Galvanometers for Variable Area Recording
G. L. Dimmick · Journal of the Society of Motion Picture Engineers · 1930
In the process of recording sound on film by the variable area method, it is necessary to modulate a narrow beam of light in such manner that its length at any instant is directly proportional to the sound pressure on a microphone diaphragm. This result has been successfully accomplished in four steps as follows: (1) A microphone converts the sound energy into electrical energy. (2) An amplifier makes it possible for the microphone to control a relatively large amount of electrical energy. (3) A galvanometer converts this electrical energy into mechanical energy in the form of rotational vibrations of a small mirror. (4) An optical system enables the mirror to control the length of a small beam of light which is focused upon the film. This paper is concerned with a description of several types of galvanometers used in this process and a discussion of factors in their design.