most important variety of the mineral is a "cupreous iron | The quantity of cupreous and other iron pyrites imported into Great Britain during the year 1883-principally from Spain and Portugal, but partly from Norway and elsewhere -was 601,288 tons, of the declared value of £1,356,083. But this quantity had been exceeded in several previous years, notably in 1880 and 1877. The quantity of iron pyrites raised in the United Kingdom in 1883 amounted to 27,672 tons, of the value (at the mine) of £17,467. See IRON, vol. xiii. pp. 280, 288; COPPER, vol. vi. p. 347; MARCASITE, vol. xv. p. 532; and MINERALOGY, vol. xvi. pp. 390, 393. For details of the Rio Tinto pyrites, see A Treatise on Ore-Deposils, by J. Arthur Phillips, 1884. 1 method and the calorimetric method were both employed by Pouillet for the accurate measurement of high temperatures before 1836. § 2. The indications obtained by any of the numerous methods which have been suggested are, as a rule, expressed in terms of Centigrade or Fahrenheit degrees. This assignment of numbers presupposes not only a definition of temperature by which the size of the degree is determined but also a physical law which gives the relation between the measured interval of temperature and the standard degree. The various definitions of the standard degree that might be employed will be found in the article HEAT, secs. 12, 24, 25, 30, 31, 32; and in sec. 35 of the same article the definition of the absolute thermodynamic scale of temperatures is given. In the same article (sec. 38) it is shown that the "absolute temperature" of a liquid in thermal equilibrium with its own vapour under a pressure p may be obtained from the formula where p is the density of the vapour, por that of the liquid, K the latent heat per unit-mass of the vapour corresponding to the saturation-pressure_p. The dynamical equivalent of heat is represented by J. We have therefore the complete theory of what may soon become a practical method of expressing temperatures in the thermodynamic scale. Sir W. Thomson, in the article mentioned (secs. 39-45), has described arrangements for measuring the pressure of the saturated vapours of various liquids which will give that measurement in a thoroughly satisfactory manner up to, at any rate, some 600° C. For the higher temperatures mercury is the liquid employed. There are, however, some experimental data still wanting before the formula quoted above can be applied to the numerical calculation of the temperature. These are (1) the density p of the saturated vapour corresponding to the series of pressures, and (2) the corresponding latent heat of vaporization. These constants have not yet been actually observed. Instruments such as those figured in the article cited can, however, be employed with convenience and accuracy as continuous intrinsic thermoscopes, whose indications can supply a numerical measure of temperature after an empirical graduation. When used thus they possess the enormous advantage that the pressure of the saturated vapour at a definite temperature is perfectly definite, so that a single observation of the pressure is all that is necessary to determine the temperature, and the instrument can be easily arranged, so that this observation is practically a very simple one. The pressure of mercury vapour has already been determined by Regnault for temperatures up to 550°. A thermoscopic method of pyrometry which is very similar to the above was sug gested by Lamy. He proposed to measure the pressure of carbonic-acid gas dissociated from calcium carbonate. There is experimental evidence to show that the pressure of the dissociated gas is definite at a definite temperature. The recombination of the dissociated gas with the solid is, however, a slow process, and the method has been pronounced by Weinhold3 to be practically unsatisfactory. PYRMONT. See WALDECK. PYROMETER, an instrument for measuring high temperatures. As long ago as 1701, in a paper 1 published anonymously in the Philosophical Transactions, Newton gave the results of attempts to estimate the temperature of red-hot iron by noting the time it took to cool to an observed temperature, assuming what has since been called Newton's Law of Cooling. The numerical results are given in terms of the degrees of a linseed-oil thermometer constructed by Newton. Its zero was the temperature of melting ice and its second fixed point the normal temperature of the human body, denoted by 12°. About the same time Guillaume Amonton in Paris made somewhat similar attempts to determine the temperature of the red-hot end of an iron bar, using for reference a rudimentary air-thermometer-the first of its kind in which the variation of atmospheric pressure was allowed for. Since the middle of the last century the different methods and instruments suggested