In a sound camera, an electromagnetic device that swings a mirror in response to signal variations (1985).
These mirror galvanometers vary the position of a light beam of constant intensity and became the variable area system associated with R.C.A.
The design of a mirror galvanometer comprises a coil of wire wound on a soft iron core suspended in the magnetic field of a permanent magnet – this coil is suspended generally by means of a strip of phosphor bronze wire and located centrally between the poles of the permanent magnet. The coil will react to a coupling force the extent of which is proportional to the current flowing through the coil, and this will cause the coil to rotate.
A small mirror is attached to the coil so that a beam of light reflected by this mirror will sweep through an angle which is proportional to the current passing through the coil.
Case, D., 1985, Motion Picture Film Processing, Media Manuals, Focal Press.