. American telephone practice . berapidly revolved between the poles of a permanent horse-shoe mag-net. Its theory of action is very simple and depends on the prin-ciples of magneto electricity discovered by Faraday and Henry, 104 MAGNETO CALLING APPARATUS. 105 and pointed out in a previous chapter—that if the number of linesof force passing through a closed coil be varied, currents of elec-tricity will be generated in this coil, the direction of these currentsdepending upon the direction of the lines of force and on whethertheir number is decreasing or increasing. In Fig. 77 is shown a simple
. American telephone practice . berapidly revolved between the poles of a permanent horse-shoe mag-net. Its theory of action is very simple and depends on the prin-ciples of magneto electricity discovered by Faraday and Henry, 104 MAGNETO CALLING APPARATUS. 105 and pointed out in a previous chapter—that if the number of linesof force passing through a closed coil be varied, currents of elec-tricity will be generated in this coil, the direction of these currentsdepending upon the direction of the lines of force and on whethertheir number is decreasing or increasing. In Fig. 77 is shown a simple loop of wire, a, which may be re-volved about a horizontal axis in the field of force of a permanentmagnet. The horizontal arrows represent the direction of the linesof force set up by the magnet through the loop. Suppose the loopto be turned in the direction of the curved arrow. When it is inthe horizontal position no lines of force will pass through it. As itapproaches the position shown by the full line it will include a. FIG. 77.—FIELD OF FORCE IN MAGNETO GENERATOR. larger and larger number of these lines. The current induced inthe coil will then be in the direction indicated by the arrows, x, andwill so continue until the loop is in its vertical position. The num-ber of lines passing through the loop then begins to decrease, andthe current therefore takes the opposite direction, as indicated bythe arrows, z. The current increases in strength in this new direc-tion until the coil is horizontal. At this point the rate at which thenumber of lines through the coil is changing is greatest, and the cur-rent is therefore a maximum. As the coil passes through the horizon-tal position the number of lines passing through it begins to increaseagain. This would cause another change in the direction of the 106 AMERICAN TELEPHONE PRACTICE. current, were it not for the fact that the direction of the lines offorce through the coil also changes. The same cycle of events takesplace dur
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