# An elementary book on electricity and magnetism and their applications . generator tends to generate an e.m.f. which rises fromzero to a certain maximum value and falls to zero again, thenreverses in direction, rising to a maximum value and returningagain to zero. PRINCIPLES OF ALTERNATING CURRENTS 309 In figure 212 let us suppose that an armature coil is revolvingaround the axis O between the poles N and S; in making one completerevolution of 360° suppose it makes 12 equal steps of 30° each. Letus plot also the instantaneous values of the voltage generated in thecoil at these 12 positions in

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An elementary book on electricity and magnetism and their applications . generator tends to generate an e.m.f. which rises fromzero to a certain maximum value and falls to zero again, thenreverses in direction, rising to a maximum value and returningagain to zero. PRINCIPLES OF ALTERNATING CURRENTS 309 In figure 212 let us suppose that an armature coil is revolvingaround the axis O between the poles N and S; in making one completerevolution of 360° suppose it makes 12 equal steps of 30° each. Letus plot also the instantaneous values of the voltage generated in thecoil at these 12 positions in one revolution or cycle. On the horizontalline we lay off 12 equal spaces of convenient size, which will representequal time intervals. When the coil is in position A, it is cutting no lines of magneticforce and hence no voltage is induced in it; therefore we start thee.m.f. curve at the base line B. When the coil has moved 30° itis cutting flux and there is an e.m.f. induced out at 1. We maylay off a vertical line ex upward to represent the instantaneous voltage. k N M le, !* f3 h Time ! r5lS0°210° 240° 270 300° 330° 360° 0 B ?MJ 60? 90u 120° 150° e, ! ! ! /;i e ! ! &lt; /! i ei % 7 I Fig. 212. — Alternating-current curve of bipolar generator. when the coil is at 30° and another line e2 to represent the instantaneouse.m.f. when the coil has moved 60° to position 2. When the coil hasmoved 90° from the zero position, it is at that instant cutting flux atthe maximum rate, since it is moving directly across the lines of force, and the line e3 represents the instantaneously induced voltage. Asthe coil moves on from this point the induced voltage begins to de-crease and at length becomes zero again as the coil reaches a point180° from the zero position. Lines e4 and eh show the instantaneousvalue of the e.m.f. at 120° and 150° respectively. As soon as the coilpasses the 180° point, it begins to cut flux in the opposite direction andso the induced e.m.f. is

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