PRESSURE ON RESISTANCE OF METALS. 629 



the details of measurement ; in view of his remark it is possible that he 

 extrapohited as did Jaej^er and Diesseliiorst. On referring to v. 

 Aubel it is found that he does indeed emphasize the constancy of the 

 temperature coefficient, but that his numerical values show a con- 

 sistent increase from 0.00412 between 0° and 19.5° to 0.00450 between 

 0° and 99.7°. It would seem, therefore, that the temperature coeffi- 

 cient of perfectly pure bismuth has not yet been definitely established, 

 but that there is no reason to suspect that the electrolytic bismuth 

 measured above contains enough impurity to sensibly affect the result 

 under pressure. 



For the pressure coefficient there is only one other determination, 

 by Williams, ^^ over a pressure range of 300 kg. at 0°. Within the 

 limits of error he found the relation between pressure and resistance 

 to be linear, and the coefficient to be +0.0.il91. He does not give the 

 temperature coefficient of his specimen, but states that it was a spiral 

 of electrolytic bismuth from Hartmann and Braun. The initial value 

 which I found above was O.O4I58. It is significant that the impurer 

 grades gave a higher initial coefficient; that of the Kahlbaum "K" 

 specimen was O.O42I. 



The distinctive features of the behavior of bismuth are as follows; 

 the average pressure coefficient is positive, increasing in numerical 

 value with increasing pressure and decreasing with increasing tempera- 

 ture, and the instantaneous coefficient at kg. is nearly independent 

 of the temperature, but at higher pressures it decreases at the higher 

 temperatures. This last point means that as pressure increases the 

 temperature coefficient of resistance decreases. 



General Survey of Results. 



In Figure 25 are collected curves for all the metals measured, except 

 Te, Bi, Sb, and Mg, giving the average pressure coefficient to 12000 kg. 

 as a function of temperature. The most obvious and striking feature 

 is the slight variation of coefficient with temperature; the variation 

 is in all cases much less than the change of resistance itself. To see 

 the significance of this, let us for the moment suppose that the coeffi- 

 cient is strictly constant with temperature. If this is true, the curve 

 of resistance against pressure at any temperature may be obtained 

 from that at any other temperature merely by changing the scale of 

 the ordinates by the proper factor. It would therefore follow that the 

 temperature coefficient of resistance would be strictly independent of 



