TEMPERATURE. 11 



coincident with the work scale. Formerly the air scale was the standard, 

 but now the hydrogen scale, as used at the Bureau International des 

 Poids et Mesures has taken its place. At the Bureau there is a hydrogen 

 thermometer in which the pressure at 0. is 1 metre of mercury, and in 

 which the density is kept constant. The degrees of the scale of this 

 instrument are increments of pressure, each T J^th of the increase of 

 pressure between C. and 100 C. We shall return to the subject of 

 gas thermometry in chapter iv. 



Platinum Resistance Thermometer. The electrical resistance of 



pure metals increases almost in direct proportion to the rise of tempera- 

 ture as indicated by the mercury-glass and gas scales. Siemens was the 

 first to employ the resistance of a platinum wire to indicate temperature, 

 and the method has been thoroughly investigated by Callendar ("On 

 the Practical Measurement of Temperature," Phil. Trans., 1887, A., 

 p. 161 ; PTiil. Mag., 1899, xlviii., p. 519). He has shown that it gives 

 an instrument convenient in form, easy to use, and applicable through a 

 far wider range than any other. The platinum wire he used in his 

 original experiments was '017 cm. diameter, about a metre long, and 

 about 5 ohms resistance. This was wrapped as a spiral on a glass 

 tube, and the ends soldered into thicker platinum leads '073 cm. diameter, 

 the tube being inserted in the enclosure of which the temperature was to 

 be measured. The resistance of the leads could easily be allowed for, 

 and the resistance of the platinum spiral itself at any temperature could 

 be found. The platinum scale is defined as giving equal degrees by 

 equal increments of resistance, 100 of such degrees making the interval 

 from C to 100 C. Let E, be the resistance at any temperature, R 

 and R 100 the resistances at C. and 100 0. If we denote a tempera- 

 ture on the platinum scale by pt, then 



Callendar compared this scale with the air scale, and found that if t is 

 the temperature on the latter, then to a close approximation 



where 8 is constant for a given wire and has nearly the same value, 1'57 

 for all specimens of pure platinum. 



It will be seen that the difference between the two scales must by 

 definition vanish at 0. and 100 C. At 50 0. it is a maximum t 

 being less than pt by about 0'4. Above 100 t is the greater, at 200 

 by about 3, at 300 by about 9, at 500 by about 31. 



One great advantage of the platinum thermometer lies in its easy 

 use for the determination of low temperatures. 



Thermo-Electric Thermometer. This thermometer makes use of 

 the fact that when a circuit consists of two different metals, A and B, 

 an electric current in general flows round the circuit when the two 

 junctions are at different temperatures. The driving E.M.F. depends 

 solely on the nature of the metals A and B and on the temperatures of 

 the two junctions. Further, if a galvanometer be included in the 

 circuit, with wire of another metal C, inserted, say, in the course of the 

 wire B, then so long as the temperatures of the junctions of C with B 



