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16 RICHARDSON'S BLUEBOOK OF PROJECTION
therefore electrically "dead/' (41) This very weak force is termed "residual magnetism." Generators which have non-residual magnets must, at starting, have their magnets excited by a small generator, usually attached to the machine itself. Residual magnetism is much too weak to enable the machine to build up a commercial voltage. It merely has strength to enable the machine to start operation.
In Fig. 3 we see field winding FF coiling around a portion of magnet NS. With field rheostat G as shown there is no resistance in series with field winding FF. The engine or other power source starts armature B rotating whereupon, with the line switch open and therefore practically no resistance in the circuit, the armature coils begin cutting such lines of force as the residual magnetism supplies. A very weak current is thus generated, all of which flows around field coil FF.
(42) Now if alternating current is sent around magnet NS through such a coil as is shown (FF), the immediate result will be to strengthen the magnet itself, and thus to increase the number of lines of magnetic force cut by the armature coil each revolution or per second, so that the magnetic strength will be further increased. (43) This proceeding, if unchecked, will go forward until the magnet has reached the "point of saturation," which means that its strength cannot be further increased.
(44) In practice, however, the process goes forward until the commercial voltage has been built up, whereupon the switch controlling outer circuit EE is closed, and sufficient resistance cut in series with winding FF (by moving the lever just below G, Fig. 3) to supply sufficient magnetic strength to enable the armature to maintain the required line voltage. In practice this latter is, in modern machines, done automatically by the machine itself.
(45) Modern generators for the most part have more than two magnets, but in effect only one N and S pole. (46) The added magnets enable the machine to produce the required amount of electrical energy at the desired