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.