Electrical Thermometry

The Resistance Thermometer.

In practice the resistance thermometer is almost invariably made of platinum, since there is very seldom any advantage to be gained by the substitution of baser metals. The instrument is for this reason often referred to simply as the "platinum thermometer." It is important that the platinum should be pure, both for the sake of uniformity and also because the change of electrical resistance with temperature is greatly diminished by impurities. The observation of the fundamental coefficient, which is .00390 (or rather larger than the coefficient of expansion of a gas) for the purest metal hitherto obtained, is one of the most delicate tests of the purity of the metal. In addition to the constancy and infusibility of the metal, a special advantage which is secured by the use of platinum is the close agreement of the thermodynamical scale with the platinum scale of temperature, as defined by the formula (20) in which the symbol pt stands for the temperature on the platinum scale centigrade, and R, and R. are the observed resistances of the thermometer at the temperatures pt, ioo° and o° C respectively. A platinum thermometer is generally arranged to read directly in degrees of temperature on the platinum scale, just as a mercury thermometer is graduated in degrees on the mercury scale. The reduction to the scale of the gas thermometer is most conveniently effected by the difference formula in which d is a constant, called the difference-coefficient, the value of which for pure platinum is 1.5o, but varies somewhat for commercial specimens. This formula was first given by H. L.

Callendar as the result of a series of comparisons of different platinum wires with each other and with other metals, and also vith an air thermometer over the range o° to C. The plati num wire in these comparisons was enclosed inside the bulb of the air thermometer itself, and disposed in such a manner as to be at the mean temperature of the bulb in case the temperature was not quite uni form throughout (Phil. Trans. A. 1887, p. 161). The formula was subsequently veri fied by C. T. Heycock and F. H. Neville (lown. Chem. Soc. February 1895), by the observation of a number of higher points up to the freezing-point of copper at 1,082° C, which they showed to agree with the most probable means of all the best determinations by various methods of gas thermometry.

The difference-coefficient d in formula (21) could evidently be determined by reference to any fixed point on the abso lute scale which was known with sufficient accuracy by observations with a gas-ther mometer. The boiling-point of sulphur hap pens to be most convenient for this pur pose, as first proposed by Callendar and Griffiths (Phil. Trans. A. 1891, p. 119) and adopted by all subsequent observers. The annexed figure 6 shows the form of appara tus they proposed for the standardization of platinum thermome ters on this basis. Similar forms are still employed with slight

variations in points of detail. Sulphur is boiled in the bulb B at the base of a hard glass tube about 4cm. in diameter, such as is used for vapour-density determinations by the method of Vic tor Meyer, and is often called a "Meyer" tube. An asbestos board CD prevents the flame reaching the sides of the tube above the level of the liquid, and minimises the risk of superheating of the vapour. The upper part of the tube is lagged with about 2 cm. of asbestos wool to reduce external loss of heat, except for 3 or 4 cm. at the top which is left bare to serve as a condenser. The top is covered with a sheet of asbestos through which the thermometer M is inserted. The gas is adjusted to keep the sulphur vapour at a steady level A near the top of the tube. A cone of asbestos board surrounds the lower half of the lagging to give it greater stability and further diminish the rate of cool ing. The condensed liquid trickles down the sides of the tube, and also to some extent down the stem of the thermometer. It was found that a naked thermometer inserted in the tube under these conditions might indicate a temperature nearly 2° lower than that of the condensing vapour owing to loss of heat by radiation to the sides in addition to the cooling effect of the liquid. These losses could be practically eliminated by fixing an umbrella E on the upper part of the thermometer tube to divert the liquid stream, and two concentric screens round the bulb to protect it from loss by radiation to the sides. The bulb was also protected from the boiling liquid below by two perforated screens H, but this seemed to make little difference, as the liquid was little if at all superheated, and boiled very quietly. This apparatus gave very consistent results, to nearly o.or° C, with three different thermometers constructed from the wire spiral which had been directly compared with the gas-scale in the bulb of the air thermometer employed in 1887, and which gave the value d =1.57. This value was again verified by comparing one of these thermometers with the same air thermometer in a bath of sulphur vapour. Employing this value, the temperature of the boiling point of sulphur at a pressure of 760 mm. was found to be C on the scale of the constant-pressure air ther mometer. Chappuis and Harker (1902) found the value 445.2° C on the scale of the constant-volume nitrogen thermometer, but this was subsequently corrected to C to allow for a prob able error in the expansion of the bulb. Eumorfopoulos (1908) found the value 444.55° using a constant-pressure air thermometer of the type shown in figure 5, with a glass bulb, but later in using the same thermometer with a bulb of fused quartz, having a much smaller coefficient of expansion, he found 444.13 on the same scale, and 444.61 on the thermodynamic scale.