number of turns of insulated wire. It will be understood that if the magnet m, fig. 2, were a permanent magnet instead of an electro-magnet, movement would still occur, and an electromagnet can be made much more powerful than a permanent magnet. The power of such a motor may consequently thus be increased.
The ring shown in fig. 3 represents a Gramme ring, which was one of the earliest and simplest forms of armature, i.e. revolving portion, employed in electric motors. In practice the current is not brought to it as shown in fig 3, but a device called a commutator, shown at k, fig. 2, is employed. The ring is mounted on a spider or on a centre which revolves in suitable bearings. The commutator k consists of a ring of conducting segments r r, separated by insulating pieces o o. Each one of the conducting segments r r is joined up by a wire or rod to equidistant portions of the armature winding w, so that the current supplied to the commutator by the brushes b, b1 enters the winding w, in the same manner as shown in fig. 3.
Fig. 2 shows a simple form of electric motor, of what is called the 'separately excited' type—that is to say, the electro-magnet is rendered magnetic or excited by an electric current proceeding from some separate source of electricity, such as a battery separate from that which supplies the electric current to the rotating part or armature. Fig. 3 represents, as already stated above, the Gramme ring. This is a form of armature which is but little used in electric motors, some form of drum armature being now almost universally adopted. The drum armature is a development of the Siemens shuttle armature, and will be best understood from the inspection of a section of that arrangement, fig. 4. In this section x is the spindle of the armature, c being the iron core, shown in the rounded h section, and w the coil of wire covered with insulating material i. The whole arrangement is very much longer than it is thick, and really does resemble a shuttle. The two poles are formed by the sides of h. Instead
