Magnetic and Electric Saturation Values. 355 



Electric Current Density. 



At ordinary temperatures and for pure metals in which 

 the resistivity is proportional to the absolute tempeiature 

 the law of conduction of electric currents is analogous to the 

 gas law. 



Let E = Fall of potential per cm. in c.g.s. units,, 

 i = Current in c.g.s. units per sq. cm., 

 T = Absolute temperature, 

 and S = a constant. 



Then ?=ST (6) 



If a is the resistivity in c.g.s. units, Ohm's law is 



E 



a 



CO 



and therefore a /£n 



o = fj KP) 



This quantity S has the same importance in electrical 

 theory as Curie's constant has in the theory of magnetism. 



If, however, i can reach a limiting value i then equa- 

 tion (6) must be written more generally as in magnetism, 

 thus 



e (K)= 8t (9) 



Putting {•=■- i the equation becomes 



5-(n-l)= ST, (10) 



h 



and multiplying throughout by ? 2 we have 



Ei (n-1) = ST* 2 (11) 



The left side of this equation when E is constant is the 

 variation with temperature of electrical energy per unit of 

 time, and according to a theory of metallic conduction to be 

 referred to later on we may write this change of energy per 

 unit of time in terms of its 1 hernial equivalent, as in the 



