[Illustration: FIG. 19.--Diagram showing principle of Dynamo producing Alternating Current.] Having seen something of the underlying principle and of the history of the dynamo, we must turn our attention to its actual working. Fig. 19 is a rough representation of a dynamo in its simplest form. The two poles of the magnet are shown marked north and south, and between them revolves the coil of wire A¹ A², mounted on a spindle SS. This revolving coil is called the armature. To each of the insulated rings RR is fixed one end of the coil, and BB are two brushes of copper or carbon, one pressing on each ring. From these brushes the current is led away into the main circuit, and in this case we may suppose that the current is used to light a lamp. In speaking of the induction coil we saw that the currents induced on making and on breaking the circuit flowed in opposite directions, and similarly, Faraday found that the currents induced in a coil of wire on inserting and on withdrawing his magnet flowed in opposite directions. In the present case the magnet is stationary and the coil moves, but the effect is just the same. Now if we suppose the armature to be revolving in a clockwise direction, then A¹ is descending and entering the magnetic field in front of the north pole, consequently a current is induced in the coil, and of course in the main circuit also, in one direction. Continuing its course, A¹ passes away from this portion of the magnetic field, and thus a current is induced in the opposite direction. In this way we get a current which reverses its direction every half-revolution, and such a current is called an alternating current. If, as in our diagram, there are only two magnetic poles, the current flows backwards and forwards once every revolution, but by using a number of magnets, arranged so that the coil passes in turn the poles of each, it can be made to flow backwards and forwards several times. One complete flow backwards and forwards is called a period, and the number of periods per second is called the periodicity or frequency of the current. A dynamo with one coil or set of coils gives what is called “single-phase” current, that is, a current having one wave which keeps flowing backwards and forwards. If there are two distinct sets of coils we get a two-phase current, in which there are two separate waves, one rising as the other falls. Similarly, by using more sets of coils, we may obtain three-phase or polyphase currents. [Illustration: FIG. 20.--Diagram showing principle of Dynamo producing Continuous Current.]