In Fig. 68 the armature, moving at right angles to the lines of force, cuts a maximum number in a given time, and the current induced in the coil is therefore now most intense. Here we must stop a moment to consider how to decide in which direction the current flows. The armature is revolving in a clockwise direction, and y z, therefore, is moving downwards. Now, suppose that you rest your left hand on the N. pole of the magnet so that the arm lies in a line with the magnet. Point your forefinger towards the S. pole. It will indicate the direction of the lines of force. Bend your other three fingers downwards over the edge of the N. pole. They will indicate the direction in which the conductor is moving across the magnetic field. Stick out the thumb at right angles to the forefinger. It points in the direction in which the induced current is moving through the nearer half of the coil. Therefore lines of force, conductor, and induced current travel in planes which, like the top and two adjacent sides of a box, are at right angles to one another.
While current travels from z to y—that is, from the ring C1 to y—it also travels from x to w, because w x rises while y z descends. So that a current circulates through the coil and the exterior part of the circuit, including the lamp. After z y has passed the lowest possible point of the circle it begins to ascend, w x to descend. The direction of the current is therefore reversed; and as the change is repeated every half-revolution this form of dynamo is called an alternator or creator of alternating currents. A well-known type of alternator is the magneto machine which sends shocks through any one who completes the external circuit by holding the brass handles connected by wires to the brushes. The faster the handle of the machine is turned the more frequent is the alternation, and the stronger the current.
Fig. 69.
CONTINUOUS-CURRENT DYNAMOS.
An alternating current is not so convenient for some purposes as a continuous current. It is therefore sometimes desirable (even necessary) to convert the alternating into a uni-directional or continuous current. How this is done is shown in Figs. 69 and 70. In place of the two collecting rings C C1, we now have a single ring split longitudinally into two portions, one of which is connected to each end of the coil w x y z. In Fig. 69 brush B has just passed the gap on to segment C, brush B1 on to segment C1. For half a revolution these remain respectively in contact; then, just as y z begins to rise and w x to descend, the brushes cross the gaps again and exchange segments, so that the current is perpetually flowing one way through the circuit. The effect of the commutator[17] is, in fact, equivalent to transposing the brushes of the collecting rings of the alternator every time the coil reaches a zero position.
Figs. 71 and 72 give end views in section of the coil and the commutator, with the coil in the position of minimum and maximum efficiency. The arrow denotes the direction of movement; the double dotted lines the commutator end of the revolving coil.
Fig. 70.