RESISTANCE AND ELECTROMOTIVE FORCE. 37 



the generator in volts. The abvolt or c.g.s. unit of electromotive 

 force is defined in Art. 52. 



Unsatisfactory character of the fundamental definition of electro- 

 motive force. The definition of any physical quantity consists, in 

 every case, of a concise statement of the fundamental method of 

 measuring that quantity, and when this fundamental method of 

 measuring a quantity involves operations which are not feasible 

 under ordinary conditions of practical work, the definition seems 

 more or less unsatisfactory. Thus, the above definition of elec- 

 tromotive force as units-of-work-per-second-per-ampere (/'//) 

 assumes that the rate of doing work in a pushing current through 

 a circuit is to be measured directly in mechanical units,, and no 

 method is specified for doing this. The simplest definition of 

 electromotive force is based on Ohm's Law as explained in the 

 following article. 



19. Ohm's Law. The current produced by a voltaic cell, or, 

 in general, by any electric generator, is inversely proportional to 

 the resistance of the circuit.* This relation was discovered by 

 G. S. Ohm in 1827 and it is called Ohm's Law. A complete 

 statement of Ohm's Law together with a clear specification of the 

 conditions under which the law applies may be derived as fol- 

 lows : The power output of an electric generator is equal to El 

 according to equation (6). If the whole of this power is used to 

 heat the circuit in accordance with Joule's Law, then we must have 



EI= 

 according to equation (3). Therefore we have 



E=RI ' (7 a) 



or 



/= | . (7*) 



*This statement and the statement given in the previous article to the effect that 

 the power output of a generator is proportional to the current, are not exactly true, 

 because of the fact that the electromotive force of a generator usually falls off in value, 

 to some extent, when the generator is called upon to give an increased current. 



