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.