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CINEMATOGRAPHIC ANNUAL
normal width. The intensity of the light is unchanged. A fixed source of light shines upon a loop, the sides of which open and close and the width of the image as it varies from zero to one mil varies the time it takes for the film to pass the exposure point.
FIGURE 1
Fig. 1 shows a photograph of the light valve, invented in 1922 by E. C. Wente of the Bell Telephone Laboratories. Essentially, it consists of a loop of duralumin tape suspended in a plane at right angles to a magnetic field. The tape, 6 mils wide and 0.5 mil thick, is secured to windlasses A and A l and stretched tight by the spring held pulley B. At points C and C l insulated pincers confine the central portions of the tape between windlasses and pulley to form a slit 1 mil wide. Supporting this loop and adjusting devices is a slab of metal with central elevation D, which constitutes the armature of an electromagnet. The central portions of the loop are supported on insulating bridges to lie 3 mils above the face of D ; here the sides of the loop are centered over a tapered slot, 8 mils wide by 256 mils long in this plane, opening to 204 mils by 256 mils at the outside face of the armature. Viewed against the light, the valve appears as a slit 1 mil by 256 mils.
The electromagnet core has a similar elevation opposing D across an air gap of 8 mils which closes to 7 mils when the magnet is energized from a 12 volt battery. A tapered slot in the magnet core begins 8 mils wide by 256 mils long and opens with the same taper as the slot in the armature. When the assem