At 90°, the electromotive force is at a maximum; hence, by using a pressure scale such that the length of the perpendicular 2 2′ for 90° will measure the maximum voltage the length of the perpendicular at any other point will represent the actual pressure at that point.

The curve lies above the horizontal axis during the first half of the revolution, and below it during the second half, which indicates that the current flows in one direction for a half revolution and in the opposite direction during the remainder of the revolution.

The application of the sine curve to represent the alternating cycle, is further illustrated in figs. 169 to 173, which show the position of the armature at each quarter of the revolution.

Figs. 169 to 173.—The sine curve with view of armature for each 90° of the revolution, showing progressively the application of the sine curve to the alternating current cycle.
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In fig. 179, the loop A B C D is in the vertical position at the beginning of the revolution. At this instant the electromotive force is zero, hence the sine curve as shown begins at E, the zero point—that is, on the axis or line of no pressure.

As soon as the loop rotates out of the vertical plane, the electromotive force rises and the current begins to flow in the direction indicated by the arrows, going out to the external circuit through brush M, and returning through brush S.

Continuing the rotation, the electromotive force increases in proportion to the sine of the angle made by the plane of the loop with the horizontal, until the loop comes into the horizontal position illustrated in fig. 170. This increase is indicated by the gradual rise of the sine curve from E to F. The loop has now made one quarter of a revolution and the electromotive force reached its maximum value.

As the loop rotates past the horizontal position of fig. 170, the electromotive force gradually decreases in intensity, reaching the zero point at the end of the second quarter—that is, when the loop has turned one half revolution. This is indicated by the gradual fall of the curve from F to G.

When the loop turns out of the vertical position shown in fig. 171 the current reverses, because the movement of A B and C D is reversed; at this instant the brush M becomes negative, and S positive. This reversal of current is indicated by the curve falling below the axis from G to I.