188 BRIDGMAN. 



Pure drawn wire, At 30° — = - 10" 7 (5.29 - 2.1 X lO^p) p 



AV 

 At 75° — = - 10- 7 (5.35 - 2.1 X KHp) p. 



I 



A rough value for the average compressibility to 12000 kg. of the 

 forging of pure nickel was 5.16 X 10 -7 . 



The values above for the two samples are seen to be nearly the same, 

 and it is therefore probable that the usual impurities, such as cobalt 

 and iron, have little effect on the compressibility. 



The initial compressibility has been found by Richards to be 4.3 X 

 10~ 7 . His material was in the form of cubes, in which the pure metal 

 is often supplied by chemical houses. Richards states that the nickel 

 as originally provided was full of flaws into which the mercury was 

 forced by pressure, and that the effect of these flaws was got rid of by 

 heavy forging. My experience with the cast specimen above would 

 strongly indicate (because of the hysteresis shown by this sample) 

 that the forging must have introduced considerable internal strain, 

 which is doubtless responsible for Richards' low value. The effects 

 of hysteresis would be especially pronounced over a small pressure 

 range, and would be in the direction to account for Richards' low value. 



Cobalt. This I owe to the kindness of Professor C. C. Bidwell of 

 Cornell University, by whom it was in turn obtained from Dr. Herbert 

 T. Kalmus, who had prepared it for the Canadian Government. It 

 was of high purity, and had the following analysis: Fe 0.14, Ni 0.00, 

 S 0.019, Si 0.02, C 0.09, Co 99.73. It was in the form of wire about 

 0.075 cm. in diameter and 6 cm. long, and was mounted as a tension 

 specimen in the lever apparatus for long specimens. The material 

 had been formed into wire by swaging followed by drawing at a red 

 heat. It was not perfectly straight as supplied. It is rather brittle 

 to bending, but I was able to make it perfectly straight by rolling 

 it between red hot iron plates, thus at the same time annealing it. 



The two visual runs at 30° and 75° were made. The results were 

 gratifyingly regular, considering the smallness of the effect. The 

 average arithmetical departure from a straight line of the 28 readings 

 (no discards) was 1.3% of the maximum pressure effect; 1.3% on the 

 difference of compressibility means 0.1% on the actual compressibility. 

 It was not possible to detect any departure from linearity. 



The final results are expressed by the formulas : 



AV 



At 30° — - = - 10- 7 (5.39 - 2.1 X I0~ 5 p) p 

 y o 



