-body OPTICAL PYROMETERS 193 conditions, the precautions in this respect being identical with those necessary for total-radiation pyrometers, given on page 163. In some special cases the connection between the apparent and true temperatures has been worked out for a given type of pyrometer, but, owing to the different emissive powers of different substances, no general relation can be given. Special Uses of Optical Pyrometers. — The advantageous use of optical pyrometers is restricted to observations at temperatures beyond the scope of instruments which have the working part in the furnace ; or to cases in which occasional readings of temperature suffice. To follow a changing tempera- ture continuous adjustment is necessary, involving labour, and therefore costly. Amongst workshop uses may be mentioned: (1) ascertaining the tempera- ture of pottery-kilns and glass and steel furnaces ; (2) in the treatment of steels at very high temperatures, to which end the pyrometer may be set to a given reading, and the process carried out when the steel is observed to attain such assigned temperature ; (3) to take casual readings when a number of furnaces are in use, or when a number of sighting-holes are provided, as in large brickmaking furnaces ; and (4) for occasional observations of the firing end of rotary cement kilns. As an instrument of research in the laboratory, a good form of optical pyrometer is very useful, as, for example, in investigating the working temperatures of electric lamps, and taking 13 194 PYROMETRY observations in electric furnaces. It is a great drawback that records cannot be taken by optical pyrometers, as much valuable information can be gathered from an accurate knowledge of temperature fluctuations in most operations. This disadvantage must always militate against the general use of these instruments. CHAPTER VII CALORIMETRIC PYROMETERS General Principles. — If a piece of hot metal, of known weight and specific heat, be dropped into a known weight of water at a temperature t v which rises to t 2 in consequence, the temperature of the hot metal, ( Q , can be obtained by calculation, as shown by the following example : — Example. — A piece of metal weighing 100 grams, and of specific heatcri, is heated in a furnace and dropped into 475 grams of water, con- tained in a vessel which has a capacity for heat equal to 25 grams of water. The tem- perature of the water rises from 5° to 25 C. To find the temperature of the furnace. The heat lost by the metal is equal to that gained by the water and vessel. Equating these, iooxcri X (4,-25) = (475 + 25) X (25 -5) from which /„ = 1 025 C. The above calculation, which applies general!)' to this method, depends for its accuracy upon a correct knowledge of the specific heat of the metal used. 195 196 PYROMETRV This value is far from constant, increasing as the temperature rises, and the result will only be correct when the average value over a given range is known. The metal used in the experiment should not oxidise readily, and should possess a high melting point. Platinum is most suitable, but the cost of a piece sufficiently large would considerably exceed that of a thermo-electric or other outfit. Nickel is next best in these respects, and is now generally used for the calorimetric method, up to 1 ooo" C. The specific heat varies to some extent in different speci- mens, but can be determined for the ranges involved in practical use. This may be done by heating a given weight to known temperatures and plunging into water, the result being obtained as in the fore- going example, t in this case being known and the specific heat calculated. From a series of such determinations, a curve may be plotted connecting specific heat and temperature range, from which intermediate values ma)' be read off. Regnault, who first suggested the calorimetric method for high temperature measurement, attempted to measure the specific heat of iron over different ranges, with a view to using this metal in the process. Owing to the absence of reliable means of determin- ing the experimental temperatures, however, Reg- nault's values were considerably in error. For the range o to 1 000 ° C. he gave the average specific heat of iron as cvi26, a figure much below the truth. CALORIMETRIC PYROMETERS 197 Thus, if a piece of iron be heated to 970 C. , as measured by the thermo-electric method, and dropped into water, the temperature calculated from an assumed specific heat of CV126 will be found to be 1 210 , or 240° too high. The values now employed are obtained 0/4 0/3 Q. - — method for measurement of E. M. F. , 63. Prinsep, gas pyrometer, 3. Protecting sheaths for pyrometers, 34- Pyrometer, definition of, 1. Pyromike, 189. Radiation pyrometers, 134. — - — calibration of. 157. indicators for, 156. — management of, 161. — — recorders for, 161. special uses of, 164. Rasch, luminosity formula, 168. Recalescence of steel, 4, 94. Recorders for radiation pyrometers, 161. resistance pyrometers, 124. thermo-electric pyrometers, 75- Resistance, measurement of, 102. — of platinum, 105. Resistance pyrometers, iot. indicators for, 118. management of, 130. recorders for, 124. special uses of, 132. — pyronietry, terms used in, 111. Roberts- Aitsien , recorder, 76. Salts, melting points of, 204. Scel'vek, discovery of thermo-electri city, 3, 20. Seger, pyramids or "cones," 205. 224 PYROMETRY Siemens, calorimetric or "water" pyrometer, 201. — indicator for resistance pyro- meter, 118. thermo-electric pyrometer, 48. — optical pyrometer, 182. — recorder, 81. — - resistance pyrometer, 114. Specific heat of nickel, 197. Standardizing of indicators, 54, 108, 157- Standards of temperature, 9. Steam, measurement of temperature of, 98. Stefan -BoI/znia/i?i, fourth -power law, 139. Stune, pyrometer, 209. Surface temperatures, measurement of, 97. Suspended-coil galvanometers, 45- 48. Temperature differences, measure- ment of, 99. — scales, 7-9. - fixed points of, 16-17. Thermal junctions, changes in, 29. choice of metals for, 21. — E.M.F., developed by, 31. — — methods of joining, 26. — ■ used in pyrometers, 27. Thermo-electric circuits, 22. — pyrometers, 20. calibration curves for, 59. for surface temperatures, 97. Thermo-electric circuits, indicators for > 45-53- management of, 91. — — practical forms of, 32. recorders for, 75-87. — — standardization of, 54. Thermodynamic scale of tempera- tures, 9. Thermometer, constant volume gas, 11. — mercury, 1. Thermophones, 220. Thread recorder, 78. Uehling, pyrometer, 218. Uni-pivot galvanometer, 49. Vapour-pressure pyrometers, 217. Wiunier, optical pyrometer, 178. Water-cooled cold junction, 33. Water equivalent of calorimeter 200. "Water" pyrometer, 201. Water-jet pyrometer, 217. Watkiu, heat recorder, 207. Wedge pyrometer, 190. Wedgwood, pyrometer, 211. — test-pieces, 205. Wheatstone bridge for measuring resistance, 104-114. Whipple, indicator, 120. W/iipf>/e-Ft'ry, pyrometer, 154, Wiborgh, gas pyrometer, 219. — thermophones, 220. Wien % luminosity law, 171. PRINTED IN GREAT BRITAIN BY NEILI. AM. LTD., EDINBURGH.