ELEMENTARY ELECTRICITY AND MAGNETISM. 23 



in which P is the power delivered by an electric generator, / is 

 the current, and E is the electromotive force between the gener- 

 ator terminals. The electromotive force between the terminals 

 of a generator is analogous to the pressure-difference between 

 inlet and outlet of a fan blower. An increase of current deliv- 

 ered by a generator * usually causes a decrease of electromotive 

 force between the terminals of the generator very much as an in- 

 creased delivery of air by a fan, by opening the outlet for ex- 

 ample, causes a decrease of pressure difference between inlet and 

 outlet of the fan. 



Unsatisfactory character of fundamental definition of electro- 

 motive force. The definition of a 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 the ordinary conditions of practical work, the definition 

 always seems more or less unsatisfactory. Thus the above defini- 

 tion of electromotive force, as units-of-work-per-second-per -ampere, 

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

 a circuit is to be measured directly in mechanical units. This can 

 indeed be done, but it is an operation of great difficulty and one 

 that is seldom performed. 



Definition of electromotive force based on Ohm's law. When 

 all the work which is delivered to an electrical circuit is used to 

 heat the circuit in accordance with Joule's law, a simple relation, 

 called Ohm's law, exists between current, resistance, and electro- 

 motive force, and the simplest method of measuring electromotive 

 force (and therefore the simplest definition of electromotive force) 

 is based on Ohm's law. 



17. Ohm's law. An important relation between electromotive 

 force, current strength, and resistance exists in the case of a cir- 

 cuit in which all of the work delivered by the generator is used 

 in heating the circuit in accordance with Joule's law. Thus the 



* Such as a separately excited dynamo, or a shunt dynamo. 



