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Oct., 1941] MECHANICAL AND OPTICAL EQUIPMENT 355
roller which presses the film against the sound roller with sufficient force to prevent film slippage. This roller bears only on a narrow portion of the film along its center line. It is mounted in a fork which is pivoted at p\ so that the roller can seek a position in which it will not exert sidewise thrust on the film. The axis of this pivot is rigidly connected to the lever arm a\t which in turn is pivoted on the horizontal axis £2. The pressure roller tension is controlled by adjustment of the spring Si connected to the lower end of the lever arm a\. The purpose of 8 is to give the film a relatively large angle of wrap around the sound roller, thus providing good seating for the film at the translation points and greater insurance against slippage. A damping roller is rigidly mounted on the sound roller shaft.
While the power needed to drive the sound roller under steadystate conditions is small, the driving torque must be increased considerably when the roller is being brought up to speed from rest because of the relatively high moment of inertia of the damping roller. During this time, the roller 6 must be pressed against the film with increased force if film slippage is to be prevented. The linkage, shown by the dashed lines in the figure, is provided for this purpose. The lever / of this linkage is pivoted at p%. A loose-fitting rod r connects the arm a\ to the lower arm of the lever /. The upper end of the lever / carries the supporting shaft of the flanged roller 9. The spring $2 forces this shaft against a stop so that, in normal operation, the axis oi roller 9 is virtually fixed. When the sound roller is being accelerated, the tension-of the film between it and the leading sprocket will increase, the upper arm of the lever / will be pulled down from its stop, the slack in the bearings of the rod r will be taken up, and pressure of roller 6 on the film will be increased until the sound roller is up to speed, when the film tension will become normal and the shaft of roller 9 will again be pulled back against the stop. With this arrangement, the sound roller is brought up to speed in about four seconds.
Flanged reels may be used in reproducing, but in recording the film must be wound in a light-tight magazine. It is practically impossible to avoid a certain amount of rubbing of the film against the side walls by the time most of the film has passed into the magazine. Consequently, a greater torque must be applied to the take-up shaft when the magazine is nearly full than would be necessary for a flanged reel holding the same amount of film. If the take-up shaft were driven by a slipping clutch, the torque would be the same for the