equals the rate of variation of Ali. i.e. equals Al ; so. if the indneed E. Al. F. is unity, Al must (4111:1I one. Tin- energy required to produce the eillrellt comes front whatever agency changes the mag n•tic field. except when a currant is made ur broken, i.e, except cases involving self-induction. t hi breaking a circuit which earries a current, the cell or other source of E. AI. E. ceases to furnish energy; and that required for the 'extra current' mines from the surrounding medium; so this current continues only so long as this supply lasts. This energy of the medinin is associated with the field around the eonduct,r due to the current. when a merest is 'made' by joining the two ends of the broken conductor. the cell or source of E. Al. E. begins to furnish energy: and this is spent iu two ways at first, producing the magnetic field, and in over vowing the resistance of the conductor. When the current rises to its full value, as given by Ohm's law. the ntorneti• field reaches its final value, and all the energy now goes to overcoming the resistance ( FRO . This energy of a mag
netie field—around either a current or a per manent magnet—is undoubtedly kinetic; the ether and the portions of matter, if they are magnetic, are in motion. Clerk Maxwell has shown how nmgnetie action: can be explained if a line of magnetic fo•ee is a linear vortex. the ether and the very small portions of matter spin ning around this line as an axis, and if these different vortices are made to depend on one another by means of sonic kind of mechanism connecting them.
The applications of induced currents are al most too numerous to mention; earth inductors to measure the direction and intensity of the earth's magnetic field; induction coils to produce sparks, etc.: transformers as used commercially to reduce or increase the E. AI. E. of current:: all forms of dynamos; the telephone; all apparatus that depends 1111011 the use of alternating cur r•nts. etc.