RESISTANCE AND ELECTROMOTIVE FORCE. 39 



the actual electromotive force between the terminals of the cell while 

 it is delivering current. Therefore, we have 



=A-^/ (9) 



in which E x is the electromotive force across the terminals of 

 the cell while it is delivering current, and, inasmuch as E x = R x l, 

 and E x l = R X I 2 , we may write : 



/= 



and 



in which P x is the power delivered by the cell to the external 

 circuit. In these equations E t is the total electromotive force 

 of the voltaic cell (or generator), R a l is the portion of this total 

 electromotive force which is used to 'overcome the resistance of 

 the cell (or generator), E x ( = E t R a -T) is the electromotive 

 force between the terminals of the cell (or generator), and P x is 

 the power delivered to the external circuit which does not include 

 the power developed in heating the cell (or generator). 



Equations (6) and ( 7 ) are , _ ^ 



nearly always used in practice in _[_ 

 their application to a portion of ^ B 

 a circuit. Thus, Fig. 16 shows "^ 

 a battery B supplying current to 



a lamp Z,, the electromotive force between the terminals of the 

 lamp is E, the current flowing in the circuit is /, the power de- 

 livered to the lamp is El, and the- current is equal to the elec- 

 tromotive force between the terminals of the lamp divided by 

 the resistance of the lamp, according to equation (7). 



Voltage drop in a generator. The electromotive force R a l 

 required to overcome the resistance of the generator (or voltaic 

 cell) in the above discussion is subtracted from the total electro- 

 motive force of the generator to give the electromotive force be- 

 tween the generator terminals, as indicated in equation (8). This 

 electromotive force RI which is used to overcome the resistance 



