BRIDGMAN. — A SECONDARY MERCURY RESISTANCE GAUGE. 241 



nearly the same values as may be deduced from the formula for p 

 in terms of p. 



The above formula holds only when the mercury resistance is en- 

 closed in a glass capillary of Jena glass No. 3880 a. If a different 

 glass is used, it will be possible to use the formula by introducing a 

 correction factor. This factor for one other glass, hard Jena com- 

 bustion tubing No. 3883, was determined by comparing two mercury 

 resistances. The comparison was made not so much with the idea 

 that this hard glass would prove more convenient for practical use, 

 but rather in the hope that these two different kinds of glass, one very 

 infusible and the other very fusible, would show a comparatively 

 large difference of compressibility. Table V shows the ratio of the 



TABLE V. 

 Effect of Different Glass Envelopes. 



observed changes of resistance in the hard and soft envelopes, at differ- 

 ent pressure. The mean of the ratios, weighted according to the 

 magnitude of the effect measured, is 1.0266, while the ratio of the 

 initial resistances is 1.0253. The difference between these two 

 numbers is presumably due to the difference of compressibility of the 

 envelopes, which turns out not to be as large as was expected from 

 the character of the glass. The fact that the ratio of the change of 

 resistances is greater than the ratio of the total resistances shows that 

 vol. xliv. — 16 



