COMPRESSIBILITY OF METALS. 



177 



JV 



1 



For specimens whose compressibility was fairly high compared with 

 that of iron the complications of the lever were dispensed with, and 

 the relative change of length was measured directly by a wire attached 

 directly to the end of the specimen. This is shown in Figure 3. The 

 specimen is compressed against the lower end of the iron container 

 by a spring M and also is pressed against the sliding contact D by 

 a spring N. 



The final measurements of compressibility were made with one or 

 the other of three different pieces of apparatus; two of 

 these magnified the motion with levers, one having a 

 long envelope for long specimens (maximum capacity 

 16.5 cm.), and the other a short envelope for pieces 

 of a maximum length of 2.5 cm. The third apparatus 

 was for the direct measurement of the relative change 

 of length, as described in the preceding paragraph. 



Each form of apparatus was checked by making with 

 it blank runs, using as the specimen a piece of the same 

 iron as that whose absolute linear compressibility was 

 measured with the other apparatus. If the compressi- 

 bility of the envelope (which was made of commercial 

 bessemer steel) was the same as that of pure iron, and 

 if it were perfectly homogeneous, so that it experienced 

 a volume compression without change of figure, then 

 the changes of resistance of the wire under pressure 

 should be such that when the various corrections were 

 applied for the change in specific resistance of the wire 

 under pressure, etc., there should be indicated no out- 

 standing relative motion of specimen and surrounding 

 envelope. Of course for one thing the compressibility 

 of the pure iron and the bessemer steel was probably 

 not exactly the same, so that perfect agreement was not 

 to be expected, but nevertheless the correction so deter- 

 mined due to all these possible sources was very small 

 indeed, and was about that due to the relative com- 

 pression of the iron and the mica washers (which were 

 0.012 cm. thick) by which the fixed contact D was in- 

 sulated. It is to be noticed that after this correction 

 was applied the results gave accurately the difference fig. 3. 



Figure 3. Device with sliding electrical contact for the direct measure- 

 ment of the difference of linear compressibility between iron and the specimen S. 



£ 



PU=4 



