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MOTION PICTURE PROJECTION 35
pushing it along — as, for example, a pump — and it can not be kept flowing without continuing the pressure. Electricity will not flow in a circuit unless there is a battery or other source of electricity in the circuit. The battery is for the purpose of providing an electric pressure. To this is given the name "electromotive force" — that is, a force which moves the electricity. This is usually abbreviated to "emf." The larger the number of cells which are joined in the circuit in such a way that their pressures will add, the greater the electric pressure in the circuit and the larger the current produced, just as the rapidity of flow of the water in the pipe line may be increased by increasing the pump pressure.
Resistance. — There is always some friction in pipe, whatever its size or material, and this hinders the flow of the water to some extent. If it were not for the friction, the water would increase indefinitely in speed. Similarly, there is friction in the electric circuit. This is called the "resistance" of the circuit. The greater the resistance the smaller the current which can be produced in the circuit by a given battery, just as the greater the friction the less rapid the flow of water with a given pump acting. A resistance coil at any point in the circuit corresponds to a partially closed valve in the pipe at any point (Fig. 5.).
Steady and Variable Currents. — If a pipe is connected to a large reservoir of water maintained at the same level, the steady pressure of the constant head of water will cause a steady flow of water in the pipe. The quantity of water which will pass a given point in one second will be the same at all times. Certain sources of elec