Now, this change is just exactly what has happened in the wire (C), as we have explained. The current attempts to reverse itself and start out on business of its own, so to speak, with the result that when the brushes (D and E) contact with the negative and positive terminals, respectively, the surging current in the wire (C) is going in the direction of the dart (H)—that is, while, in Fig. [110], the current flows from the wire (C) into the positive terminal, and out of the negative terminal into the wire (C), the conditions are exactly reversed in Fig. [111]. Here the current in wire C flows into the negative (-) terminal, and from the positive (+) terminal into the wire C, so that in either case the current will flow out of the brush D and into the brush E, through the external circuit (G).
It will be seen, therefore, that in the direct-current motor, advantage is taken of the surging, or back-and-forth movement, of the current to pass it along in one direction, whereas in the alternating current no such change in direction is attempted.
Alternating Positive and Negative Poles.[p. 154]—The alternating current, owing to this surging movement, makes the poles alternately positive and negative. To express this more clearly, supposing we take a line (A, Fig. [112]), which is called the zero line, or line of no electricity. The current may be represented by the zigzag line (B). The lines (B) above zero (A) may be designated as positive, and those below the line as negative. The polarity reverses at the line A, goes up to D, which is the maximum intensity or voltage above zero, and, when the current falls and crosses the line A, it goes in the opposite direction to E, which is its maximum voltage in the other direction. In point of time, if it takes one second for the current to go from C to F, on the down line, then it takes only a half second to go from C to G, so that the line A represents the time, and the line H the intensity, a complete cycle being formed from C, D, F, then through F, E, C, and so on.
How an Alternating Dynamo Is Made.—It is now necessary to apply these principles in the construction of an alternating-current machine. Fig. [113] is a diagram representing the various elements, and the circuiting.