Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/145

PYROMETER 133 Brothers, the mass of metal employed is a copper cylinder. For a sketch and description of the instrument, see IRON, vol. xiii. p. 304 (fig. 21). 7. Continuous Intrinsic Thermoscopes. The other pyro- 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 instru- ment 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 thermo- electric 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 dif- ference 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. 1 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 expan- sion of solid bodies. An ingenious application of the relative expansion of gold, silver, and platinum was introduced by Breguet. Very narrow strips of the three metals are fastened together to form a compound ribbon- spiral, and to the end of the spiral is attached a needle, which, as the temperature changes, moves over a graduated circle. The in- strument, 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 de- scribed by him in the Bakerian lecture (Proc. Roy. Soc., 1871). " Assuming a dynamical law, according to which the electrical re- sistance 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 coeffi- cients (representing linear expansion and an ultimate minimum resistance) the resistance r for any temperature is expressed by the general formula r = aT*+pT+y, which is found to agree very closely both with the experimental data at low temperatures sup- plied 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 constants a = 0'039369, /3 = "00216407, 7= -0'24127. The experimental arrangement for practical purposes of the in- 1 See Beckert, Zeitschr. f. anal. Chem., xxi. p. 248, 1882. 2 Pocjy. Ann., cxlix. p. 206. strument as supplied by Messrs Siemens Brothers is exceedingly convenient. It is shown in fig. 4. P is the coil of platinum-wire wound on a 'cy- linder of fireclay, and connected by stout platinum wires X, X, G with three bind- ing screws at the end of a stout iron tube 6 feet long, and thereby with an arrange- ment for compar- ing its resistance with that of a standard coil X, by means of dif- ferential volta- meters V, V. A current from six Leclanche cells is divided into two parts, one going through the standard coil X, the volta- meter V, and an additional pla- tinum wire, also marked X, join- ing the other branch again at the end of the platinum coil, while the other branch includes the voltameter V~, the connect- ing wire X, and / - the coil P. The wire C is com- _. mon to both cir- ^ cuits. The amount of gas generated in the voltameters is inversely proportional to the resistances of the respective branch circuits. Thus, if V and V be the volumes of gas in the two voltameters respectively, V _ Resistance of P and its connexions V ~ Resistance of X and its connexions The leading wires from the screws of the iron tube to the com- mutator 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 adjiisted 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 observa- tion at each of six temperatures, three of these between 531 and 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 sub- mitted 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 resist- ance after being repeatedly heated and cooled. It was found that