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

Another excellent machine of a different type is that of Messrs. Siemens Brothers, exhibited in Fig. 54. There м 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

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,

and are intended to feed a number of arc lamps. The currents, therefore, like those for the Jablochkoff candle, are somewhat dangerous to life, if not properly insulated on the wires, and one or two fatal

accidents have happened with them out of carelessness. The currents from ordinary dynamos constructed to feed one or two arclamps or a number of incandescent lamps are not at all dangerous to life and limb, and hence no word of warning need be given with respect to them. The Edison machine for instance, which we illustrate in Fig. 57, does not generate currents which are injurious to life. Here the

stout pole-pieces n s, which are bored out to allow the bobbin or armature E to revolve between them. The bobbin wires run lengthwise, like those in the