for measuring high temperatures have been very numerous,in fact the variation of almost every physical property of substances which alter with change of temperature has been utilized for this purpose. Measurements of the increase of pressure produced in a quantity of gas while its volume remains constant or of the increase of volume at constant pressure, of the heat given out by a mass of metal in cooling to an observed temperature, of the expansion of a metal or graphite bar or of a mass of clay are those which have been most frequently employed; but, besides these, the change in the electrical resistance of a wire, the saturation-pressure of the steam of various liquids, the pressure of gas dissociated from various solids, the electromotive force of a thermo-electric couple, the density of the vapour of a liquid, the change of shape of a compound spiral of different metals, have been used,§3. Gas Pyrometry. Measurement of High Temperaeven the alteration in the wave-length of a note of given tures by the Expansion of Air and other Gases and Vapours. pitch has been suggested as capable of being made use-Temperatures may be expressed in the absolute thermoof for pyrometric purposes. For reasons which will be dynamic scale by the method of the gas-thermometer, given below, the numerical results obtained by one or which is available for practical purposes even at very high other of the numerous forms of the gas-thermometer have temperatures. It has been shown that the indications a more definitely intelligible value. The gas-thermometer 2 Comptes Rendus, lxix. p. 347. 1 "Scala Graduum Caloris," in Phil. Trans., xxii. p. 824. 2 3 66 Pyrometrische Versuche," Pogg. Ann., cxlix. p. 186. XX. of a nitrogen or hydrogen gas-thermometer, whether it is | In like manner v1000 may be taken from a table of the arranged to show the increase of pressure at constant coefficients of expansion of gases. The different methods volume or the increase of volume at constant pressure, give which have been suggested for the employment of this for the temperature numerical results which are practically property of gases to measure high temperatures are very identical with the corresponding numbers on the absolute numerous. We give details of a few of them. scale. It follows, therefore, that any two gas-thermometers, if similarly graduated, would give identical indications for the same temperature, no matter whether or not they are filled with the same kind of gas and whether or not the quantities of the gases are such that the pressure in the two thermometers is the same at any one temperature. This important property of gas-thermometers has been experimentally verified by Regnault1 by direct comparison up to 350° C. of instruments filled with different gases and at different pressures. For these reasons the readings of a properly arranged gas-thermometer have justly come to be regarded as furnishing the standard of temperature, at any rate outside the limits of the freezing and boiling points, and indeed may now be regarded as the temperature standard for scientific purposes throughout the whole range. The Kew standards are calibrated mercury-in-glass thermometers whose fixed points are repeatedly redetermined. Such instruments will not agree exactly with the gas-thermometer except at the freezing-point and boilingpoint. Comparisons have been made between various mercury-thermometers and air-thermometers by Regnault 2 and many others. The results obtained by different observers are not entirely concordant; but it is needless here to discuss them, for, whatever may be the divergence between the mercury and air thermometers in the freezingpoint and boiling-point, the method of measuring higher temperatures by continuing the scale of a mercury-thermometer beyond those limits is altogether untrustworthy in consequence of the very wide divergence between different mercury-thermometers at the same temperature, amounting sometimes to 10° or more 3 at a temperature of 300°. The air-thermometer readings must therefore be regarded as the standard at any rate for temperatures beyond the boiling-point. (1) The Constant-Pressure Method. The following is a very simple and practical plan of employing the method for obtaining a reading of the temperature. A glass or porcelain bulb, provided with a fine neck, is very carefully dried and filled with perfectly dry air; it is then exposed to the source of the heat whose temperature is to be investigated in such a manner that the point of the neck just projects from the furnace. When the equilibrium of temperature is reached, the neck is hermetically sealed by a blowpipe or oxy-hydrogen flame, and the bulb is withdrawn and allowed to cool, and weighed. The neck is then immersed in water or mercury and the point broken off. In consequence of