298 Steady Currents in Linear Conductors [OH. ix 



and the three conductors have definite potentials V A , V B , V c . The differ- 

 ence of potential between the two "terminals" a, b is V A V B , but the 

 peculiarity of the voltaic cell is that this difference of potential is not equal 

 to the difference of potential between the two conductors when they are 

 placed in contact and are in electrical equilibrium without the presence of 

 the liquid C. Thus on electrically joining the points a, b in the voltaic cell 

 electrical equilibrium is an impossibility, and a current is established in the 

 circuit which will continue until the physical conditions become changed or 

 the supply of chemical energy is exhausted. 



Electromotive Force. 



343. Let A, B, G be any three conductors arranged so as to form a 

 closed circuit. Let V AB be the contact difference of potential between A and 

 B when there is electric equilibrium, and let V BC) V CA have similar meanings. 



If the three substances can be placed in a closed circuit without any 

 current flowing, then we can have equilibrium in which the three conductors 

 will have potentials V A , V B , V c , such that 



Vd~'B = 'AB : > VB~~'C'BC'I VC~*A = V CA . 



Thus we must have 



V AB + V BC + V CA = Q ........................ (264), 



a result known as Voltas Law. 



If, however, the three conductors form a voltaic cell, the expression on the 

 left-hand of the above equation does not vanish, and its value is called the 

 electromotive force of the cell. Denoting the electromotive force by E, we 

 have 



We accordingly have the following definition : 



DEFINITION. The Electromotive Force of a cell is the algebraic sum of the 

 discontinuities of potential encountered in passing in order through the series 

 of conductors of which the cell is composed. 



Clearly an electromotive force has direction as well as magnitude. It is 

 usual to speak of the two conductors which pass into the liquid as the 

 high-potential terminal and the low-potential terminal, or sometimes as the 

 positive and negative terminals. Knowing which is the positive or high- 

 potential terminal, we shall of course know the direction of the electromotive 

 force. 



344. If the conductors C, A of a voltaic cell ABC are separated, and 

 then joined by a fourth conductor D, such that there is no chemical action 

 between D and the conductors C or A, it will easily be seen that the sum of 

 the discontinuities in the new circuit is the same as in the old. 



