Electric currents must always circulate in closed paths--that is, the current that starts out from a generator must return to it, and the amount coming back is the same as that which leaves. The action of an electric generator can be understood by comparing it with that of a water pump pumping into a pipe which runs around from the delivery end to the suction. With such an arrangement it can be seen that the action of the pump would be to keep the water in circulation, but the same water would be pumped through the pump and the pipe all the time. With an electric generator the action is the same, and in Fig. 22 the current flowing along any one of the tracks follows the course indicated by the arrows. The currents pass out to the several tracks through the trolley wires _T T T T_, and return through the tracks _R R R R_. The bus bar _A_ is connected with a plate _D_, which is imbedded in the ground, and is also connected with the ends of the rails _R R R R_. Suppose for a moment that the two lower generators are out of service, their switches _a a_ being turned so as to disconnect them from the bus _A_, and, further, suppose that the three lower _b_ switches are open, so that the current can only pass to the upper track; then the top generator will feed into the top road only. Tracing the path of the current under these conditions, we find that it will start from the upper side of the generator through the _a_ switch to the _B_ bus, and thence to the trolley wire at the top of the figure. On reaching the first car a portion of the current passes to the track _R_, the amount being dependent upon the speed of the car and the load. Why the whole current does not follow this path generally puzzles the layman, but the explanation is that the motors hold the current back, and only allow as much to pass through them as is necessary to perform the required work--that is to say, the current flowing through each car is not controlled by the generator or by the force of the current, but by the requirements of the motors. The amount of current delivered by the generator is governed by the demands of the motors. The current that does not pass through the first car goes on to the second one, and if there were more cars there would be current left in the trolley wire to supply them. After passing through the motors of the two cars the current returns through the rails _R_ to the plate _D_, and thus to bus _A_, from which it enters the lower side of the top generator. It will from this explanation be seen that the action of the generator is simply to keep the current circulating. If two of the generators are connected with the bus bars _A_ and _B_, the current required by the motors will be delivered by the two machines, and if the three generators are placed in service the current will be divided among them.