COMPRESSIBILITY OF METALS. 189 



AT' 



At 75° — = - 10- 7 (5.47 - 2.1 X I0~ 5 p) p. 



I 



As far as I can find, the compressibility of cobalt has not been pre- 

 viously measured. The value found above, close to that for nickel 

 and iron, is of the order of magnitude that one would expect. 



Nichrome. The reason for determining the compressibility of this 

 alloy is that the difference of compressibility between it and iron enters 

 as a correction on the observed readings. The correction is very 

 small, and at the maximum amounts to only 0.3 mm. on the slide wire 

 of the potentiometer. The material was obtained from Driver Harris 

 Company, and is of the grade known as Nichrome II. Its composi- 

 tion is Ni 80% and Cr 20%. The sample measured was from the 

 same length as the wires attached to the lever or the specimen, by 

 which the relative changes of length were measured. Its diameter 

 was 0.030 cm. and length about 10 cm. It was mounted as a tension 

 specimen in the lever apparatus for long specimens. 



Because of the smallness of the correction it was necessary to meas- 

 ure the compressibility only at 30°. Here the mean departure from 

 a smooth curve (no discards) was 1.5% of the maximum pressure 

 effect, which corresponds to 0.09% on the actual compressibility. The 

 deviation from linearity was sensibly not the same as for iron, and at 

 the maximum was 5.5% of the maximum pressure effect. The 

 results are contained in the formulas : 



AF 

 At 30° — = - 10- 7 (5.50 - 1.5 X I0~ 5 p) p. 

 I o 



Gold. Measurements were made on two samples, both of which I 

 owe to the kindness of Baker and Company. The specimens were 

 stated to be of the highest purity, but I have no analysis. The den- 

 sity was found to be 19.272 at room temperature. Both specimens 

 were in the form of drawn rod 0.75 cm. in diameter, and were annealed 

 at a bright red before the measurements. The first was 2.5 cm. long, 

 and was mounted as a compression specimen in the lever apparatus 

 for short specimens; the second was 12 cm. long, and was mounted 

 as a compression specimen in the lever apparatus for long specimens. 



The compressibility of gold is exceedingly close to that of iron, and 

 hence accurate results are not to be expected for the difference of com- 

 pressibility. Apart, however, from the smallness of the effect, the 

 apparatus did not function as well as usual during these measurements, 

 there being minor electrical troubles, probably due to short circuits, 