the previous expansion of the air the pressure in the interior is much less than the atmospheric pressure, and the liquid consequently enters the bulb. When so much has entered that the pressure is the same inside and out (the difficulty of the comparative opacity of the porcelain is not insurmountable), the end is closed by a small piece of wax, and the bulb removed and weighed, with the liquid it contains. The bulb is then completely filled with the liquid, first weighings gives a value v of formula (2), which only requires and weighed a third time. The difference between the third and correction for the expansion of the envelope, while the difference between the second and first weighings gives a value of the volume from which 2 and 100 can be calculated, using the known co-efficient of expansion of air, and thus all the requisite data for the determination of t are obtained. This method was used by Regnault 5 to determine the coefficient of expansion of air, and has since been described as "a new pyrometer. In the process just described the volume of the residual gas is measured; its pressure, after cooling, may be measured instead, by an arrangement which was suggested by Regnault. The bulb is provided with a long fine neck, to the end of which a tap is fitted and so arranged that it can be easily connected with a manometer. The bulb is exposed to the high temperature, the tap being left open, and when the final temperature is reached the tap is closed and the bulb allowed to cool; it is then connected with the manometer, and, if the tap be a threeway tap, drilled as shown in fig. 1, it is easy to expel all the air from the bulb side of the manometer, between the mercury surface and the tap. The general principle employed in the use of the gasthermometer is as follows. Let Po be the pressure of a mass of gas at 0° C., P100 the pressure of the same mass of gas at 100° C., the volume being the same, p, the observed The residual pressure pressure of the same mass of gas at some unknown temperature t, the volume still remaining the same, then 4 We require, therefore, three observations of the pressure, two to graduate the instrument and the third to measure the temperature. If the thermometer has been filled with gas of a perfectly definite kind-e.g., properly dried and purified air, nitrogen, or hydrogen—and the containing vessel has been previously thoroughly dried, the value of P100 may be obtained from tables, since P100=20(1 + 100a), where a is the tabulated coefficient of expansion of the gas at constant volume. It is practically impossible to keep the volume of the gas constant in consequence of the expansion of the envelope. A correction must be applied on this account, the value of which is derived from independent observations of the expansion of the material of the envelope. If the pressure of the gas be maintained constant, and the volumes v, 2100, Vo be observed for the three temperatures t°, 100°, 0°, we have is then measured by Fig. 1. the bulb and for the part of the connecting tube not exposed to the high temperature. Instead of measuring the volume of the residual gas in the manner thus described, Deville and Troost have pumped the hot air out of the porcelain bulb by means of a Sprengel pump, and measured the volume of air delivered by the pump. On this plan a series of observations can be made at the same temperature, a three-way tube with suitable taps serving to put the bulb alternately in connexion with a vessel to supply dry air and with the pump. Crafts and Meier have obtained results by sweeping out the air with a current of hydrochloric-acid gas, which was separated from the air it carried by being passed through water. 8 An instrument for observing the continuous variation of volume of a gas at constant pressure is figured and described by Sir W. Thomson in HEAT (Sec. 65). Arrangements have also been suggested by which the density of the gas at the high temperature can be directly measured. Regnault has described a hydrogen pyrometer based on this principle suitable for measuring the temperature of a porcelain furnace. A wrought-iron tube of known capacity is permanently fixed in the furnace; it is filled with pure dry hydrogen by passing a current of the gas through it for some time. The current of gas is then stopped, and after the gas has attained the temperature of the furnace it is swept out by a current of dry air and passed over red-hot copper oxide. The water thus 5 Mém, de l'Inst., xxi, 7 C. R., xc. 606. 6 Comptes Rendus, xc. 727, 773. 