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12 HANDBOOK OF PROJECTION FOR
Of course, it is understood that other things, such as the size and winding of the magnets, kind of armature core et cetera, have much to do with the ultimate performance of the machine, but we are merely explaining to you the principle upon which the generator operates, which, with some variations in methods, is always the same.
HOW CURRENT GENERATION IS STARTED.— The magnet of the type of dynamo shown in Fig. 3 is a "permanent" magnet, meaning that it does not become entirely demagnetized when the armature comes to rest. In effect, when lying idle it is just an enormous horse-shoe magnet, much tho same, except for its size, as the horse-shoe magnets that children play with. The magnetism retained when the machine is at rest is called "residual magnetism." It is very much too weak to enable a dynamo to build up and maintain a commercial voltage. The most we might hope to accomplish by its use would be to generate perhaps ten volts' pressure.
Examining Fig. 3 we see that wire F — F coils around the upper part of magnet. A. It may connect either directly to brushes D — D or to lines E — E a short distance from them. In other words, coil F — F and the armature form a complete circuit, which but for field resistance H would be a short circuit, and is in fact a short circuit when lever G is in position shown. Coil F — F forms what is known as the "shunt field circuit" and the generator shown is a "shunt wound" machine.
It is a well known fact that if a current of electricity is passed through a wire wound upon a magnet in the way wire F — F is wound, the magnet will have its power increased, and that the power of the magnet wrill increase proportionately as the current is increased until the point of "saturation" (iron is said to be saturated with magnetism when the point is reached where it will receive no more) is reached.
Bearing the foregoing in mind, a dynamo starts generating electro-motive force as follows : First having placed lever G in the position shown, which "cuts out" or eliminates all the resistance of the field rheostat, the switch connecting wires E — E with the outside circuit is opened, so that all current generated must flow around shunt circuit F — F, there being no place else for it to go. Power from an engine or motor is now applied and armature B is rotated at high speed, its coils cutting lines of magnetic force in the weak field of residual magnetism. This immediately creates a slight electromotive force, the current resulting from which flows around