BRIDGMAN. — A SECONDARY MERCURY RESISTANCE GAUGE. 251 



portionate effect of temperature is reduced. This is shown by the 

 temperature effect both on resistance and on pressures coefficient of 

 resistance. Thus the temperature coefficient of the pressure coeffi- 

 cient has become reduced at 6500 kgm. to 0.7 of its initial value, while 

 the temperature coefficient of resistance is reduced from 0.0009 to 

 0.0007. This latter effect shows itself in a tendency of the curves for 

 different temperatures to draw together with increasing pressure 

 toward some value of resistance greater than zero. That is, for a 

 large enough value of pressure, the resistance acts as if it might have 

 a definite value independent of temperature. 



Conclusion. 



In this paper it has been found that the mercury resistance gauge 

 is a reliable secondary standard of pressure if proper precautions are 

 used. The mercury must be pure and free from air. The irregular 

 behavior under pressure of the containing glass capillary is the principal 

 source of error. An easily fusible glass in which the strains left after 

 drawing are presumably small, is better than an infusible glass. The 

 glass must be seasoned by several applications of pressure over the 

 entire range before it becomes regular in behavior. If after this it is 

 exposed to considerable changes of temperature or to sudden changes 

 of pressure, it must be reseasoned. The maximum error that can be 

 introduced by irregularities in the glass is about 2.5 per cent. The 

 dependence of pressure on the measured proportional change of re- 

 sistance (p) and temperature is given by the equation 



p = a P 10ft» 1 - 03 [1 - ax {t - 25°) - M* - 25 ) 2 ], 

 where 



a = log- 1 4.4871 ; 



^ = log— i 9.8836- 10; 



a 1 = log- 1 7.1253- 10;' 



k = log- 1 4.4487 - 10. 



This formula, which applies to mercury in a capillary of Jena glass 

 No. 3880 a, gives the pressure correctly to -tV per cent between 500 

 and 6800 kgm. and 0° and 50° C. 



Empirical expressions have also been deduced connecting the 

 specific volume resistance and the specific mass resistance of mercury 

 with the pressure. 



