Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/629

 DYNAMO

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be correctly proportioned, the number of lines which sents a class of machines which are distinguished by the cross the interior will bear but a small ratio to those which somewhat misleading name of “unipolar” dynamos. In pass entirely through the iron, and the counter E.M.F. of this special form the (B) principle of arrangement is the internal wires will become very small; they may then seldom employed, since it is difficult to add other inductors be regarded simply as connectors for joining the external in order to increase the E.M.F. If, however, the maginductors in series. netic circuit be interrupted by another air-gap (Fig. 8), (2) The second or drum method was used in the original a single inductor can be “ shuttle-wound ” armatures invented by Dr Werner von arranged to cut the flux Siemens in 1856, and is still known as the “Siemens” twice in each revolution winding. The farther end of inductor 1 (Fig. 5) is joined in different directions as by a connecting wire to the farther end of another inductor it passes through the two 2' situated nearly diametrically opposite on the other side air-gaps. The E.M.F. which of the core and under the opposite pole-piece. The near is now produced in the inend of the complete loop or turn is then brought across the ductor will be first radially end of the core, and can be used as the starting-point for outwards, and then inwards another loop beginning with inductor 2, which is situated towards the centre, i.e., it by the side of the first inductor. The iron core may now alternates in direction just Fig. 8. be solid from the surface to the as in case (A). It then shaft, since no connecting wires becomes possible to employ a number of inductors (Fig. 9), are brought through the centre, and to add together their E.M.F.’s in series by methods and each loop embraces the entire which are analogous to the ring and drum winding already armature core (Fig. 7). By the fordiscussed under case (A), and which yield the third and mation of two loops in the ring fourth types of armature. armature and of the single loop in (3) By the discoidal or flat-ring method (Fig. 10) the drum armature, two inductors an inductor 1 is joined to its next neighbour 2, and for OHS are placed in series; the curve of this to be possible it is again necessary that there should instantaneous E.M.F. is therefore be an iron core (as A in Fig. 9) to convey the lines from on similar in shape to that of the one pole to the 180” single inductor (Fig. 4), but with other, and at the Fig 7 its ordinates raised throughout to same time to double their former height, as shown at the foot of shield the conFig. 6. necting wires, so Next, if the free ends of either the ring or drum loops, that they may not instead of being connected to two collecting rings, are themselves cut the attached to the two halves of a split-ring insulated from field. A second the shaft (as shown in Fig. 7 in connexion with a drum magnet M' (shown armature), and the stationary brushes are so set relatively dotted in Fig. 10) to the loops that they pass over from the one half of the can now be arFig. 9. split-ring to the other half at the moment when the loops ranged on the are passing the centre of the interpolar gap, and so are opposite side of the armature core to the first magnet M, giving little or no E.M.F., each brush will always remain with poles of the same sign facing each other. The greater either positive or negative. The current in the external portion of the length of the wires which previously served circuit attached to the brushes will then have a constant merely as conductors can then also be rendered usefully direction, although the E.M.F. in the inductors still remains inductive, the inactive connectors being simply the wires alternating; the curve of E.M.F. obtained at the brushes joining the two sides of each loop.3 The armature thus is thus (as in Fig. 7) entirely above the zero line. The takes the form of a flat ring (Fig. 10), and is analogous first dynamo of Pixii,1 which immediately followed Faraday’s discovery, gave an alternating current, but in 1832 2 the alternator was converted into a machine giving a unidirected current by the substitution of a rudimentary “ commutator ” in place of mercury collecting cups. On passing to case (B) we are at once reminded of the original dynamo of Faraday, yet though the disc of Fig. 1 is in reality a single inductor whose plane of movement is at right angles to its axis of rotation, it has the peculiarity that the E.M.F. generated is always in the same direction along the inductor’s length; it thus repre1 2

Ann. Chim. Phys. vol. 1. p. 322. Ann. Ohim. Phys. vol. li. p. 76. Since in Pixii’s machine the armature was stationary, while both magnet and commutator rotated, four brushes were used, and the arrangement was not so simple as the split-ring described above, although the result was the same. Saxton’s machine (1833) and Clarke’s machine (1835, see Sturgeon’s Annals of Electricity, vol. i. p. 145) were similar to one another in that a unidirected current was obtained by utilizing every alternate halfwave of E.M.F., but the former still employed mercury collecting cups, while the latter employed metal brushes. Sturgeon in 1835 followed Pixii in utilizing the entire wave of E.M.F., and abandoned the mercury cups in favour of metal brushes pressing on four semicircular discs (Scientific Researches, p. 252). The simple split-ring is described by Wheatstone and Cooke in their Patent No. 8345, 1840.

to a ring armature in which a second pair of poles is introduced into the centre of the core, each of the same sign as the external pole which it faces. (4) The fourth or disc winding was in principle em3 The advantage originally claimed for the discoidal-ring machine, namely, that the rim of the armature could also be utilized by joining the opposite poles, and that so the inactive length of the wire was considerably reduced as compared with the longitudinally-wound ring (Schuckert, Brit. Pat., No. 4464, 1877), has proved somewhat illusory, owing to the difficulty of avoiding eddy-currents in the core when the flux enters it in two directions at right angles to each other, from the sides and from the overarching pole-piece respectively. S. III. - 73