8 Throughout this article the term "density" is used whenever the mass of a unit of volume of a substance is referred to. 9 Ann. de Chim., [3], lxiii. p. 39. formed is collected in sulphuric acid tubes and its amount determined by their increase in weight, and from this observation the density of the hydrogen in the wrought-iron tube is calculated. An arrangement of taps makes the observation a very easy one when the apparatus is once set up. The formula (2) requires in this case to be slightly modified. Thus let de, doo, do be the densities of the hydrogen at the temperatures to, 100°, and 0° respectively, then for the same mass of gas m we have— (2) The formula shows how the temperature of air in any experiment may be determined when its density at that temperature is observed. It is sometimes more convenient to determine instead the density of some vapour which at ordinary temperatures would be a solid or a liquid, and to deduce from that observation the density of air at the corresponding temperature. Thus, suppose that the density de (expressed in grammes per cc.) of the vapour of any given liquid or solid is observed, and that independent observations show that the specific gravity of the vapour, referred to air at the same temperature and pressure, is o, then we have de-de/o, and, since do and doo. can be taken from tables, all the necessary quantities in equation (3) are obtained. It will be noticed that the value of o, the specific gravity of the vapour, is to be derived from independent observations. Apart from direct experimental evidence in any particular case, there is the generally accepted theory, based on the law of Avogadro, that the specific gravity of a gas or vapour referred to hydrogen at the same temperature and pressure is represented by half the number expressing the molecular weight of the substance of which the vapour is composed. For elements, with few exceptions (of which mercury is one), the ratio of the atomic weights gives the specific gravity referred to hydrogen at the same temperature and pressure. At any rate, if there are sufficient data for us to regard o as known, we may evidently deduce the value of de, and thus by formula (3) the temperature, from an observation of 8. 3 Fig. 2. No. 2. Mercury Vapour.-Regnault1 suggested the direct observation of the density of mercury vapour for the purpose of determining the temperature. The process is as follows. A quantity of mercury is placed in a wrought-iron flask provided with a perforated lid as shown in fig. 2, No. 1. The flask is then exposed to the temperature to be measured, and when thermal equilibrium is attained the small lid is slid along so that the neck is closed. The flask is then taken out and allowed to cool. The mercury is collected and weighed; the volume of the flask is determined and corrected for the expansion of the iron; and these two observations determine the density of mercury (in grammes per cc.) at the temperature in question. The specific gravity2 of mercury vapour referred to air at the same temperature and pressure is known to be 6.92. No. 1. A porcelain flask with a ball stopper, shown in fig. 2, No. 2, may be used instead of the iron flask. Iodine Vapour. Deville and Troost's Pyrometer.-Some of the best-known determinations of very high boiling points have been made by Deville and Troost, who employed iodine in a manner similar to that in which Regnault employed mercury. Some iodine was contained in a porcelain flask of about 300 cc. capacity, with a fine neck, which just protruded from the source of heat and was loosely closed by means of a stopper; when the temperature was reached and the iodine completely volatilized, the stopper was fused on to the nozzle by means of an oxy-hydrogen blowpipe. The mass of the iodine remaining in the flask was determined by weigh ing, after it had cooled; the volume of the flask had been previously determined; thus the density of the iodine vapour could be found. A correction of the volume of the flask was necessary in consequence of the expansion of the Bayeux porcelain of which it was composed. This was obtained from independent observations of the linear elongation of a rod of porcelain for temperatures up to 1500; their results gave a coefficient of cubical expansion of 0-0000108 between 0 and the boiling-point of cadmium (856°), 0-0000108 between 0° and the melting-point of silver (1000°), from 0-000016 to 0-000017 between 1000° and 1400°, reaching 000020 towards 1500. The specific gravity of iodine vapour was taken to be 8716, referred to air at the same temperature and pressure; this assumption was justified by additional observations with air and by using the number in a determination of the density of steam at the boiling point of mercury. (3) The Manometric Gas-thermometer.