INDUCTANCE AND CAPACITY. 355 



The magnetic field which is produced by an electric current 

 seems to be a state of motion of the ether, and the momentum 

 of the electric current depends upon this magnetic field. Thus, 

 a current in the circuit a, Fig. I, produces a very weak mag- 

 netic field, except in the small region between the wires, and the 

 momentum of the current is small ; the same current in circuit 

 b produces an intense magnetic field inside of the coil, and the 

 momentum of the current is correspondingly great ; and the 

 magnetism of the iron core accounts for the very great mo- 

 mentum of the current in circuit c. In the following discus- 

 sion the notion of kinetic energy is used in preference to the 

 notion of momentum for the reason, mainly, that kinetic energy 

 is expressed in mechanical units even when it is electrical kinetic 

 energy. 



2. The kinetic energy associated with a current in a circuit. 

 Definition of inductance. The ether motion which constitutes 

 the magnetic field in the neighborhood of an electric circuit, rep- 

 resents kinetic energy, and the kinetic energy which is associated 

 with an electric current is the total energy residing in the mag- 

 netic field produced by the current. We shall for convenience 

 call this the kinetic energy of the current. 



The amount of energy residing in a portion of a magnetic field 

 is proportional to the square of the intensity of that portion of 

 the field. This is analogous to the fact that the energy of a por- 

 tion of a moving liquid is proportional to the square of the 

 velocity of that portion. If the current in a circuit is doubled 

 the field intensity is everywhere doubled, so that the energy of 

 each portion of the field is quadrupled. Therefore the total 

 energy of the field is quadrupled when the current is doubled, or 

 in other words, the kinetic energy of a current in a given circuit 

 is proportional to the square of the current, that is 



W=\LP (i) 



in which W is the kinetic energy of a current, i t in the given 



