ELECTRICAL APPARATUS. 171 



done in unit of time in the part of an electric circuit lying between 

 two given points is equal to the strength of the current multiplied 

 by the difference of potential between these points. It may be 

 added here that, if the direction of the current is from a point of 

 higher potential to a point of lower potential, work is done by the 

 current ; whereas, if the current flows from a point of lower to a 

 point of higher potential, work is done in maintaining the current. 

 We know, however, that if, starting at any point A of an electric 

 circuit and following the direction of the current, we arrive at a 

 point B of lower potential, we shall, by continuing to follow the 

 current, get back again from the point B to the point A ; that is,, 

 we shall pass from a point of lower to a point of higher potential. 

 Consequently, since the strength of the current crossing any com- 

 plete section of a circuit is the same, it follows that the nett 

 amount of work done by the current in any part of a circuit is- 

 equal to the nett amount of work done in the remainder of the 

 circuit in maintaining the current. 



The different kinds of work which the electric current can do- 

 may be classified as follows : 



A. The work done by a current of constant strength traversing 

 a circuit no part of which undergoes change of position relatively 

 to another part or to any other conducting circuit or magnet is 

 entirely internal; that is, it appears within the conductors of 

 which the circuit is made up, in one or other of the following 

 forms : 



(1) As development of heat, in metallic conductors of one 



material. 



(2) As development of heat accompanied by transfer of heat 



causing inequality of temperature, in metallic con- 

 ductors not all of one material. 



(3) As development of heat together with chemical decom- 



position, in a conductor consisting of a compound 

 liquid. 



