402 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



compounds. A more detailed discussion, however, is not possible in this 

 monograph. 20 



7. The Conductance of Metals as Affected by Other Factors. 

 a. Anisotropic Metallic Conductors. As might be expected, the con- 

 ductance of many crystalline substances depends upon the orientation of 

 the crystal. Thus, the conductance of a crystal of bismuth at right 

 angles to its base at 15 is 1.78 times that parallel to its base. 21 It has 

 been shown that the conductance of a bismuth crystal may be represented 



. by means of an ellipsoid of rotation. 22 



b. Influence of Mechanical and Thermal Treatment. The conduct- 

 ance of metals is dependent upon their previous mechanical and thermal 

 treatment. Wires which are hard drawn in general exhibit a lower con- 

 ductance than do annealed wires. The thermal treatment of metals has 

 an influence on their conductance, not only in that it tends to relieve 

 mechanical stresses resulting from previous mechanical treatment, but 

 also in that it tends to induce various transformations in the body of the 

 metal, some of which are reversible and others of which are irreversible. 



c. The Influence of Pressure on Conductance. The resistance of 

 most metallic elements is decreased under the action of uniform pressure. 



The coefficient -~ -3- for solid metallic elements varies between 15.1 XlO' 7 

 R dp 



for nickel and 152 X 10~ 7 for lead. For bismuth the value of the 

 coefficient is positive and equal to + 196 X 10~ 7 . The resistance does 

 not vary as a linear function of the pressure, the pressure coefficient de- 

 creasing with increasing pressure. The resistance of manganin wire 

 varies very nearly as a linear function of the pressure. The only pure 

 liquid metal for which data are available is mercury. At 25, the value 

 of its resistance-pressure coefficient is 334 X 10~ 7 . It would be inter- 

 esting to know whether other liquid metals exhibit a similarly high value 

 of this coefficient. 



The influence of pressure on the resistance of variable conductors is 

 often extremely marked. 23 



d. Photo-electric Properties. A few substances are sensitive to the 

 action of light. Selenium is the most remarkable example of this type 

 of substances. The influence of light and various other factors on the 

 conductance of selenium has occupied the attention of a great many inves- 

 tigators. A detailed discussion cannot be given here. 24 



8. Relation between Thermal and Electrical Conductance in Metals. 



80 A very complete summary is given by Koenigsberger, loc. cit., pp. 661-680. 

 "Lownds, Ann. d. Phys. 9, 681 (1902). 



22 van Everdingen, Versl. Akad. van Wetensch. Amsterdam 3, 316 and 407 (1900), 



23 For references, see Koenigsberger, loc. cit., pp. 694-7. 

 * For references, see Koenigsberger, Iqc. cit., pp. 681-694, 



