214 THE REALITIES OF MODERN SCIENCE 



is constant, the current will depend only upon the 

 velocity with which they move. In other words, 

 velocity and current are proportional. 



The total kinetic energy represented by a current, 

 i, is therefore proportional to i 2 . Let us write it as 



K.E.=Li* (5) 



where L is a factor of proportionality which depends 

 upon the mass of the electrons and hence upon the 

 geometrical form of the circuit. The factor, L, is 

 usually called the coefficient of self-induction of the 

 circuit, because it determines the currents induced in 

 each part of the circuit by the changing electron streams 

 in all the other parts. It is a measure of the retarda- 

 tion or negative acceleration of the electrons occa- 

 sioned by their interactions. 



The self-inductance takes into account only those 

 interactions occurring between the electrons of its own 

 circuit. If there is in the neighborhood of a circuit 

 of inductance LI, with a current of ii, another circuit 

 which is carrying a current, say i%, then there are 

 possible interactions between the two circuits which 

 may either increase or decrease the energy required 

 to establish the current ii in circuit 1. Consider first 

 a single loop and let its inductance be L. If a current 

 of / is flowing in it, the kinetic energy is JL/ 2 . Now 

 suppose we divide this circuit lengthwise of the con- 

 ductor by an imaginary plane so that on one side the 

 current is ii and on the other side i%, where iz = I ii. 

 The energy may now be expressed as 



K.E. = \LP = iL(ii+i,) = JLti+Ltit,+iLi;* (6) 



