COMPRESSIBILITY OF METALS. 



175 



compressibility with pressure is thus seen to be about 8.5%. One 

 would not suspect so large a change from the slight apparent departure 

 from linearity of the curve plotting change of length against pressure. 



Compressibility of Metals Other than Iron. 



method. 



The general idea of the method is the same as that used for the 

 absolute linear compressibility of iron, namely a wire is attached to 

 the specimen, which slides over a contact fixed to a 

 comparison piece, and the relative motion of the wire 

 is obtained from a measurement of the potential dif- 

 ference between the sliding contact and the contact 

 fixed to the wire. In this case, however, the com- 

 parison piece is not the external cylinder itself, but 

 is another piece of iron within the cylinder which is 

 exposed to hydrostatic pressure all over, and which 

 therefore experiences only a uniform compression. 

 The amount of this compression is determined by 

 the measurements on iron just described. 



Several forms of apparatus were used, depending 

 on the numerical value of compressibility and the 

 shape in which the specimens could be obtained. 

 For those substances whose compressibility is near to 

 that of iron, an apparatus with a lever was used, by 

 which the relative motion of the specimen and the 

 surrounding iron was magnified about seven times. 

 A sketch of the essential parts is shown in Figure 2. 

 The specimen shown at S was, when convenient, 

 made in the form of a turned bar 6 mm. in diameter. 

 Over each end was slipped a well fitting steel cap, 

 provided with steel points. At the lower end one 

 of these points rested against the containing envelope 

 of iron, and at the upper end the other point bore 

 against the short arm of the lever. This lever had 

 three point support, one at A, and two, one behind 

 the other, at B. The lever was kept tightly pressed 

 against the specimen by a spring of flat steel G, 



s 



i 



J-U-l 



i 



Figure 2. Device with lever magnification and sliding electrical contact 

 for measuring the difference between the linear compressibility of iron and the 

 specimen S. 



