Motion picture projection : an elementary text book (1928)

30 MOTION PICTURE PROJECTION to flow through a pipe. The point to be settled in the minds of those taking up electricity is that the dynamo merely sets into motion something already existing, by generating sufficient pressure to overcome the resistance to its movement. Although we speak of alternating and direct current, it should be clearly understood that it is impossible to get a continuous current with a dynamo. The current is really a pulsating one, but the pulsations are so small and follow each other so quickly that the current is practically continuous. It is common knowledge that a battery supplies what is known as a current of electricity. To obtain the current there must be a complete, closed, conducting path from the battery through the apparatus which is to be acted on by the current, and back again to the battery. For example, when connecting up an electric bell, a wire is carried from one binding post of the battery (Fig. 1) to one of the binding posts of the bell, and a second wire is brought from the other binding post of the bell back to the remaining binding post of the battery. Any break in the wire immediately causes the current to stop and the bell to be silent. This furnishes an easy method of controlling the ringing of the bell, since it is only necessary to break the circuit at one point to stop the current, or to connect across the gap with a piece of metal to start the current going again. Thus the battery supplies the power to operate the bell, and the button opens and closes the circuit and thus controls the delivery of that power to the bell. Similar considerations apply when using the city lighting circuit. Wires