-In the constant-pressure methods of measuring temperature which have just been described one experiment gives only a single observation of the temperature. The continuous variation of temperature can be better observed by the constant-volume method. This method as used for temperatures up to that at which glass softens (about 550° C.) was thoroughly investigated by Regnault, whose normal instrument is discussed under HEAT, Sec. 24. The difference of pressure between the gas contained in the bulb and the atmosphere is measured by an open mercury-manometer. The barometric pressure must also be observed in order to obtain the values Pt, P100, and Po respectively of formula (1). Various forms have been given to the manometric apparatus in order that the mercury may be brought at each observation to the fiducial mark in the limb in connexion with the bulb. Balfour Stewart's 5 has a screw adjustment. An instrument described by Codazza is provided with an air-compression manometer, and thus the necessity of a separate observation of the barometric height is dispensed with. Various other suggestions have been made for securing the same object. The most convenient form of the instrument for general use is Jolly's (described in Poggendorff's Jubelband, p. 82, 1874), and represented in fig. 3. The two vertical tubes of the manometer are connected by an india-rubber tube properly strengthened by a cotton covering, and they can be made to slide vertically up and down a wooden pillar which supports them; they are provided with clamps for fixing them in any position and a tangent screw for fine adjustment. The connexion between the bulb and the manometer is made by means of the convenient three-way tap described above. The scale of the instrument is engraved on the back of a strip of plane mirror before silvering, and the divisions are carried sufficiently far across the scale for the reflexions of the two surfaces of the mercury to be visible behind the scale. Parallax can thus be avoided and an accurate reading obtained without the neIn order to cessity of using a kathetometer. allow for the expansion of the glass of the reservoir a weight-thermometer bulb is supplied with the instrument, made from another specimen of the same kind of glass, and the relative expansion of the mercury and the glass can thus be determined by the observer himself. The volume of the air-bulb and that of the capillary tube and the small portion of Fig. 3 the manometer tube above the small beak of glass, the point of which serves as the fiducial mark, are determined by the instrument-makers. The formula of reduction is aH-3ẞH v Ho 1+at' 9 where is the pressure at the high temperature t, H, the pressure at the temperature of the air t', v'/v the ratio of the volume of the connecting tube, &c., to the volume of the bulb, a the coefficient of expansion of the air, and 38 the coefficient of cubical expansion of the glass. A similar instrument with a bulb which will resist higher temperatures may be used beyond the softening-point of glass. Pouillet in his classical research on high temperatures used a platinum bulb and connecting tube. He employed the constantpressure method and measured in the manometer tube the variation of volume. Regnault mentions a platinum air-pyrometer and gives instructions for drawing the platinum connecting tube; but no results of measurements obtained with it are given. E. Becquerel published an account of results obtained with a platinum reservoir air-thermometer, which were objected to by Deville and Troost on the ground that platinum becomes porous at high temperatures, and their objection is supported by an experiment described by them in the Répertoire de Chimic Appliquée, 1863, p. 237, and Fortschritte der Physik, 1863, p. 84. Weinhold 10 used a Jolly's thermometer fitted with a porcelain bulb and connecting tube, and Deville and Troost are of opinion that porcelain forms the only suitable material for gas-thermometer bulbs for very high temperatures. For use at high temperatures the gas-thermometer should be filled with gas at a low pressure, so that when heated there may be no great difference of pressure between the interior and the external air. It is perhaps unnecessary here to insist upon the necessity for the complete desiccation of the interior of the bulb and of the gas employed. (4) The last modification of the gas-thermometer to which it is intended for reading high temperatures rapidly to an accuracy of necessary to call attention is that designed and used by Berthelot, 12 within two or three degrees. It consists of a small cylindrical bulb of glass or silver of 4 cc. capacity connected with a vertical stem of thermometer tubing of 0.2 mm. diameter. This stem terminates in an open vessel of mercury, and thus the pressure of the gas can be measured. Berthelot's instrument is graduated by reference to four fixed points, namely, the freezing-point and boiling-point of water, and the boiling-points of mercury and sulphur. In order that the mercury index may move easily in the tube, extreme care must be taken in drying the tube, and only