Batteries make enough electricity flow to do a good deal of work. But they are rather expensive, and it takes a great many to give a flow of electricity sufficient for really heavy work, such as running street cars or lighting a city. Fortunately there is another way of getting large amounts of electricity to flow. This is by means of dynamos. HOW A DYNAMO MAKES A CURRENT FLOW. To understand a dynamo, you must first realize that there are countless electrons in the world--perhaps all things are made entirely of them. But you remember that when we want to get these electrons to do work we must make them flow. This can be done by spinning a loop of wire between the poles of a magnet. Whenever a loop of wire is turned between the two poles of a magnet, the magnetism pushes the electrons that are already in the wire around and around the loop. As long as we keep the loop spinning, a current of electricity flows. [Illustration: FIG. 114. Spinning loops of wire between the poles of a magnet causes a current of electricity to flow through the wire.] If only one loop of wire is spun between the poles of a magnet, the current is very feeble. If you loop the wire around twice, as shown in Figure 114, the magnet acts on twice as much of the wire at the same time; so the current is stronger. If a very long piece of wire is used and is looped around many times, and the whole coil is spun rapidly between the poles of a powerful magnet, myriads of the electrons in the wire rush around and around the loops--a powerful current of electricity flows through the wire. [Illustration: FIG. 115. The more loops there are, the stronger the current.] Now suppose you bring one loop of the long wire out, as shown in Figure 115, and suppose you spin the rest of the loops between the poles of the magnet. Then, to flow through the loops by the magnet the electricity will have to go clear out through the long loop and back again. While it is flowing through this long loop, we can make it work. We can cut the long loop and attach one broken end to one part of an electric lamp and the other end to the other part, so that the electricity has to flow through the lamp in order to get back to the spinning coil of wire, as shown in Figure 116. Such an arrangement as this is really an extremely simple dynamo. [Illustration: FIG. 116. If the electricity passes through a lamp on its way around the circuit the filament of the lamp glows.] [Illustration: FIG. 117. A dynamo in an electric light plant.]