A Wilde's machine 14. ton in weight, measuring about 5 ft. in leno,h and height, and 20 in. wide, driven by a steam-engine pro duces a most brilliant electric light, and exhibits the most astonishing heating powers.
Wheatstone and Siemens gave a new interpretation to 1Vilde's principle. Their important discovery is of the following nature: Suppose the upper machine in fig. 1 removed, and that we have ing but the electro-magnet and armature left. If the wires ing from the binding-screws of the armature be joined up with the electro-magnet, we might fancy that, there being no permanent magnetism, no result would follow on 'the armature being moved. Sitcli. however, is not the case. If the armature be moved at any velocity, it will soon be brought to a halt by the mutual action ing: In the clectro-inagnet there is always some magnetism left. This induces a feeble current in the coil, but this is sufficient to make the magnet stronger and able to induoe a stronger current, and this ciprocal action continues until it grows to an enormous intensity. So great, indeed, would it becomc, that if we had sufficient mechanical energy at our disposal to persist in the motion, the coils of armature and electro-magnet would be melted, and the machine destroyed. This startling discovery may, however, be thought of little value, as a machine that consumes its own electricity is of no external usc. All machines now -work on this reciprocal principle, and a description of them will best show how it is Aurnecl to account. Ladd was the first to construct a machine on Wheatstone and Siemens's principle. In the armature there are two unequal coils, the larger for fur nishing the external current, the smaller for exciting the electro-magnet. These two coils revolve together, the one at right angles to the other, in the saine magnet cylinder. Iu large machines he uses two magnet-cylinders, one at each end of the electro-magnet; or rather, he uses two electro-magnets, and the two armatures cora plete the magnetic circuit. Ferguson of Edinburgh alters Ladd's arrangement in using only one piece of iron for the armature of the machine with two grooves cut in it, a larger one for the coil giving the external current, and a smaller one for the exciting current. This offers the advantage that the heating, of the solid iron of
the armature by repeated magnetism is lessened by being transformed into an electric current. The electro-magnet is thus fed by a current obtained not by an additional expenditure of energy, but by the utilization of force that would be otherwise converted into useless or even hurtful heat.
The great drawback of all the forms of the machine just described is the enormous velocity at which they rotate—some 2,000 or more revolutions in the minute. At this speed a machine soon wears itself out. Another disadvantage is the heating of the armatures in Wilde and Ladd's machine. Ferguson's has never been tried on a large scale. It is found necessary to keep the armatures cool by a flow of cold water. This heat, how ever removed, is manifestly a mere squandering of the energy of motion, and a loss to the current rriven off. A third objection is the loss that always takes place when the side springs e'shange from the one ring to the other, sparks more or less bright accompanying the change. For the electric light; however, the alternate currents are used, and this source of loss is not experienced. These defects are removed in the latest form of the electro-magnetic machine by Gramme of Paris. In it, instead of a solid armature of iron, a ring is employed on which a great number of bobbins of wire are set. Fig. 4 is intended to explain the rudimentary prin . ciple of it. The ends of the wires of two • conticruous bobbins are soldered to strips of maal called sectors. These are shown as radii in the s figure. In the machine itself they are first brought down radially, then turned at right angles so as to be parallel to .the axis of the machine. They are very numerous (though few in the figure), and being separated from each -other by sheets of silk, form a compact whole. Metallic brushes, B, B, rub on the end face of the sectors, and form the poles of the revolving armature. The principle of action may be thus under .stood: Suppose we first ascertain what . . .