22 NEW METHOD FOR DETERMINING COMPRESSIBILITY 



exceedingly sensitive electrical contact, can be estimated with great 

 accuracy. For the pressures, on the other hand, we have had to 

 depend chiefly upon the hydraulic dial gauge made and guaranteed by 

 Schaeffer and Budenberg. In view of the extensive experience of 

 these manufacturers and the fact that the gauge is vouched for by the 

 Societe Genevoise it seemed hardly possible that we could improve 

 upon the accuracy of their work. The gauge registers as far as a 

 thousand atmospheres, and has only a very small temperature coeffi- 

 cient, according to their testimony and our careful trial. We tested 

 this by means of our new liquid manometer, described on page 41, 

 keeping the latter constant in temperature and varying the temperature 

 of the dial gauge. The temperature of the room used for the experi- 

 ments varied ordinarily but little from 20 , hence no trouble could 

 have arisen from change of temperature, even had the coefficient been 

 considerable. 



Several other indications point to the accuracy of the gauge. For 

 example, the great regularity to be seen in the various curves, particu- 

 larly in that representing /5' /$", points toward consistency in the 

 indications. 



A number of comparisons of the gauge with known weights placed 

 at the end of the lever arm of the press confirmed to some degree this 

 conclusion. In order wholly to eliminate friction, and thus test this 

 method more thoroughly, a rotary piston might have been provided. 

 This change would however have involved a fundamental dismantling 

 of the apparatus, an act which at present we were not willing to per- 

 form. Yet another indication of the accuracy of the gauge is found in 

 the essential agreement of our work upon water, described later, with 

 the work of others. 



After the work was completed, our gauge was returned to Schaeffer 

 and Budenberg, in order to be thoroughly tested anew. Their report 

 was very satisfactory, indicating inessential errors in the lower part of 

 the scale, and giving the error at 500 atmospheres as only 0.5 atmo- 

 sphere. 



Whatever may have been the error of the gauge, the results are ac- 

 curate relatively to one another ; moreover, they may be easily cor- 

 rected at any future time with the help of the liquid manometer 

 already mentioned. 



In view of the facts above stated, it seems certain that the possible 

 inaccuracies of the gauge must be so small as to affect only the second 

 differential coefficient, and not the average value of the compressibility. 

 Hence even if the possible errors in the gauge were never found, the 

 following results would be significant. 



