The revolving coils of the dynamo are called the armature, and the fixed electro-magnets are called the field magnets, and these latter may be two or more in number. When two are used they are arranged on opposite sides of the armature, and form what is known as the bipolar machine. A larger number constitutes the multipolar machine. The field magnets in the multipolar machine usually are arranged in radial position around the entire circumference of the revolving armature, and are held in a fixed circular frame. To give a clear idea of the principles of the dynamo, the bipolar machine is best suited for illustration, and is here given in Figs. 20 and 21, in which Fig. 20 represents the dynamo complete, and Fig. 21 a detail of the end of the armature and commutator. This armature consists of coils or bobbins of insulated wire, each section having its terminals connected with separate insulated plates on the hub, which plates are known as the commutator. When any section of the armature approaches the pole of a field magnet, the current induced in that section of the armature coils by the field magnet, is taken off from a corresponding plate of the commutator by flat springs, seen in Fig. 20, and known as brushes. The field magnets A and B, Fig. 20, are shown with only a few turns of wire about them for clearer illustrations of the connections, which are made as follows: The wire _a_ is extended in coils around the field magnet B, and thence around field magnet A, and thence to the upper brush on the commutator, thence through the wire coils or bobbins of the rotary armature C, and thence by the lower brush to the wire _b_. The terminals of the wires _a_ and _b_ extend to the point of utilization of the current, whether this be electric lights, motors, or other applications. In this illustration, the circuit, it will be seen, passes through both the coils of the field magnets and the coils of the armature, involving the principle of mutual excitation. [Illustration: FIG. 20.--BIPOLAR DYNAMO.]