perfectly pure mercury can be used. § 4. The results obtained by any of the air-pyrometric methods just described may be employed to express directly the temperature of the pyrometer in numbers agreeing closely with the thermodynamic scale. The other instruments to which we now turn our attention can only be regarded as intrinsic thermoscopes, which, in order to give intelligible numerical results, must be graduated by direct comparison with an air-thermometer. Some of them may indeed be used by extrapolation to give a numerical measure of temperatures outside the practical range of the air-thermometer, employing for that purpose a formula verified for temperatures within the range. A case in point is the determination of the temperature of fusion of platinum by the calorimetric method described below. These intrinsic thermoscopes are frequently much more convenient in practice than any of the modifications of the air-pyrometer. § 5. Discontinuous Intrinsic Thermoscopes.—The best example of the measurement of temperature by a discontinuous intrinsic thermoscope is that suggested by Prinsep.1 He formed a series of definite percentage alloys of silver and gold and of gold and platinum. The melting-points of these alloys give a series of fixed temperatures lying between the melting-points of silver and gold and of gold and platinum respectively. An observation is taken by exposing in the furnace, upon a small cupel, a set of small flattened specimens of the alloys, not necessarily larger than pin heads, and noticing which of them are fused. The temperatures of fusion of these alloys have been determined by Erhard and Shertel2; their results are given in the following table, taken from Landolt and Börnstein's Physikalisch-chemische Tabellon. Table I.-The Fusing-Points of Prinsep's Alloys. temperature of a furnace. Certain "pyrometrical beads" or "trials” -i.e., small hoops or gallipots of clay-indicated the temperature by their tint much in the same way as the proper temperature is indicated by the colour of steel in tempering. § 6. The Calorimetric Method. This is a very convenient method and is often practically employed for measuring the temperature of furnaces. The observation consists in determining the amount of heat given out by a mass of platinum, copper, or wrought-iron on cooling in water from the high temperature. The theory is simple. Let m be the capacity for heat of the calorimeter and of the water contained in it, M the mass of metal, T the temperature required, t the initial temperature of the water in the calorimeter, Ø the final temperature of the water after the introduction of the metal, and κ the mean specific heat of the metal between the temperatures and T. Then T— 0 —m(0 − t) M.K K The value of x, the mean specific heat of the metal between the temperatures occurring in the experiment, must be determined by precisely similar calorimetric experiments, in which the high temperature 7 is determined by the application of one of the air-pyrometer methods. The following table (II.) gives the best-known determinations of the mean specific heat of platinum for different ranges of temperature. It is said, however, that some difficulty is met with in the use of Prinsep's alloys in consequence of the property possessed by silver of taking up oxygen when melted and ejecting it on solidifying and of molecular changes in the alloys which make it unadvisable to use the same specimen more than once. A similar method has recently been employed by Carnelley and Carleton Williams, in which metallic salts with high fusing-points were employed instead of alloys, the fusing-points being initially determined by a calorimetric method. These methods recall an old empirical method sometimes employed in porcelain manufacture for estimating the 1 Phil. Trans., 1828, p. 79. 2 Jahrb. für das Berg- und Hütten-Wesen in Sachsen, 1879. 3 Determinations of temperature by a porcelain air-thermometer. Errors in general less than 20°. 4 See Chem. Soc. Jour., 1876, i. 489; 1877, i. 365; 1878. Violle's results give, if c be the mean specific heat between 0° and to, c=0·0317+000006t. Assuming this formula to hold beyond the verified limits, he obtains by calorimetric observations 1779° C. as the temperature of the melting-point of platinum. The true specific heat of wrought-iron at temperature t is according to Weinhold (1.c.) given by the formula cco+at+ßt, where Co=0·105907, a=0.00006538, p=0·000000066477, and the total heat obtained from unit-mass of wrought-iron cooling from to to t2° is therefore (c+at+ẞt2)dt. The specific heat of copper does not appear to have been accurately determined for high temperatures. The determinations by Bède, quoted by Landolt and Börnstein (op. cit., p. 178) are 15°-100° mean specific heat 0·09331; 16°-172° 17°-247° 0'09483; 0.096$0. There