Two electro-magnets are shown in Fig. 234, in which the North pole of one magnet is near the South pole of the other, and the magnetic field between the two lies in the approximately straight lines between the two magnets, as indicated by the dotted lines. If the wire _CD_ be moved across this field and its ends be joined, as by the metallic circuit _CEFD_, a current will flow in this circuit. The wire _CD_ may be made to revolve around the wire _EF_, passing in front of one pole and then in front of the other pole, as in Fig. 235. The current in the circuit will pass in one direction when the wire is passing one pole, and in the other direction when it is passing the other pole. The connection between this elementary arrangement and the dynamo is easily recognized. In the dynamo a magnetic field is produced by electric magnets, called “pole pieces,” and a considerable number of wires similar to the wire _CD_ are placed upon an armature so that they revolve in front of these poles. Each individual wire produces current first in one direction and then in another direction, as explained above; but if there be many wires there will always be the same number in front of the North, or positive pole, and the same number in front of the South, or negative pole, so that the total or resultant action is practically uniform, and may be made to produce a continuous current. Such a machine is the common direct current dynamo, or motor. [Illustration: FIG. 235.] A dynamo transforms mechanical into electrical energy, and a motor transforms electrical into mechanical energy. The two operations are reversible, and may be effected in the same machine; a dynamo may be used as a motor, or a motor may become a dynamo. _A dynamo is a motor when it is driven by a current of electricity, and it is a dynamo when it is driven by mechanical power and produces an electric current._ If a motor be driven by an engine, it can deliver a current of electricity which is able to operate other motors or electrical apparatus or lights. A simple form of electric machine is shown in Fig. 236, which is a general form of the electric motor. In this there are two projections of steel, _H_ and _G_, which are made electro-magnets by the current flowing through the wires wound around them from any source of electricity, such as a battery at _I_ and _J_. These magnets have poles facing toward an armature, _K_, on a shaft. The poles _G_ and _H_ are called the “salient” poles; the poles _M_ and _P_ are called the “consequent” poles. The magnetic flow or field is shown by the dotted lines. On the periphery of the armature are wires in the slots shown. As this armature revolves, there will be a tendency for electricity to flow through the wires. [Illustration: FIG. 236.]