260 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



any possible distortion of the comparator by the lengthening of the 

 cylinder under pressure. The close contact of cylinder and comparator 

 insured the practical equality of temperature of the two, and the co- 

 efficients of expansion of the two pieces proved so close that the few 

 tenths of a degree variation which occurred in the temperature of the 

 room introduced no appreciable error. The microscopes were focussed 

 on fine fortuitous scratches on the cylinder at the points H and I 

 (Figure 1). Change of length was measured by a micrometer eyepiece 

 in either microscope, which had been previously calibrated. Settings on 

 the fine scratches could be made with a maximum error of 0.0003 mm., 



1000 



2000 



3000 



4000 



5000 



6000 



7000 



Figure 3. The elongation of the cylinder of figure 1, as a function of the 

 pressure. Q, observations at increasing pressures ; CD. at decreasing pressures. 

 The ordinates give the proportional elongation multiplied by 10 6 . That is 

 at a pressure of 6400 kgm. per sq. cm. the elongation of the cylinder is 0.000056 

 per unit length. 



thus introducing a possible error of reading of the change of length of 

 0.0006 mm. The total change of length was found to be 0.02 mm. 

 at 6000 kgm. The maximum error here possible on the extension 

 coefficient of the cylinder is, therefore, 6 parts in 200. The mean of 

 several readings, of course, has a much less probable error. 



The results obtained are shown in Figure 3, in which extension of 

 the cylinder is plotted against pressure. The pressure was measured 

 here, as in all subsequent work in this paper, by a secondary gauge 

 depending on the variation of the resistance of mercury under pressure. 

 The justification and calibration of this gauge has been made the sub- 

 ject of another paper. The figure shows distinct evidence of hysteresis, 

 the extension under decreasing pressure being greater than the corre- 

 sponding extension under increasing pressure. This is the more sur- 

 prising as the total extension of the cylinder is only ^ of the value of the 



