[Illustration: FIG. 185.--Gramme ring armature with four coils. The electromotive force induced in coils A, A′ reaches the zero point at the instant that of coils B, B′ is at a maximum; hence, sine curve No. 1, beginning at zero, and No. 2, at the maximum, show the pressure changes for A, A′ and B, B′, respectively. The summation of these curves gives _the resultant curve_ No. 3, showing changes in pressure of current delivered to the external circuit.] =Ques. What may be said with respect to the four coil Gramme ring armature shown in fig. 185?= Ans. According to the laws of electromagnetic induction, with the north pole of the field at the left and clockwise rotation, the induced currents flow _upward_ on both sides of the ring, hence, _the electromotive forces oppose each other at only two of the junctions, namely: at the one connected to brush M where the pressures on either side are both directed toward the junction and the other at the junction connected to brush S, at which the pressures are both directed from the junction._ [Illustration: FIG. 186.--Gramme ring armature with six coils. The sine curves 1, 2 and 3, represent the conditions due to coils AA′, BB′ and CC′, respectively, and 4, the resultant pulsations.] It is evident, then, that the pressure at M is higher than at S; that is, M is positive and S negative; consequently, the current flows from M to the external circuit and returns through S. =Ques. In what other way may the four coils of the armature in fig. 185 be regarded?= Ans. They may be considered as two pairs A A′ and B B′, the action of either pair being identical with the two coil armature shown in fig. 183; this, in turn, produces the same effect as the one coil armature of fig. 182, with the exception that the amplitude of the current generated with two coils is twice as great as that with one coil of the same number of turns. Again considering the action of the four ring coil shown in fig. 185, and starting at the beginning of the revolution, the variation of electromotive force induced in coils AA′ is indicated by the dotted sine curve 1, and of BB′ by dotted curve 2. It will be seen that 1 begins at the axis or line of no pressure, and 2 at maximum pressure. [Illustration: FIG. 187.--The resultant curves of figs. 183, 185 and 186 are here shown for comparison to illustrate the approach to uniform pressure as the number of coils are increased. It should be noted that the number of pulsations per cycle depends on the number of coils, and that as the pulsations increase in number, the variation in pressure decreases.]