404 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



very great. Thus the value of - for graphite, silicon and hematite 



(Fe 2 3 ) is 2.5 X 10 12 , 6.8 X 10 14 and 7.3 X 10 14 respectively. For ordi- 

 nary metals the value is in the neighborhood of 6.7 X 10 10 at room tem- 

 peratures. 29 In this connection it is interesting to note that the thermal 

 conductance of some nonmetallic crystals is greater than that of many 

 metallic substances. Thus, the thermal conductance of rock salt is 

 0.0137 and that of quartz -L to its axis is 0.0263, while that of bismuth is 

 0.0194. While thermal and electrical conductance are intimately related, 

 the fact that some nonmetals are likewise excellent thermal conductors 

 should not be lost sight of. 



9. Thermoelectric Phenomena in Metals. We have to consider three 

 related thermoelectric phenomena, namely: 1, the electromotive force 

 arising in a metallic system as a result of a temperature difference be- 

 tween the junctions of two metals; 2, the Peltier effect which is a heat 

 transfer taking place when a current passes through a junction between 

 two different metallic conductors; and, 3, the Thomson effect which is a 

 heat transfer accompanying the passage of the current through a con- 

 ductor in which a temperature gradient exists. From a practical point 

 of view, the first of these effects is the most important and has been 

 investigated most extensively. 



The thermoelectric force of a thermocouple may be expressed very 

 nearly as a function of the temperature by means of an equation of the 

 form: 



Usually a quadratic equation suffices. For smaller temperature differ- 

 ences, the sign of the constant a corresponds with the direction of the 

 thermoelectric force. The sign of this electromotive force depends upon 

 the nature of the metals. Let us call the effect positive for the two metals 

 AB, when the current flows from A to B at the cold junction. The metal 

 A will then be said to be positive with respect to B. The values of the 

 coefficients a and |3 for different metals with respect to lead, the cold 

 junction being kept at a temperature of C., are given in Table CLXII. 

 As may be seen from the table, metals which are closely related often 

 have thermoelectric constants which are opposite in sign; thus, lithium 

 and potassium stand in reverse order to lead, which, in the table, is taken 

 as a standard. So, also, the closely related elements, antimony and 

 bismuth, which exhibit a relatively high thermoelectric power, lie near 



n See Koenigsberger, IQC. eft., p. 720-, 



