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 DYNAMO minute, and j?=the number of pairs of fields. In the ring and drum alternators, p is also the number of pairs of poles, but in the discoidal ring and the disc types the pairs of poles must be reckoned on one side only, since each pair of poles facing each other on opposite sides of the armature only corresponds to a single field. The above reasoning has, so far, led us in all cases to as many coils as there are fields and an armature core only half covered with winding, and such is the form taken by the single-phase alternator. But since only one-half of the armature core is utilized, an entirely distinct but similar set of coils may be wound to form a second armature circuit between the coils of the first circuit. The phase of this second circuit will differ by a quarter of a period from that of the first, and it may either be used to feed an entirely separate external circuit, possibly at a different pressure, or, if it be composed of the same number of inductors and therefore gives the same pressure, it may be inter-connected with the first circuit to form a quarter-phase alternator. By an extension of the same process, if the width of winding in each ring coil be reduced to

one-third of the pitch, or the width of each side of the drum coil be reduced to one-sixth of the pitch, three armature circuits can be wound on the same core, and a three-phase alternator is obtained, giving waves of E.M.F. differing in phase by 120°. Reverting to the simple dynamo of Fig. 7, we have still to consider the effect of the addition of more armature loops in dynamos of this type, which give a unidirected current in virtue of their split-rings. If the loops form a single coil, wider than the interpolar gap and connected to a single split-ring, they will, just as in the alternator, act differentially against one another during part of a revolution. Hence the coil must be more or less concentrated into a narrow band. Such an arrangement in the case of drum winding gives the H-form “ shuttle ” armature invented by Dr Werner von Siemens. Although its E.M.F. has a much higher maximum value than that of the curve of Fig. 7, it still periodically varies during each revolution, and so gives a pulsating current. To avoid the pulsation, recourse must be had to a new principle peculiar to the closed-coil continuous-current machine, which is by far the most largely used of all dynamos. Since a second coil placed at right angles to the original coil will yield a wave of E.M.F. of which the crests will coincide with the hollows of the first wave, the E.M.F.’s of the two coils when in series will supplement each other, and the

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fluctuations of the resultant curve will be greatly reduced, although its maximum may be no higher. Instead, therefore, of concentrating the new loops into a narrow coil, as in the case of the alternator, the opposite course must be pursued, and the loops be divided between several coils equally spaced round the armature. Given two coils at right angles and with their split-rings displaced through a corresponding angle of 90°, they may be connected in series by joining one brush to the opposite brush of the second coil, the external circuit being applied to the two remaining .brushes.1 The same arrangement may again be repeated with another pair of coils in parallel with the first, and we thus obtain Fig. 14 with four split-rings,

their connexions to the loops being marked by corresponding numerals; the four coils will give the same E.M.F. as the two, but they will be jointly capable of carrying twice the current, owing to their division into two parallel circuits. How, since the effect of joining brush 2 across to brush 3 is virtually to connect the end of coil A with the beginning of coil B, and so on until they are closed upon themselves in a continuous helix, the four split-rings may be replaced by the simpler arrangement of Fig. 15, consisting of four segmental portions of a cylinder, insulated from one another and from the shaft, and requiring only two brushes, which form the terminals of the external circuit. Each sector replaces a pair of halves of adjacent split-rings, in the order of sequence, 2 and 3, 4 and 5, 6 and 7, 8 and 1. The function of this structure2 being not merely to collect the current, but to commute its direction in any coil as it passes through the interpolar gap, it is known as the commutator. Each coil is successively short-circuited, as a brush bridges over the insulation between the two sectors which terminate it; and the brushes must be so set that the period of shortcircuit takes place when the coil is generating little or no E.M.F., i.e., when it is moving through the zone between the pole-tips. The effect of the four coils in reducing the percentage fluctuation of the E.M.F. is very marked, as shown at the foot of Fig. 14 (where the upper curve is the resultant obtained by adding together the separate curves of coils A and B), and the levelling process may evidently be carried still further by the insertion of more coils and more corresponding sectors in the commutator, until the whole armature is covered with winding. For example, Fig. 16 shows a ring and a drum armature, each with eight coils and eight commutator sectors; their resultant curve, on the assumption that a single inductor 1 Such was the arrangement of Wheatstone’s machine (Brit. Pat., No. 9022) of 1841, which was the first to give a more nearly continuous current, the number of sections and split-rings being five. 2 Its development from the split-ring was due to Pacinotti and Gramme (Brit. Pat., No. 1668, 1870) in connexion with their ring armatures.