=164. Electric Cars=, as well as boats, automobiles, etc., etc., are moved by the power that comes from electric motors, these receiving current from the dynamos placed at some "central station." We have already seen how the motor can do many kinds of work. By properly gearing it to the car wheels, motion can be given to them which will move the car. [Illustration: Fig. 237.] Fig. 237 shows two dynamos which will be supposed to be at a power house and which send out a current to propel cars. From the figure it will be seen that the wires over the cars, called trolley-wires, are connected to the positive (+) terminals of the dynamos, and that the negative (-) terminals are connected to the tracks. In case a wire were allowed to join the trolley-wire and track, we should have a short circuit, and current would not only rush back to the dynamo without doing useful work, but it would probably injure the machines. When some of the current is allowed to pass through a car, motion is produced in the motors, as has been explained. As the number of cars increases, more current passes back to the dynamos, which must do more work to furnish such current. _Trolley-poles_, fastened to the top of the cars and which end in grooved wheels, called _trolley-wheels_, are pressed by springs against the trolley-wires. The current passes down these through switches to _controllers_ at each end of the car, one set being used at a time. [Illustration: Fig. 238.] [Illustration: Fig. 239.] =165. The Controllers=, as the name suggests, control the speed of the car by allowing more or less current to pass through the motors. The motors, resistance coils and controllers are so connected with each other that the amount of current used can be regulated. [Illustration: Fig. 240.] [Illustration: Fig. 241.] When the motorman turns the handle of the controller to the first notch, the current passes through all of the resistance wires placed under the car, then through one motor after the other. The motors being joined in series by the proper connections at the controller, the greatest resistance is offered to the current and the car runs at the slowest speed at this first notch. As more resistance is cut out by turning the handle to other notches, the car increases its speed; but as the resistance wires become heated and the heat passes into the air, there is a loss of energy. It is not economical to run a car at such a speed that energy is wasted as heat. As soon as the resistance is all cut out, the current simply passes through the motors joined in series. This gives a fairly slow speed and one that is economical because all the current tends to produce motion.