are two obvious sources of error of considerable amount in the use of the calorimeter for pyrometrical purposes, viz, (1) the liability of the metal to lose heat during its passage from the furnace to the calorimeter, and (2) the evaporation of water from the calorimeter. With the small mass of platinum generally used, the former source of error is likely to be very important, for the temperature of a mass of 50 grammes of mercury at 100° C. may fall a full degree in being carried to a calorimeter 3 feet away. It does not appear that any estimates of the amount of loss which may be so produced in calorimetric determinations have been published; but in order to reduce the loss Sallerons suggests the employment of a platinum or copper carrier in which to heat the mass of metal, and J. C. Hoadly uses a graphite crucible for that purpose. The second source of loss is more easily disposed of. Weinhold (1.c.) uses a calorimeter closed by a lid and quite filled with water. This is provided with a broad tube passing nearly to the bottom of the calorimeter, and the latter is tilted while the platinum mass is being introduced; whereas Violle 10 gets over the same difficulty by the use of a calorimeter provided with a platinum "éprouvette, so that the heat is imparted more slowly to the water. In a calorimetric pyrometer for technical purposes, made by Messrs Siemens 5 C. R., iii. p. 786 (1836). 7 Phil. Mag., [5], iv. p. 318. 6 Pogg. Ann., cxlix, 8 Chem. News, xxvii. 77. 9 Jour. of Franklin Inst., xciv. p. 252. 10 Phil. Mag., [5], iv. p. 318. Brothers, the mass of metal employed is a copper cylinder. For a sketch and description of the instrument, see IRON, vol. xiii. p. 804 (fig. 21). §7. Continuous Intrinsic Thermoscopes.-The other metric methods to which we have space to refer are those which depend on the continuous variation of some property of a body with variation of temperature. Each instrument of this kind requires graduation by direct or indirect comparison with an air-thermometer. The methods may be grouped under three heads, (1) the expansion of a rod of metal or earthenware; (2) the variation of electrical resistance of a wire; (3) the electromotive force of a thermoelectric junction. (1) Expansion of Metals and Earthenware. -The necessity for the measurement of high temperatures has been most felt perhaps in pottery manufacture, and in consequence many attempts have been made by potters to establish a system of pyrometry based on the permanent contraction which clay undergoes when exposed to a high temperature. The action of Wedgwood's pyrometer described in the Phil. Trans., 1782, 1784, and 1786, depends on this property of clay. The linear contraction of a clay cylinder was measured by means of a metal groove with plane sides inclined to each other at a small angle, and the temperature was estimated numerically by comparing the contraction with that produced by a known difference of temperature. The results were not very satisfactory, since the clay would contract the same amount by long-continued heating at a lower temperature as by a short exposure to a higher one. Wedgwood's estimate of the melting-point of cast iron was 20,577° Fahr. The measurement of temperature by the expansion of a metal rod has been very frequently attempted. The first instrument to which the name of "pyrometer" was given was of this kind, and was devised by Muschenbroek, and others were devised in the early part of the century by Des Aguliers, Ellicot, Graham, Smeaton, Ferguson, Brogniart, Laplace, and Lavoisier, and later by Pouillet. We may say here that the only acccurate methods of measuring the extremely minute elongations of metal rods are those in which the expansion is referred by some optical arrangement to a scale kept quite uninfluenced by the source of heat which causes the expansion. In this respect Pouillet's method of employing the expansion of a rod is superior to those previously employed. The relative expansion of a metal in an earthenware socket was employed by Daniell in his well-known pyrometer. The relative expansion was indicated by an index of porcelain which was pushed forward when the bar expanded and left behind when it contracted, so that after the apparatus had cooled the expansion could be measured at leisure by the scale provided; due allowance was made for the expansion of the index itself. Quite recently the expansion of graphite has been employed for pyrometry by Steinle and Harting. As the result of his experience, however, Weinhold? states that it is not possible to obtain trustworthy measurements of temperature from an instrument depending on the relative expansion of solid bodies. 