BRIDGMAN. 



A SECONDARY MERCURY RESISTANCE GAUGE. 



>49 



Finally, the variation of specific resistance with temperature may be 

 calculated from the formula given for the variation with temperature 

 of p as determined by the measurement of p. Retaining only the 

 term of the first degree in p, we have to the degree of experimental 

 accuracy reached in these results: 



AR s (p,t) __&R s (p,to) 



R s (0, 



R (0, h) 



+ a 2 a ia pW h P % (t — t ), 



where a, a v a, and b have the values already assigned, and t equals 

 25°. In the deduction of this formula the variation of the compressi- 



1000 



2000 



3000 



4000 



5000 



COOO 



7000 



Figure 10. The resistance of mercury at various temperatures and pres- 

 sures in terms of the resistance at zero pressure and 25°. 



bility of the glass with the temperature was neglected. This variation 

 is beyond the limits of error if the glass used has a temperature co- 

 efficient of the same order as that found by Amagat,5 who found a 

 change of 10 per cent for 100°. From this formula R(p, t) was calcu- 

 lated for a number of pressures and for the temperatures 125°, 25°, 

 and — 75°, assuming R (0, 25°) equal to unity, and taking for the 

 temperature coefficient of specific conductivity the value 0.000888. 

 These results are given in Table IX and plotted in Figure 10. This 

 large temperature range was taken merely for convenience in showing 



5 Amagat, C. R., 110, 1248 (1890). 



