33 TABLE 403. 



RESISTIVITIES AT HIGH AND LOW TEMPERATURES. 



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 (Junes finds very sudden and great drops in p. e.g. for Mercury, p, 5^ <C4 XIO ~ 10 P and for Sn., p, K <io-'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 -f- temp. coef. (Zn., Cd.) ; those where p suddenly decreases and thereafter steadily increases (Sb., Bi.). 

 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 aie origi ally 

 given as ratios to p . (Ann. d. Phys. 45, p. 706, 1914, 46, p. 176, 1915.) Resistivities are in ohms per cm. cube unless stated. 

 Italicized figures indicate liquid state. 



Au. below o, Niccolai, Lincei RenH. (5), i6,p. 757,906, 1907; above, Northrnp, Jour. Franklin Inst. 177, p- 85, 1914. 

 Cu. below, Niccolai, 1. c. above, Northrup, ditto, 177. p. i, 1914. Ag. below, Niccolai, I.e. above Northrup, ditto, 178, 

 p. 85, 1914. Zn. below, Dewar, Fleming, Phil. Mag. 36, p. 271, 1803 ; above, Northrup, 175, p. 153, 1913. Hg. below 

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

 p. 1474, 1008, 148, p. 204, looo- Above, Northrnp, Tr. Am. Electroch. Soc. p. 185, 1911. Na, below, means, above, 

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

 Mag. 36, p. 271, 1893. 



SMITHSONIAN TABLES. 



