196 PROPAGATION OF ELECTRICITY. 



The second equation may be written 



i-i i t' I 



71 



R 



and we deduce from it 



Thus the inverse of the resistance of a number of conductors 

 terminating at the same points, is the sum of the inverses of the 

 resistances of the several separate conductors ; in other words, the 

 conductivity of a multiple conductor is the sum of the conduc- 

 tivities of the several conductors of which it is made up, which 

 indeed is evident. 



210. HETEROGENEOUS LINEAR CONDUCTORS. The existence of 

 electromotive forces of contact between metals slightly modifies 

 Ohm's law. Consider two points P l and P 2 (Fig. 53), separated by 



Fig- 53- 



two different conductors A and B whose resistances are a and b. 

 Let V 1 and V 2 be the potentials at the points P l and P 2 , V a and V & 

 the potentials at the point P on each side of the surface of sepa- 

 ration of the metals. The current from P : to P 2 is 



a + b 



Let H a& be the sudden rise which the potential experiences 

 between the metals A and B going with the current that is to say, 

 the electromotive force of contact V b - V a = B|A of the conductors 

 B and A, we get 



