and wire is wound upon this in a particular manner so as to form a continuous coil, which may (as it generally does) or may not surround some soft iron. When the armature is removed from a machine, it has somewhat the appearance of a short bolster with a spindle pushed through it. The bolster part, on close examination, will be found to consist of the coil of wire already mentioned. This wire-coil portion revolves between the magnet-poles. The spindle, at one end, carries a number of copper plates, placed radially and close together, so as to form in appearance a solid cylinder, termed the commutator. Each plate is insulated from the next and from the spindle. By insulation is meant that some substance through which the electricity will not pass is placed between plate and plate, and between plates and spindlee. Upon this commutator rest two brushes, to which are attached the wires that lead to any point where electrical energy is required. Every plate in the commutator has a wire joined to it, the other end of the wire being in connection at certain points of the armature-coil. It will thus be seen that any current produced in the armature will flow to the copper plates and enter the brushes which press upon them, and so pass on into the wires (often termed cables, mains, leads, or lines) and travel to the point required. When the armature is revolved, the plates of the commutator successively pass the brushes, consisting of a group of fine copper wires or thin plates, which should be of considerable length, in order to give them elasticity and to allow for wear and tear. A portion of the current produced is employed to excite the electro-magnets. Permanent magnets may be used,—in fact, they are still used in France, for dynamos employed in light-houses,—but for general purposes the electro-magnets are cheaper and have many other advantages. A piece of soft iron, wound round and round with wire which has a current passing through it, becomes a powerful magnet; and the iron is then said to be excited. When the electro-magnet of the dynamo is excited, the armature is in an extremely powerful magnetic field; which means that it is strongly under magnetic influence. If the spindle is revolved under these conditions and the wires leading from the brushes are connected so as to form a closed circuit, it will be found that the power required to turn the spindle increases as the speed is raised. The best comparison to make is that of turning a fan in a barrel of treacle: the more quickly the fan is turned the greater will be the resistance to its motion.
The very fact of revolving the armature, which is nothing more than a specially-wound coil of wire close to the poles of a magnet, produces a current of electricity in the coil; and the faster this coil is turned the greater will be the pressure of the current produced. The quantity produced is dependent upon the resistance of the circuit. The resistance is made up, in the simple case imagined, of the wire in the armature, the length of which is invariable, and the length of the wire joined to the brushes, which is variable. Evidently the longer this wire is, the more resistance will be offered to the passage of the current; and if the pressure were to remain unaltered for two different lengths of this wire, the quantities of current which would flow through it myst necessarily vary with its length,—i.e., if the armature-
