CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL 

 LABORATORY, HARVARD UNIVERSITY. 



THE MEASUREMENT OF HIGH HYDROSTATIC 



PRESSURE. 



II. A SECONDARY MERCURY RESISTANCE GAUGE. 



By P. W. Bridgman. 



Presented by W. C. Sabine, December 9, 1908. Received December 16, 1908. 



In the introduction to the first part of this paper it was stated that 

 the end sought in designing the primary gauge was the calibration by 

 means of it of some secondary gauge which should be easily repro- 

 ducible. The secondary gauge that it was proposed to adopt is one 

 involving the variation of mercury resistance with pressure. This is 

 of an entirely different character from the type of secondary gauge 

 in common use, which is usually some form of metallic deformation 

 gauge like that of Bourdon. Undoubtedly the Bourdon is one of the 

 most convenient forms of secondary gauge that it would be possible 

 to devise, being almost immediate in its action, and capable of stand- 

 ing considerable rough handling. If it were applicable over the wide 

 pressure range contemplated for the mercury gauge, its greater con- 

 venience would certainly overbalance the fact that every such Bourdon 

 gauffe must be initially calibrated against some direct standard. 



It seems to be a fact, however, that any elastic deformation gauge 

 becomes unsuitable at high pressures, even when once calibrated, 

 because of the entrance of hysteresis effects. It is true that the ex- 

 istence of elastic hysteresis effects has frequently been doubted, and 

 it has even been stated that proof of their existence would give us 

 knowledge of a new elastic property. It nevertheless seems to be a 

 fact that hysteresis may be inappreciable at low values of the stress, 

 but become increasingly important at higher pressures. This is not 

 the place, however, to enter into a discussion of this point, which will 

 afford the subject for another paper. But in this paper there will be 

 given a somewhat detailed examination of the behavior under pressure 

 of one Bourdon gauge, which will at least show that this type of gauge 

 is irregular at high pressures, whatever the true explanation of this 



