SILVER WIRE GALVANOMETER:
Another method of varying the light falling on the photo film on the cylinder consists in mounting two very minute overlapping shutters one on each of the two wires of an electric circuit suspended in a strong magnetic field. On these overlapping shutters a light source is focused, so that greater or lesser displacement of the shutters, by reason of varying strengths of current in the adjacent runs of the wire, allows more or less light to pass there between.
Another lens images these tiny shutters onto the film covered cylinder, so that when the shutters are opened by the incoming currents in the two wires, the light is concentrated on the film.
As the exposure depends on (a) the shutter openings, and the shutter opening on (b) the incoming current strength, and the incoming current strength on (c) the light values at the sending station, development of the film again gives a duplicate of the picture at the sending station.