Translations:Introduction to Dielectricity & Capacitance (Eric P. Dollard)/20/en

Mechanism of Storing Energy Magnetically

The process of pushing these lines or loops outward, causing them to stretch, represents storing energy as in rubber band. A given current strength will hold a loop of force at a given distance from conductor passing current hence no energy movement. If the flow of current increases, energy is absorbed by the field as the loops are then pushed outward at a corresponding velocity. Because energy is in motion an E.M.F. [ electro-motive force ] must accompany the current flow in order for it to represent power. The magnitude of this EMF exactly corresponds to the velocity of the field. Then if the current ceases changing in magnitude thereby becoming constant, no EMF accompanies it, as no power is being absorbed. However, if the current decreases and represents then a negative velocity of field as the loops contract. Because the EMF corresponds exactly to velocity it reverses polarity and thereby reverses power so it now moves out of the field and into the current. Since no power is required to maintain a field, only current, the static or stationary field, represents stored energy.