152 BRIDGMAN. 



since it was the only one he had left, I was glad to have the chance to 

 make the measurements. I have now obtained indirectly, through 

 the kindness of Professor C. C. Bidwell of Cornell University, one of 

 the purest of Kalmus's specimens. Since the preparation of cobalt in 

 the form of wire is a matter of some difficulty, involving the swaging 

 instead of drawing of the metal in the early stages, and since there 

 seems to be no commercial use for pure cobalt, it is not likely that 

 material other than that of Kalmus will be available for some time. 



The analysis of this sample was: Co 99.73, Fe 0.14, Ni 0.00, S 0.019, 

 Si 0.02, and C 0.09. 



The total impurity is seen to be 0.27%, against 1.30% of the former 

 sample. 



The diameter of this piece was 0.10 cm. and the length about 6 cm. 

 Its compressibility had been previously measured; for this purpose 

 it had been annealed and straightened by rolling between red hot iron 

 plates. For these measurements of resistance, current and potential 

 terminals were soft soldered near the ends, and measurements made 

 with the potentiometer. The technique was exactly like that used 

 with those metals formerly measured with the potentiometer, and has 

 been fully described in the previous paper. Because of the low re- 

 sistance of this sample and the smallness of the pressure coefficient, 

 the individual readings did not have as great regularity as those on 

 the previous sample, and I did not think it worth while to try for the 

 temperature coefficient of the pressure coefficient by making readings 

 at different temperatures. 



Measurements were made at 30° over the pressure range of 12000 

 kg/cm 2 . The accuracy was not high enough to detect departure from 

 linearity with pressure. Discarding one point, the average deviation 

 of the observed points from a straight line was 2.0% of the maximum 

 pressure effect. The average pressure coefficient over the range of 

 12000 kg. was -0.0 6 934, against -0.0 6 S65 of the former sample. The 

 purer sample has the numerically larger coefficient. This seems to be 

 true in the majority of cases, although there is no general rule here as 

 in the case of the temperature coefficient. 



The resistance was measured at 30° and 75° at atmospheric pressure. 

 Assuming the relation between temperature and resistance to be 

 linear, as it was for the former sample, the temperature coefficient 

 between 0° and 100° is 0.00439, against 0.00365 for the other sample. 

 The highest value which I find listed for cobalt is 0.0033. 2 The higher 

 coefficient of the new sample is what would be expected because of its 

 greater purity. 



