COMPRESSIBILITY OP METALS. 185 



Richards has found for the initial compressibility of molybdenum 

 in the form of fused buttons of density 10.21 the value 4.5 X 10~ 7 . 

 This again is considerably higher than the value found above, but the 

 discrepancy is not more than is to be expected when the small size of 

 Richards' sample is considered, and the fact that one of his two results 

 was 40% higher than the other. Richards' object in his measurement 

 was only to obtain the order of magnitude. 



Tantalum. This was in the form of a drawn wire 0.062 cm. in 

 diameter and 10 cm. long, mounted as a tension specimen in the lever 

 apparatus. I am indebted for this material to the kindness of the 

 Fansteel Co., of North Chicago. It was stated by them to be of 

 unusually high purity, but I have no analysis. As a partial means of 

 estimating its purity I determined its temperature coefficient of re- 

 sistance between 0° and 100°. The relation between temperature and 

 resistance is sensibly linear over this range, and the average coefficient 

 is 0.00335. This is materially higher than the coefficient of a piece 

 which I had formerly obtained from the General Electric Company, 

 namely 0.00293, and for which I have determined the effect of pressure 

 on electrical resistance. The coefficient is less than that of a sample 

 of Holborn, 8 0.00347. 



Two runs were made on this specimen, as usual, at 30° and 75°. 

 The results at 30° were appreciably more regular than at 75°. At 30° 

 the average arithmetical departure of the observed points from a 

 smooth curve (no discards) was 1.4% of the maximum pressure effect, 

 and at 75° it was 3.2%. The departure from linearity was sensibly 

 the same at the two temperatures, and at its maximum was 6.8% of 

 the pressure effect. It must be remembered that because of the small 

 difference of compressibility between tantalum and iron a large per- 

 centage error in the difference of the two compressibilities need not 

 mean a large error on the absolute compressibility. 



The final results are given by the formulas : 



AV 

 At 30° =f = - 10- 7 (4.79 - 0.25 X 10" 5 p) p 



I 



o 



AV 

 At 75° ^r = - 10- 7 (4.92 - 0.25 X 10~ 5 p) p. 



I 



The close approach of the compressibility to constancy with pressure 

 is to be noted. 



The compressibility of tantalum at 20° over a small pressure range 



