Similar considerations apply to the case of the simple circuit and the magnet shown in Fig. 12. In this circuit there is no current flowing so long as the magnet is at rest; but if the magnet be moved up toward the circuit so as to increase the number of lines of force that pass through the circuit, there will be a momentary "inverse" current induced in the circuit and it will flow in the negative direction. While if the magnet were moved away the decrease in the number of lines of force would result in a transient "direct" current, or one flowing in the positive direction.
It would be possible to deduce these results from an abstract consideration of the matter from the point of view of the principle of conservation of energy. But we prefer to reserve this point until a general notion of the action of dynamo-electric machines has been given.
The following principles or generalized statements follow as a matter of the very simplest consequence from the foregoing considerations:
(a) To induce a current in a coil of wire by means of a magnet there must be relative motion between coil and magnet.
(b) Approach of a magnet to a coil or of a coil to a magnet induces currents in the opposite direction to that induced by recession.
(c) The stronger the magnet the stronger will be the induced currents in the coils.
(d) The more rapid the motion the stronger will be the momentary current induced in the coils (but the time it lasts will, of course, be shorter).
(e) The greater the number of turns in the coil the stronger will be the total current induced in it by the movement of the magnet.
These points are of vital importance in the action of dynamo electric generators. It remains, however, yet to be shown how these transient and momentary induction currents can be so directed and manipulated as to be made to combine into a steady and continuous supply. To bring a magnet pole up toward a coil of wire is a process which can only last a very limited time; and its recession from the coil also cannot furnish a continuous current since it is a process of limited duration. In the earliest machines in which the principle of magneto-electric induction was applied, the currents produced were of this momentary kind, alternating in direction. Coils of wire fixed to a rotating axis were moved past the pole of a magnet. While the coil was approaching the lines of force were increasing, and a momentary inverse current was set up, which was immediately succeeded by a momentary direct current as the coil receded from the pole. Such machines on a small scale are still to be found in opticians' shops for the purpose of giving people shocks. On a large scale alternate current machines are still employed for certain purposes in electric lighting, as, for example, for use with the Jablochkoff candle. Large alternate-current machines have been devised by Wilde, Gramme, Siemens, De Meritens, and others.--Engineering.