CHAP. X] ENERGY AND INDUCTANCE 179 



period is less than that which corresponds to the i 2 /? loss in the 

 circuit, so that practically the same amount of steam is saved 

 which was expended before in increasing the magnetic field. 



These phenomena may be explained, or at least expressed in 

 more familiar terms, by assuming the magnetic field to be due to 

 some kind of motion in a medium possessed of inertia (Art. 3). 

 When the field strength is increased it becomes necessary to accel- 

 erate the parts in motion, overcoming their inertia. When the 

 field is reduced, the kinetic energy of motion is returned to the 

 electric circuit. One can also conceive of the energy of the mag- 

 netic field to be static and in the form of some elastic stress. 

 Under this hypothesis, when a current increases, the magnetic 

 stress also increases at the expense of the electric energy. In 

 either case, when the current is constant no energy is required 

 to maintain the field, any more than to maintain a constant rota- 

 tion in a fly-wheel or a constant stress in an elastic body. 



It seems the more probable that the magnetic energy of a 

 circuit is stored in some kinetic form, because the current which 

 accompanies the flux is itself a kinetic phenomenon. On the other 

 hand, it appears more likely that electrostatic energy is due to some 

 elastic stresses and displacements in. the medium, and thus it 

 may be said to be potential energy. Electric oscillations and 

 waves consist, then, in periodic transformations of electrostatic into 

 electromagnetic energy, or potential into kinetic energy, and rice 

 versa, similar to the mechanical oscillations and waves in an 

 elastic body. In the familiar case of current or voltage resonance 

 the total energy of the circuit at a certain instant is stored in the 

 form of electrostatic energy in the condenser (permittor) con- 

 nected into the circuit, or in the natural permittance of the circuit ; 

 the current and the magnetic energy at this instant are equal 

 to zero. At another instant, when tne current and the map HI i< 

 field are at their maximum, the energy stored is all in the form 

 of magnetic energy, and the voltage across the condenser and 

 the stress in the dielectric are equal to zero. An oscillating 

 pendulum offers a close analogy to such a system. The resistance 

 of the electric circuit, and the magnetic and dielectric hysteresis, 

 are analogous to the friction and windage which accompanies 

 the motion of the pendulum. 



As it is of importance in mechanical machine design to know 

 the inertia of the moving parts of a machine, so it is often necessary 



