INTRINSIC ENERGY OF THE CURRENT. 501 



The term Mil' is the relative energy of the two currents ; it is 

 the work which would have been necessary to bring the circuits 

 traversed by the currents I and I' from an infinite distance to their 

 actual position. Each of the other terms within the parenthesis may 

 be called the intrinsic energy of the corresponding current; it is 

 equal to half the product of the coefficient of self-induction by the 

 square of the strength, and represents the cost of the work of creating 

 the actual current in each circuit (this not being subject to any foreign 

 action), or the external work which this current could develop if it 

 were left to itself, and vanished under the same conditions. 



It may be observed that we may write 



T T2 T 'T'2 T T 



- + Mil' + - - = - (LI + MI') I + - (LT + MI) I'. 



2 22 2 



Each of the two terms on the right hand side represents the 

 potential energy of the corresponding circuit ; it is the work which 

 must be spent in each circuit when the field is brought from zero to 

 its actual state, and is therefore the work which it would produce if 

 all the currents were simultaneously annulled. In each of the 

 circuits this work is equal to half the product of the strength by the 

 flow of magnetic force which traverses it. 



That part of the energy which we have been here considering 

 is in a form which it is not possible to define in the present state 

 of science. We cannot say, for instance, whether it is in the state 

 of ordinary potential energy, like the tension of an elastic body, 

 or of an actual energy consisting in the motion of a particular 

 fluid, or again in both of these at once ; nor further, whether it is 

 localised in the circuit traversed by the current, or diffused through 

 the whole medium, in accordance with the ideas of Faraday and of 

 Maxwell. 



525. In the general case in which the factors L, M and L' are 

 variable, the first term of the expression (17) always represents the 

 variation in the potential energy of the two circuits ; the whole of 

 the other terms 



1 IVL+IIVlI+-IVL f 



2 2 



represents the work done by the electrodynamic actions of the 

 conductors, in consequence of their changes of form, or of relative 

 position. 