1 An ingenious application of the relative expansion of gold, silver, and platinum was introduced by Bréguet. Very narrow strips of the three metals are fastened together to form a compound ribbonspiral, and to the end of the spiral is attached a needle, which, as the temperature changes, moves over a graduated circle. The instrument, of course, requires empirical graduation. A modification of it is sometimes used to measure the temperature of the hot blast of an iron furnace. (2) Variation of Electrical Resistance. A pyrometric method founded on the variation of the electrical resistance of a platinum wire has been practically carried out by Siemens, and was described by him in the Bakerian lecture (Proc. Roy. Soc., 1871). Assuming a dynamical law, according to which the electrical resistance increases according to the velocity with which the atoms are moved by heat, a parabolic ratio of increase of resistance with increase of temperature follows, and in adding to this the coefficients (representing linear expansion and an ultimate minimum resistance the resistance r for any temperature is expressed by the general formula r=aT+BT+y, which is found to agree very closely both with the experimental data at low temperatures supplied by Dr Matthiessen and with the experimental results varying up to 1000 C." The details of the experimental verification are not given in the abstract of the lecture, nor are the numerical values of the constants for platinum. But Weinhold gives the information, obtained by letter from the lecturer, that T is the absolute temperature, and the numerical values of the constantsa=0 039369, 8=0·00216407, y= -0-24127. The experimental arrangement for practical purposes of the in See Beckert, Zeitschr. f. anal. Chem., xxi. p. 248, 1882. strument as supplied by Messrs Siemens Brothers is exceedingly long, and thereby with an arrange ment for comparing its resistance with that of a standard coil X, by means of dif ferential voltameters V, V. A current from six Leclanché cells is divided into two parts, one going through the standard coil X, the voltameter V, and an additional platinum wire, also marked X, joining the other branch again at the end of the platinum coil, while the other branch includes the voltameter V, the connecting wire X, and the coil P. The wire C is common to both cir P Fig. 4. 80 80 100 喝 cuits. The amount of gas generated in the voltameters is inversely proportional to the resistances of the respective branch circuits. Thus, if V and 'be the volumes of gas in the two voltameters respectively, Ꮴ = Resistance of P and its connexions The leading wires from the screws of the iron tube to the commutator BBC are bound together in one cable, so that they have the same resistance; thus the observed variation in the ratio of the resistances may be regarded as entirely due to the variation in the resistance of P. The height of the liquids in the two voltameters can be adjusted by the short glass tubes S, S" sliding vertically on the wooden support to which the voltameters are attached. They are connected by means of india-rubber tubing with the voltameters. The commutator BBC is used to reverse the direction of the current every ten seconds during the observation, which lasts long enough to give a sufficient supply of gas in the voltameter tubes. By this artifice the error due to variation in the polarization of the electrodes is avoided. The voltametric arrangement for comparing the resistances simplifies very greatly the apparatus required. In a laboratory the resistances may be, of course, more accurately compared by means of resistance-coils and a galvanometer. For technical purposes the temperatures up to 1400° are reduced from the observations by means of a very convenient slide rule. For temperatures beyond 1400° the calculation has to be gone through. The experimental data upon which the verification of the formula and the determination of the constants rest are not very numerous. Besides the measurements of Siemens referred to above, there is an experimental comparison by Weinhold of the results obtained from the instrument and those of an air-thermometer. For these observations the iron cover of the coil was removed. The results up to 500°, which in each case are the mean of from five to ten observations, show an agreement within 9°; those between 500° and 1000°, comprising one observation at each of six temperatures, three of these between 531° aud 553° and three between 933 and 992, show differences of about +26° at the lower limit and -53° at the upper. The arrangement for comparing the resistances was found to be satisfactory and sufficiently sensitive. Specimens of this instrument were also submitted to experiment by a committee of the British Association (Report, 1874), but their attention was confined to the resolution of the question whether the platinum coil gave the same resistance after being repeatedly heated and cooled. It was found that |
