330 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 lov er 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 mi.d- 

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

 the siime order as the coefficients of expansion of gases. For temperature resistance (i, 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, ^k <4Xio-"' p^ and for Sn., p, ^j- <icr-'p 

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

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

 then possessing nearly linear t, p graphs (Al., Cu., Sn., Au., Ag., Pb.) ; tiiose where p suddenly increases and then the 

 + temp, coefficient is only approximately constant ; (Hg., Na., K.); those about doubling p then having a -, slowly 

 changing to a + temj). 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 ol metals. 'I'lie 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. 40?, iqii for temperature resistance thermometry). The Shimank and Pirrani values are originally 

 given as ratios to py. (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 oO, Niccolai, Lincei Rend. (5), 16, p. 757,906, 1907; above, Northrup, Jour. Franklin Inst. 177, p. 85, 1914. 

 Cu. below, Niccolai, 1. c. above, Northrup, ditto, 177. p. i, i9i4^ 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, i<)n. Hg. below 

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

 p. 1474, iqo8, 148, p. 204, 1909. Above, Northrup, Tr. Am. Electroch. Soc. p. 185, 191 1. Na, below, means, aboveJ 

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

 Mag. 36, p 271, 1893. ■■ 



Smithsonian Tables. 



