PRESSURE ON RESISTANCE OF METALS. 



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surprising here; for Pb, Tl, In, and Sn, the substances with the largest 

 average pressure coefficient, the curvature is greatest at the highest 

 temperature, but for all other normal substauces it is less. For some 

 substances, such as cobalt, the magnitude of the variation is surpris- 

 ingly large; half the relative curvature disappears between 0° and 100°. 

 This effect, although small, cannot be explained by errors in the pres- 



FiGURE 26. The ratio, at 0°, of the maximum departure from linearity to 

 the change of resistance under 12000 kg. plotted against the change of resist- 

 ance under 12000 kg. The diagram shows that in general the relative curva- 

 ture of the resistance-pressure curves increases as the absolute value of the 

 pressure coefficient increases. 



sure measurements, because it will be remembered that the manganin 

 measuring coil was always at the same temperature. 



The position of the pressure of maximum deviation from linearity 

 is another characteristic datum ; this has already been tabulated under 

 the individual substances. This pressure shows a distinct tendency 

 to fall below the mean pressure, that is, below, 6000 kg. This is as 

 one would axpect if the instantaneous pressure coefficient tends toward 

 constancy, that is, if the equation of resistance against pressure can 

 be written R=Roe~"'P. As a matter of fact, if the equation could be 



