632 BRIDGMAN. 



Since ordinarily the line of constant volume has a greater slope than 

 that of constant resistance, it will result that at higher temperatures 

 the two lines approach each other. If this tendency persists, it means 

 that at high enough temperatures the resistance of a solid will decrease 

 instead of increase along a line of constant volume with increasing 

 temperature. This is an important point for theoretical considera- 

 tions. 



To a closer degree of approximation the pressure coefficient is not 

 independent of temperature. The manner of dependence is plainly 

 obvious from Figure 25; the coefficient may rise or fall with tempera- 

 ture. Furthermore, the relation between coefficient and temperature 

 need not be linear; there are six examples of non-linear relation, Tl, 

 Cd, Zn, Al, Ag, and Cu. The departure from linearity is so slight that 

 it is not obvious on the scale of Figure 25. As a general rule, the 

 coefficient increases with temperature for metals of low melting point, 

 and decreases for those with higher melting points, although there are 

 several exceptions. Except for this, there seems no obvious connection 

 between the manner of departure from constancy and other physical 

 properties. 



At any constant temperature the relation between pressure and 

 resistance is not linear, but the slope of the resistance-pressure curve 

 becomes less at the higher pressures. This is true without exception 

 for all the metals with a negative pressure coefficient, and is only what 

 one would expect. The manner of departure from linearity varies from 

 metal to metal, however. The variation is not regular, so the simple 

 types of formula hitherto proposed to represent the dependence of 

 resistance on pressure cannot be valid. There is a general tendency, 

 however, for the maximum departure from linearity to be greater for 

 those metals with the larger coefficient, as one would expect. Fur- 

 thermore, the ratio of the maximum departure from linearity to the 

 pressure coefficient is also greater for the greater coefficients. This 

 means that as the effect of pressure on resistance increases from metal 

 to metal, the relative curvature of the resistance-pressure curves in- 

 creases also. This ratio is roughly proportional to the value of the 

 pressure coefficient, as is shown by Figure 26, but there are several 

 well marked exceptions, particularly at the smaller coefficients. 



For any one substance, the variation with temperature of the ratio 

 to the pressure coefficient of the maximum departure from linearity 

 with pressure is of interest. This has already been mentioned under 

 the individual substances: a curve with greater curvature at the higher 

 temperature means a larger value for the ratio. The facts are a trifle 



