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excite it still more and strengthen the magnetic field. The result is yet stronger currents in the coil, which in turn excite still stronger magnetism in the field, and thus, by a reciprocal process of give and take the electro-magnetism is built up to a high degree of intensity, and a powerful current is generated in the coils. Part of this powerful current can be led away by the commutators, for some external purpose, leaving part to feed the electro-magnets; or the whole current after it has fed the electro-magnets can be led away for other uses, such as the production of light in an electric lamp.

Fig. 52 is a skeleton diagram of a generator of this

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kind drawn by Professor Silvanus P. Thompson, of Bristol, and it explains the action of the Gramme, Brush, and other machines. Here N and s are the soft iron pole pieces of the two electro-magnets, between which the armature A revolves. This armature consists of a ring or "core" of soft iron wound with

coils of wire, but for the sake of simplicity only twelve turns are shown, all the turns being in one circuit, and each of them connected by a wire to a metal bar of the commutator c, carried by, but insulated from, the axle x of the armature. Two metal brushes bb' rub on these bars as they rotate past them and tap the current generated in the coils as the armature revolves. It will be seen that one of the brushes b is connected by wire to the coils of one of the electro-magnets, the other brush being connected to one wire running to the lamp, while the other electro-magnet is connected to the other wire running to the lamp. Now, as the armature is rotated from left to right, in the direction of the large arrows, currents of electricity are generated in its coils in the direction of the small arrows, and these being collected by the brushes, pass through the electro-magnets, heightening their inductive power, and then flow out to the electric lamps.

One of the best dynamo-electric generators is that of M. Gramme, shown in Fig. 53. It consists of two powerful electro-magnets м м and м' м', furnishing a strong magnetic field between the poles N s. In this space a series of coils of wire A, wound on an iron ring or hoop, and technically called an "armature," are mounted on an axle and rapidly rotated by a belt, from a motor running on the pulley P. The currents thus generated in the coils are led to a series of copper bars set round the axle at c, and a pair of metal brushes bb' rub upon these bars and carry off the current as the axle revolves. Each pair of bars as it passes the brushes delivers up its charge of current, and these successive charges make up the

continuous current which is led from the brushes by the conducting wires to the lamps.

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Another excellent machine of a different type is that of Messrs. Siemens Brothers, exhibited in Fig. 54. There M and M' are the electro-magnets, A is the revolving armature in which the currents are generated; c are the commutator bars, and b one of the two brushes for collecting the currents. In this machine the two magnetic poles are curving iron plates N S, and the coils of the armature are wound longitudinally along the axis.

The generator of Mr. Brush, employed by the AngloAmerican Brush Company, differs from these in points of detail. Fig. 55 is a sketch of the Brush armature, with four of the bobbins of wire removed

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to show the construction of the iron ring, which is deeply grooved to allow the heat generated in the metal by the action of the machine to radiate away. The complete machine is illustrated in Fig. 56, where the ring with its bobbins is shown mounted on an axle between two pairs of electro-magnets, another pair being behind those seen in front. The front pair have two curving or crescent poles of soft iron, both being "north" poles, while the back pair have similar pole-pieces of "south" polarity. These sickle-shaped poles nearly meet, and thus bring the entire ring with the exception of two opposite bobbins within their influence. There are eight bobbins on the ring of a machine for feeding sixteen arc lamps, and six of these are under the influence of the magnets at a time, three on each side. Each opposite pair of bobbins is connected together, and the free ends are connected to four contact pieces on the axle of the armature on which run four collecting brushes. The opposite bobbins are joined together because they give currents in the same direction, one by descending between a pair of "north" poles, and the other by ascending between a pair of "south" poles. As each pair of connected bobbins traverses the space between the "north" and "south" poles it is cut out of circuit, and therefore becomes idle until it comes under the influence of one or other pair of poles. The electro-magnets derive their current from the armature in the way we have described. The armature is rotated by a belt from a revolving shaft, or as in the figure by being coupled direct to the shaft of a steamengine E.

The Brush machines give very powerful currents,

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