Table 491 

 RESISTIVITIES AT HIGH AND LOW TEMPERATURES* 



417 



The electrical resistivity (p, ohms per cm. cube) of good conductors depends greatly on chemical purity. Slight con- 

 tamination 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 mod- 

 ify p. For liquid metals this last cause of variability disappears. The + temperature coefficients of pure metals is of 

 the same order as the coefficients of expansion of gases. For temperature resistance (t, p) plot at low temperatures the 

 graph is convex towards the axis of t and probably approaches tangency to it. However for extremely low temper- 

 atures Onnes finds very sudden and great drops in p. e.g. for Mercury, p, d k <C4 xio ~ 10 P u an <l f° r Sn., p, „ K <io- 7 p _ 

 The t, p graph for an alloy may be nearly parallel to the t axis, cf. constantan ; for poor conductors p may decrease with 

 increasing t. At the melting-points there are three types of behavior of good conductors: those about doubling p and 

 then possessing 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.j. 

 The values from different authorities do not necessarily fit because of different samples of metals. The Shimank values 

 (t given to tenths of °) are for material of theoretical purity and are determined by the a rule (see his paper, also Nernst, 

 Ann. d Phys. 36, p. 403, 191 1 for temperature resistance thermometry). The Shimank and Pirrani values are originallv 

 given as ratios to p n . (Ann. d. Phys. 45, p. 706, 1014, 46, p. 176, 1915) Resistivities are in micro-ohms per cm. cube 

 unless stated. Italicized figures indicate liquid state. 



Au. below o°, Niccolai, Lincei Rend. (5), 16, p. 757,906, 1907; above, Northrup, Jour. Franklin Inst. 177, p. .85, 1914. 

 Cu. below, Niccolai, I. c. above, Northrup, ditto, 177. p. «, i9>4- Ag. below, Niccolai, I.e. above Northrup, ditto, 178, 

 p. 85, 1914. Zn. below, Dewar, Fleming, Phil. Mag. 36. p. 271, 1893 ; above, Northrup, 175, p. 153, 1913- Hg below 

 Dewar, Fleming, Proc. Roy. Soc. 66, p. 76, 1900 ; above, Northrup, see Cd. K. below Guntz, Broniewski, C. R. 147, 

 p. 1474, 1908, 148, p. 204, 1909. Above, Northrup, Tr. Am. Electroch. Soc. p. 185, 191 1. Na, below, means, above 

 see K. Fe., Manganin, Constantan. Niccolai, I.e. German Silver, 90% Pt. 90% Rh., Dewar and t leming — Phil. 

 Mag. 36, p. 271, 1893. * See also page 413. 



Smithsonian Tables. 



