While current travels from _z_ to _y_--that is, _from_ the ring C^1 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. [Illustration: 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 C^1, 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 B^1 on to segment C^1. 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. [Illustration: FIG. 70.] PRACTICAL CONTINUOUS-CURRENT DYNAMOS. The electrical output of our simple dynamo would be increased if, instead of a single turn of wire, we used a coil of many turns. A further improvement would result from mounting on the shaft, inside the coil, a core or drum of iron, to entice the lines of force within reach of the revolving coil. It is evident that any lines which pass through the air outside the circle described by the coil cannot be cut, and are wasted. [Illustration: FIG. 71.] [Illustration: FIG. 72.] The core is not a solid mass of iron, but built up of a number of very thin iron discs threaded on the shaft and insulated from one another to prevent electric eddies, which would interfere with the induced current in the conductor.[18] Sometimes there are openings through the core from end to end to ventilate and cool it. [Illustration: FIG. 73.]