Nitrogen Thermometer from Zinc to Palladium. 101 



y 



this ratio — is unity, but in the experimental constant-volume 



thermometer it always varies considerably from 1. We have 



preferred therefore to treat equation (4) as the fundamental 



v 

 equation, introducing in place of — , however, the proper 



function of the expansion coefficient of the bulb material. 



Since apparatus designed for high-temperature work is not 

 suited for the most accurate determination of a, a has been 

 treated in this discussion as a separately determined constant. 



In the experimental gas thermometer, there is always a 

 small space in the tube connecting with the manometer which 

 is at various temperatures other than t. The pressure (p' or 

 p/) actually measured is not therefore the p> 01 * p of 'the for- 

 mula. Imagine that this supplementary space is heated up to 

 the uniform temperature t, without any change in its volume, 

 and let the resulting corrected pressure be p (or p as the case 

 may be). Furthermore, let 



V = volume of bulb at t°. 



V — " " " n° 



v^= " " unheated space" which is at tem- 

 peratures other than t (or than 0°). 

 t 1 = temperature of this space. 

 (3 = linear expansion coefficient of the bulb material. 



Under these conditions, formula (4) becomes : 



i r v + v x _ -i 



p tt a L P 'V a + v 1 Po J 



3/3* 



U[*V + i+i> )- A ] (5) 



P.- <- ^ - . y 



In this formula ^ is a very small correction term ; while the 







important quantities to be measured arej? , p 7 a and /3. The 

 ratio ~ becomes of importance, however, in determining the 







corrected pressure p from the measured pressure p' '. The 

 derivation of this correction is as follows : 



The mass of the gas in the unheated volume under the 



actual conditions of measurement is proportional to — — — ; 



1+ v; 1+ v7 1 + v„ 



