400 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



which are metallic in a crystalline state and which are nonmetallic 

 when precipitated from solution. 



The specific conductance of the metals of this class is often relatively 

 low. In the following table are given values of the specific conductance 

 of a few of these metals. 



TABLE CLXI. 

 SPECIFIC CONDUCTANCE OF VARIOUS SUBSTANCES AT 0. 



Conductor Specific Conductance 



Graphite (Siberia) ................... 8.71 X 10 2 



Silicon (+ 3.3% impur.) ........ . ...... 10.0 



Titanium ............................ 2.8 X 10 3 



Zirconium ........................... 5 X 10 3 



CuS . ............................... 8.5 X10 3 



Pb0 2 ............................... 4.3 X 10 3 



CdO ................................ 8.3 X 10 2 



PbS ................................ 4.2 X 10 2 



Fe 3 4 ............................... 1.16 X10 2 



FeS 2 (Pyrite) ....................... 0.42 X 10 2 



FeS 2 (Marcasite) ... ................. 0.06 



The resistance of metals of this class at lower temperatures decreases 

 greatly with increasing temperature, approximately as an exponential 

 function. At higher temperatures, the conductance reaches a minimum 

 value, after which it increases approximately as a linear function of the 

 temperature. A familiar example of this type of substance is carbon. 

 It is uncertain, however, that the observed conductance curves of this 

 type actually relate to pure substances. Kammerlingh Onnes and Hof 17 

 have shown, for example, that graphite may be purified to a point where 

 its resistance decreases with temperature down to approximately 173 

 with a coefficient of 0.0029. At lower temperatures the resistance de- 

 creases somewhat more rapidly. Similar results have been obtained in 

 the case of bismuth. In the earlier experiments of Dewar and Fleming/ 8 

 bismuth was found to exhibit a minimum resistance at temperatures 

 varying from room temperatures to 80 C., depending upon the purity 

 of the sample. Later, however, this element was purified to a point where 

 its resistance decreased throughout with decreasing temperature down to 

 liquid hydrogen temperatures. 19 Since many of the substances of this 

 class cannot be prepared readily in a pure state, it follows that the pecu- 



17 K. Onnes and Hof, KoninkUjke Akad, van Wetensch. Amsterdam 11, 520 (1914). 

 "Dewar and Fleming, Phil. Mag. W, 303 (1895). 



18 J. Clay, Dissertation, Leiden (1908) ; Jahrb. f. Rad. 8, 391 (1911). 



