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.
PNEUMATIC VALVE:
An interesting scheme of picture reception is known as the pneumatic light valve, the vibration of which causes a shadow band to oscillate across a lens opening into the camera.
In a circular center opening in a magnetized iron diaphragm is suspended an iron disc somewhat smaller than the opening. This small disc is magnetically held by its edge to the inside edge of the opening in the diaphragm, with its plane in the plane of the magnetic field of the diaphragm.
Upon the disc is mounted a tiny mirror, and as the suspension of the disc is in the magnetic field held there by the strength of the field itself, it is extremely easily disturbed, so that a small beam of light reflected from the mirror can be vibrated with a very little current through great amplitude.
As the beam of light has a transverse shadow band therein of a width to normally close the lens opening into the camera, the varying amplification of the vibration of the mirror, and therefore, of the shadow, admits a proportional amount of light.
SPARK GAP:
One of the very simplest light sources for exposing the film on the receiving cylinder consists of a minute spark-gap located in contact with the moving film.
The strength of the incoming current charges a small condenser until the gap breaks down and the passing spark exposes the film (or perhaps perforates it). If the current is strong the sparks pass the gap at a high frequency, while if the current is weak the frequency is less. The range may be from 500 to 5,000 per second perhaps, depending on the current strength, and, of course, the film exposure correspondingly varies, and the different degrees of density of the picture results.