Richardson's handbook of projection (1930)

990 HANDBOOK OF PROJECTION FOR recording machine is almost exactly in the simple fundamental form just described. In this machine, a powerful electromagnet with tapered pole-pieces creates an intense magnetic field in the space between its poles. Stretched side by side across this space are two tiny flat ribbons of duralumin a few thousandths of an inch thick. These ribbons lie so close to each other that only two-thousandths of an inch separates their adjoining edges. Under this condition, on passing a current up one ribbon and down the other, they will both move sideways, but, be it noted, in opposite directions, because the current is flowing through one in the opposite direction to what it is flowing in the other. The result is that with current flowing one way, the ribbons come closer together; the other way, they separate further apart. Therefore the alternating current which represents sound waves causes them to vibrate, and the separation between them increases and decreases correspondingly with the back and forth movement of the air in the sound waves. An opening is provided in each pole face, opposite the separation between the ribbons, therefore if a light be caused to shine into one opening, the width of the beam that passes through will depend on the separation of the ribbons. In film recording by the variable density method the basic problem is to vary the exposure of the sound track in accordance with the frequency and loudness of the recorded sound. Photographic exposure can be varied either by changing the intensity of the illumination or by changing the duration of exposure. The former method is, of course, the one followed with the Aeo