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WESTERN ELECTRIC SYSTEM
373
bly of magnet and armature is complete, the valve constitutes a slit 1 mil by 256 mils, its sides lying in a plane at right angles to the lines of force and approximately centered in the air gap.
The windlasses A and A1 , one of which is grounded, are connect
PLANE OF RIBBON OF LIGHT
CONDENSING LENS SYSTEM
PLANE OF
VALVE
RIBBONS
fO.OOl "X 0.256"')
^ SLIT *
PLANE OF
IMAGE ON
FILM
( 0.0005X0.128")
IMAGE
OBJECTIVE LENS SYSTEM
DIAGRAM OF OPTICAL SYSTEM IN STUDIO RECORDING
FIGURE 2
ed to the output terminals of the recording amplifier. If the magnet is energized and the amplifier supplies current from an oscillator, the duralumin loop opens and closes in accordance with the current alterations. Length and tension of the vibrating part are so chosen that its resonance is at 8500 cycles which puts it out of the range of the conspicuous cycles in speech and music.
If this appliance is interposed between a light source and a photographic film we have a camera shutter of unconventional design. Fig. 2 shows a diagram of the optical system for studio recording. At the left is a light source, a ribbon filament 18 ampere projection lamp, which is focussed on the plane of the valve. The light passed by the valve is then focussed with a 2 to 1 reduction on the photographic film at the right. A simple achromat is used to form the image of the filament at the valve plane, but a more complicated lens, designed to exacting specifications by Bausch and Lomb, is required for focussing the valve on the film. The undisturbed valve opening appears on the film as a line % mil by 128 mils, its length at right angles to the direction of film travel. The width of this line varies with the sound currents supplied to the valve, so that the film receives a varying exposure: light of fixed specific intensity through a varying slit. (See Fig. 6 on page 381.)
Both the aeo light and the light-valve result in variable den