CAPACITY FOR HEAT OF METALS AT LOW TEMPERATURES. 
325 
The procedure in other respects was the same as before. The rise in temperature 
of the wire above the mean temperature of the block is given by the relation 
0 = O’10 n 2 , 
where n is. the number of standard cells balanced at the ends of the heating coil. 
Thus, from the temperature coefficient of resistance at any temperature, the 
correction could be calculated. 
Our practice of determining' the resistance of the heating coil on the completion of 
an experiment, when the temperature of the block was a few degrees above the 
surroundings, rendered the correction of small magnitude and in most cases it could 
be neglected. 
In the case of sodium it was found that the increase of resistance due to increase of 
current was not appreciable. 
(4) Measurement of Temperature. 
The calibration of a platinum thermometer for use at low temperatures presents 
some difficulty, since the parabolic formula 
t- pt = s\(-Lf-L\ 
1 l\100/ 100J 
does not hold true below about —40° C. 
At first we endeavoured to approximate to the true scale below 0° C. by means of an 
expression of the form 
R = R 0 (l -\-at—(3t 2 +yf), 
where the constants a, /3, and y are determined by standardisation in liquid oxygen, 
ice, steam and vapour of boiling sulphur. 
Recently, however, Henning* has effected a direct comparison down to liquid air 
temperature of some platinum thermometers with the hydrogen gas thermometer, and 
as our standardisations enabled us to utilize the data obtained by Henning for the 
purpose of a reduction to the gas scale, we base our calculations on the results 
obtained in that comparison. He found that thermometers constructed of wires 
having different values of dt do not give consistent results at low temperatures, their 
indications differing by as much as half a degree. 
If, however, the platinum temperatures pt' and pt deduced from observations with 
two thermometers are compared at the same true temperature t, then the following 
relationship holds good 
pt'—pt = cpt (pt — 100),.(l) 
where c is a constant determined by a comparison of the two thermometers at one 
temperature, as for example, in a liquid air bath. 
* ‘ Annalen d. Physik,’ vol. 4, p. 635, 1913; vol. 10, p. 1064, 1913. 
f Henning determined S by the freezing-point of zinc (419°-40). 
