CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL 

 LABORATORY, HARVARD UNIVERSITY. 



AN EXPERIMENTAL DETERMINATION OF CERTAIN 



COMPRESSIBILITIES. 



By P. W. Bridgman. 

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



In a preceding paper the change of resistance produced by hydro- 

 static pressure on a fine thread of mercury in a capillary of a specified 

 glass was measured. This change of resistance is the sum of two 

 effects: the change of resistance produced by the changed dimen- 

 sions of the glass capillary, and the change of resistance due to the 

 changed electrical properties of the mercury under pressure. The 

 change of resistance produced by the distortion of the glass is simul- 

 taneously an increase of resistance because of the decreased bore of 

 the capillary, and a decrease because of the decreased length. The 

 total fractional change of resistance is easily seen to be the linear com- 

 pressibility of the glass. The change of resistance due to the changed 

 electrical properties of the mercury may be further divided into two 

 effects: that due to the change in the conducting power of the sepa- 

 rate molecules, and that due to the change in the number of molecules 

 occupying a given space. This latter effect is determined directly by 

 the compressibility of the mercury. 



A complete description of the phenomena involved in the measured 

 change of resistance of the mercury involves, therefore, a knowledge 

 of the compressibility of both the glass and the mercury. This paper 

 is occupied with a description of the methods by which these were de- 

 termined. As the pressure range employed here (6500 kgm.) is some- 

 what higher than that usually used, modifications of the methods in 

 common use were necessary. It seemed undesirable, however, to 

 bury a description of these methods in a paper on the unrelated sub- 

 ject of the electrical resistance of mercury, and the matter has there- 

 fore been made the subject of a separate paper, although the method 

 has been applied to only a few substances, and all the data have been 

 collected solely with a view to the above discussion of the effect of 

 pressure on the resistance of mercury. However, the paper contains 



