BRIDGMAN. — A DETERMINATION OF COMPRESSIBILITIES. 



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extension at the elastic limit under pure tension. The departure of 

 the points from a straight line representing the mean is comparatively 

 slight, however, and in applying the corrections determined in this way 

 the relation between extension and pressure was assumed to be linear. 



With this apparatus the linear compressibility of a piece of com- 

 mercial aluminum rod and several specimens of iron and steel were 

 made. In Figure 4 is shown 

 the fractional change of s 5 J_ 

 length of the aluminum rod 

 corrected for the extension 

 of the steel cylinder, plotted 

 against pressure. This fig- 

 ure does not include the 

 first observation which was 

 made with a pressure slightly 

 higher than any subsequently 

 reached. The rod took a 

 distinct set on this first ap- 

 plication, being permanently 

 shortened by one part in 

 30,000. No evidence of 

 further set was found on 

 subsequent applications of 

 pressure. This is the first 

 occasion on which a set 

 in any dimension by the 

 application of hydrostatic 

 pressure has been directly 

 observed. No attempt was 

 made to find whether this 

 linear set is accompanied by 

 volume set. The displace- 

 ment was measured from the 

 mean of several determina- 

 tions of the position of the ring at zero pressure. But this determina- 

 tion is obviously affected by the same errors as displacement measure- 

 ments at higher pressures. It is evident from the figure that within 

 the limits of error the points lie on a straight line. This was assumed 

 to be of the form a + bp, and a and b determined by least squares, 

 discarding the most discordant results, a is the true zero position and 

 b the pressure coefficient of contraction. In this way every measure- 

 ment at any pressure contributes to the more accurate determining of 



.0005 



1000 2000 3000 4000 5000 6000 7000 



Figure 4. The observed proportional 

 change of length of an aluminum rod 

 plotted against pressure. 



