Motion picture projection : an elementary text book (1928)

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MOTION PICTURE PROJECTION 39 physiological effects caused by the passage of the current. The electric lamp has an effect on the eye. We do not, however, see the electric current in the lamp, but the effect on the eye is due to the light waves sent off by the hot filament. The energy of the current has been changed over into heat in the lamp. When we hear a sound in the telephone receiver it is not the electric current we hear, but merely the vibration of the thin diaphragm. The electric current has used some of its energy in causing the diaphragm to vibrate. The acid taste noticed when the tongue is placed across the poles of a dry battery is due to the chemical decomposition of the saliva into other compounds as a result of the passage of the current through it. Electromotive Force. — When a difference of electrical potential exists between two points, there is said to exist an electromotive force, or tendency to cause a current to flow from one point to the other. This electromotive force is analogous to the pressure, caused by a difference in level of two bodies of water connected by a pipe. The pressure tends to force the water through the pipe, and the electromotive force tends to cause an electric current to flow. Electromotive force is commonly designated by the letters E. M. F. or simply E. It is also referred to as pressure or voltage. Current. — A current of electricity flows when two points, at a difference of potential, are connected by a wire, or when the circuit is otherwise completed. Similarly, water flows from a high level to a lower one, when a path is provided. In either case the flow can take place only when the