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May, 1937] NEW MOTION PICTURE APPARATUS 527
all subsequent models of Photophone studio recorders, the magnet is connected to the driving motor and driven at about 10 per cent higher speed than the drum."1 By this expedient the continuous component of torque due to the difference in mean speed is caused to help rotation instead of producing a drag. Enough torque is supplied to the drum shaft by the magnet to overcome the bearing friction and relieve the film of the work of rotating the drum. As the magnet current is increased, the loop of film that pulls the drum is gradually relieved of its steady tension until this becomes zero, and with still higher magnet current, the slack film is thrown into this loop and the loop above the drum begins to exert a retarding tension. Conditions for guiding the film are better when the film approaching the drum is under a slight tension. The recorders are therefore operated with more than enough magnet current to overcome friction losses. Before a recording is made a piece of "daylight film" is run through the recorder and the magnet current is adjusted to the value that throws the slack film into the lower
NEGATIVE
DRUM WITH ROTARY
STABILIZER
\ JT^WL I A I '
PRESSURE ROLLER
EXPOSED FILM —
FIG. 15. Principle of the non-slip printer.
loop (film leaving the drum) and the upper loop under very slight tension, such that it forms an easy bow (see Fig. 12) which possesses a high degree of flexibility but can still exert enough tension to effect the final control of the drum speed. Such a loose loop of film can transmit little or no disturbance from the sprocket. During starting, the lower film loop is under tension for accelerating the drum, but the magnet assists in the acceleration. When the drum is up to speed the film loops are relieved of tension, and with the oil films in the bearings established, the drum almost floats along, held at constant speed by its flywheel with no appreciable forces to disturb the speed constancy. The oscillation produced by starting dies out in a few seconds, The path of the film and location of all guiding rollers has been carefully worked out to facilitate the formation of easy film loops, to lay the film upon the drum accurately directed and guided, to prevent edgewise movements or weaving, and to insure its lying snugly against the drum at the recording point. Fig. 12 is a front view of the PR-4 recorder, showing the drum and film path. The film loops shown are fairly representative of conditions during running. Fig. 13 shows the most recent type of magnetic drive recorder.