=Ques. What occurs during the second half of the revolution?= Ans. The wire A B (fig. 166), which before was moving in a downward direction, moves in an upward direction; hence, the current is reversed and flows around the loop in the direction A D C B (fig. 166), going out through brush S and returning through brush M. This makes M negative and S positive. [Illustration: FIG. 167.--Illustrating the increase and decrease in the rate magnetic lines are cut by a revolving loop. The initial position of the loop is taken at right angles to the direction of the lines of force. Since the loop rotates at a constant speed, it is evident that it does not cut the magnetic lines at uniform rate, because the intercepted arcs 0-1, 1-2, etc., are unequal. These arcs, rectified at the right by the horizontal lines 0-1, 1-2, etc., show more clearly the increase and decrease in the rate at which the magnetic lines are cut.] =Ques. What may be said of the electromotive force during the second half of the revolution?= Ans. It varies in a similar manner as in the first half of the revolution; that is, the magnetic lines are cut _with increasing rapidity_ during the third quarter, _and with decreasing rapidity_ during the fourth quarter of the revolution, which causes the electromotive force to increase and decrease during these intervals. The cycle of events just described may be summed up as follows: During the revolution of the loop: 1. From 0° to 90°, the electromotive force increases from 0 to maximum; 2. From 90° to 180°, the electromotive force decreases from maximum to zero; 3. From 180° to 270°, current reverses and the electromotive force increases from zero to maximum; 4. From 270° to 360°, the electromotive force decreases from maximum to zero. It was stated that, during the revolution of the loop, the magnetic lines were cut “with increasing or decreasing rapidity,” causing the electromotive force to rise or fall. The reason for this is illustrated in fig. 167. The loop is here shown in a horizontal position at right angles to the direction of the magnetic field; the latter, as indicated by the even spacing of the vertical arrows representing the magnetic lines, is assumed to be uniform. The wire C D of the loop, as it rotates at _constant speed_, cuts the magnetic lines at the points 0, 1, 2, 3, etc., but the distances 0-1, 1-2, 2-3, etc., between these points, are unequal; that is, the wire C D travels farther in cutting the lines 0 and 1, than it does in cutting 1 and 2, and still less in cutting the lines 2 and 3. After cutting the line 4, which passes through the axis of revolution, the opposite conditions obtain.