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732 Transactions of S.M.P.E., Vol XII, No. 35, 1928
many considerations the valve is tuned to 7000 cycles per second. Under these circumstances about 10 milliwatts of A.C. power are required for full modulation at a frequency remote from resonance ; about one one-hundredth of this power at the resonant frequency. The impedance of the valve with protecting fuse is about 12 ohms. If this appliance is interposed between a light source and a photographic film we have a camera shutter of unconventional design. Figure 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
PLANE OF RIBBON OF LIGHT
CONDENSING LENS SYSTEM
PLANE OF VALVE RIBBONS f0.002"X 0.256" ^ SLIT
PLANE OF
IMAGE ON
FILM
fO.OOf X 0.128' ^
v IMAGE J
OBJECTIVE LENS SYSTEM
Fig. 2. Diagram of the optical system for studio recording.
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 1 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.
Figure 3 shows a studio recording machine with the door of the exposure chamber open. In this machine the film travels at 90 feet per minute, and the sound track is made at the edge away from the observer. The line of light, the image of the valve, overruns the perforations by 6 mils, extending toward the center of the film 122 mils inside the perforation line. The right-hand sprocket serves to draw film from the feed magazine above and to feed it to the take-up magazine below; this sprocket is driven from the