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MANAGERS AND PROJECTIONISTS 989
Light Valve Recording
TWO methods for the recording of sound on the film are now being used in connection with Western Electric equipment, viz. : the Aeo lamp, as already described, and what is called a "Light Valve," in which is employed a fundamental principle of great importance and wide application. It is as follows:
Suppose we have what is called a magnetic field, that is, a space subject to the influence of a magnet, such as, we will say, the gap between the poles of an electromagnet. Now imagine that in this space we hold a conductor, across the direction of the field, a straight piece of copper wire, for example, held in the space between the pole-pieces. Then on passing a current through this wire, the wire will try to move sideways, that is, parallel to the face of the magnet pole. The wire will continue to exert a force, if it be held, or to move if movement be permitted, as long as it stays in the magnetic field and the current keeps flowing. If the direction of the current is reversed, the direction of the force or movement is also reversed.
This simple principle, or method of obtaining force and motion by electrical means, is utilized in some form in all the electric motors in the world, from those that drive fans and dentists' drills to those that propel battleships and haul the transcontinental trains over the Rocky Mountains. It is also the basis of the dynamic loud speaker, of the oscillograph, and of some types of galvanometers. Its use in the Western Electric light-valve