ELECTRIC MOMENTUM. INDUCTANCE. 143 



Discussion of equation {48]. It was pointed out in Art. 53 

 that to double the strength of the current in a circuit is to double 

 everywhere the intensity of the magnetic field which is due to the 

 current, and it was shown in Art. 44 that the kinetic energy per 

 unit volume of a magnetic field is proportional to the square of 

 the field intensity. Therefore to double the strength of the cur- 

 rent in a circuit is to double everywhere the intensity of the mag- 

 netic field due to the current, and to quadruple everywhere the 

 energy of the magnetic field, so that to double an electric current 

 is to quadruple the total energy of its magnetic field. 



Units of inductance. If W in equation (48) is expressed in 

 joules, and / in amperes, then L is expressed in terms of a unit 

 of inductance which is called the henry, that is to say, a circuit 

 has an inductance of one henry when a current of one ampere in 

 this circuit represents one half of a joule of kinetic energy. 



If W in equation (48) is expressed in ergs and / in abam- 

 peres, then L is expressed in c.g.s. units of inductance. The 

 c.g.s. unit of inductance is sometimes called the abhenry* A cir- 

 cuit has one abhenry of inductance when a current of one abam- 

 pere in that circuit represents one half of an erg of kinetic energy. 

 There are io 9 abhenrys in one henry. 



Inductance of a coil. Strictly, one cannot speak of the in- 

 ductance of anything but an entire circuit, inasmuch as every 

 portion of a circuit contributes its share to the magnetic field at 

 each and every point in the surrounding region; it. is, however, 

 allowable to speak of the inductance of a coil when the terminals 

 of the coil are not too far apart, and when the remainder of the 

 electrical circuit does not produce any perceptible magnetic field 

 in the region occupied by the coil. 



Non-inductive circuits. A circuit is said to be non-inductive 

 when the inductance of the circuit is negligibly small, that is, 

 when the electromotive force L x dildf\ is negligibly small as 

 compared with the electromotive force RI which overcomes the 



*The c.g.s. unit of inductance is often called the centimeter. 

 f See next article. 



