ELECTRIC MOMENTUM. INDUCTANCE. 145 



current in the coil is doubled ; and consequently the total kinetic 

 energy of a rotating wheel is proportional to the square of its 

 angular velocity, in the same way that the total kinetic energy 

 of a current in a given coil is proportional to the square of the 

 current. That is, we may write 



W= 



in which W is the kinetic energy of a rotating wheel, o is the 

 angular velocity of the wheel, and (J^) is a proportionality 

 factor. The quantity K is called the moment of inertia of the 

 wheel. 



77. Electromotive force required to cause a current to increase 

 or decrease. To maintain a constant current in a circuit an elec- 

 tromotive force equal to Ri must act upon the circuit to over- 

 come the resistance of the circuit. If the electromotive force 

 which acts upon the circuit is greater than Ri, the current in- 

 creases in value, and if the electromotive force which acts upon 

 the circuit is less than Ri, the current decreases in value. Let 

 the electromotive force which acts upon a circuit exceed Ri by 

 the amount e ; then we have 



'-ij (49) 



in which L is the inductance of the circuit and difdt is the 

 rate at Which the current increas'es. When e is negative (elec- 

 tromotive force less than Ri} then dijdt is negative, that is, 

 the current decreases. 



Mechanical analogue of equation (49). To keep a body in 

 uniform motion a force sufficient to overcome the drag of friction 

 must act upon the body. If the force which acts upon the body 

 is greater than the drag of friction, the body gains velocity, and 

 if the.force which acts upon the body is less than the drag of fric- 

 tion, the body loses velocity. Let the force which acts upon the 

 body exceed the drag of friction by the amoijmt e, then we have 



* 



ii 



