RESISTANCE AND ELECTROMOTIVE FORCE. 35 



gas at constant pressure. That is to say, the resistance of a wire 

 made of pure metal is approximately proportional to the absolute 

 temperature. 



ELECTROMOTIVE FORCE. 



18. Power delivered by an electric generator. Definition of 

 electromotive force. From Faraday's laws of electrolysis it is 

 evident that the amount of zinc consumed per second in a voltaic 

 cell by voltaic action is proportional to the strength of the cur- 

 rent. Therefore the available * energy developed per second by 

 the chemical action in the cell is proportional to the strength of 

 the current, or in other words, the electrical energy developed 

 per second by a given type of voltaic cell in the maintenance of a 

 current is equal to a constant multiplied by the current. That is, 



P=EI (6) 



in which P is the electrical energy developed per second by a 

 voltaic cell, / is the current produced by the cell, and E is a 

 constant for the given type of cell. This constant E is called 

 the electromotive force of the cell. This definition of electromotive 

 force applies to any form of electric generator. Imagine a 

 dynamo driven at constant speed, and having a field magnet of 

 which the strength is invariable. Ignoring friction, the only 

 opposition to the motion of the dynamo is that which is due to 

 the current flowing through the armature wires. Therefore to 

 double the current output of such a dynamo would double the 

 force required to drive it,f and therefore double the rate at which 

 work would be expended in driving it, its speed being constant ; 

 but the work which would be expended in driving such a dynamo 

 would all go to maintain the current, so that the rate at which 



* Available, that is, for the production of current. In some voltaic cells the whole 

 of the energy developed by the voltaic action goes to maintain the current ; but, in 

 general, a definite fractional part only of this energy is available for the production of 

 an electric current. See Physical Chemistry, H. C. Jones, pages 376-405. See also 

 papers by H. S. Carhart "On the Thermodyaamics of the Voltaic Cell," Physical 

 Review, Vol. XI, p. I, Vol. XVI, p. 248, and Vol. XXVI, p. 209, March, 1908. 



f See Art. 52, on the side push of a magnetic field on an electric wire. 



