TABLE 398.— RESISTIVITIES AT HIGH AND LOW TEMPERATURES 



393 



The electrical resistivity (p, ohm-cm) of good conductors depends greatly on chemical purity, 

 Slight contamination even with metals of lower p may greatly increase p. Solid solutions of good 

 conductors generally have higher p than components. Reverse is true of bad conductors. In solid 

 state allotropic and crystalline forms greatly modify p. For liquid metals this last cause of varia- 

 bility disappears. The -f temperature coefficients of pure metals is of the same order as the coef- 

 ficients of expansion of gases. For temperature resistance (t, p) plot at low temperatures the 

 graph is convex toward the axis of t and probably approaches tangency to it. However for 

 extremely low temperatures Onnes finds very sudden and great drops in p, e.g., for mercury, 

 p 3 .«K <4 X 10" Io p o and for Sn, Pz.sk <10" 7 p<>. The /, p graph for an alloy may be nearly parallel to 

 the t axis, cf. constantan ; for poor conductors p may decrease with increasing /. At the melting- 

 points there are three types of behavior of good conductors ; those about doubling p and then pos- 

 sessing nearly linear t, p graphs ( Al, Cu, Sn, Au, Ag, Pb) ; those where p suddenly increases and 

 then the -f temp, coefficient is only approximately constant (Hg, Na, K) ; those about doubling p 

 then having a — , slowly changing to a + temp. coef. (Zn, Cd) ; those where p suddenly decreases 

 and thereafter steadily increases (Sb, Bi). The values from different authorities do not neces- 

 sarily fit because of different samples of metals. Resistivities are in microhm-cm unless other- 

 wise stated. Italicized figures indicate liquid state. 



SMITHSONIAN PHYSICAL TABLES 



