ELECTRIC CIRCUITS 103 



one complete cycle; it is equal to the maximum value divided by 

 V2 only in the case of sine waves. 



Alternating-current voltmeters and ammeters indicate the 

 effective values of voltage and current regardless of the wave 

 form. 



74. Inductance in Alternating-current Circuits. When a cur- 

 rent flows in a conductor a magnetic field is produced in the space 

 surrounding it; as long as the current remains constant this field 

 does not react on the electric circuit, but when the current varies 

 the flux linking with the circuit also varies and induces in the 

 conductor an e.m.f. opposing the change in the current and con- 

 sequently the change in the flux. This action is due to the 

 inertia of the magnetic field and is analogous to the action of the 

 flywheel in mechanics. The inertia of the flywheel opposes any 

 change in speed just as the inertia of the magnetic field opposes 

 any change in current. Energy is stored in the flywheel as the 

 speed increases and given back as it decreases and the only loss 

 of energy is that due to friction. Similarly energy is stored in the 

 magnetic field as the current increases and is returned to the elec- 

 tric circuit as the current decreases, and the only loss of energy 

 is that due to hysteresis and eddy currents in the iron parts of the 

 magnetic circuit. 



The energy stored in the flywheel is 



(147) 



where / is its moment of inertia and co is its angular velocity. 

 The energy stored in the magnetic field is 



W = -x- watt-seconds, 



where L is the inductance of the circuit in henrys and i is the 

 current in amperes. 



The inductance of the coil opposes the change in current by 

 generating a back e.m.f. 



T di 

 eb= ~ L di- \f 4 }**+**( 



