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214 A. G. ZIMMERMAN [j. S. M. P. E.
synchronous motor and a train of gears driving two sprockets. As the film passed through the recorder, it was drawn over a cylindrical drum, to which a flywheel was attached by means of a shaft. This drum was isolated from the sprockets, and the sprocket-tooth pulsations, by means of loops of film threaded around suitable rollers, although the film was relied upon to drive the drum. Acoustical power recording methods could no longer be used; the method of recording on the film, in the PR-1 recorder, was an outgrowth of Hoxie's work with the telegraphic recorder and other work done in the laboratories of the General Electric Company on oscillographs and vibrators. In the field, the PR-1 recorder performed remarkably well, notwithstanding the obstacles with which it had to cope. It was the first commercial film recorder to be called upon to withstand the rebuffs of a slightly unsympathetic film industry, to say nothing of some rather belligerent directors and directors' staffs. A new era had dawned in the film world, and the PR-1 took its place in the front ranks of the invaders.
Recollections are not all that remain of the hectic days during the nascency of "sound" and its adoption by the already mature and undoubtedly independent silent film industry. Much was to be learned from the conditions prevailing in the studios under which the sound equipment would have to work. Where laboratory experiments had sufficed for the original developments, field experience was now to be had; and in so far as the recorder was concerned, commercial film presented problems, both chemical and mechanical. Only the mechanical difficulty of shrinkage and its relation to the attainment of constant speed will be discussed. In the PR-1 recorder, the isolating loops between the sprocket and the film drum were of such a nature that when a disturbance occurred in the speed of the film during recording, more filtering or damping action was required than the flywheel alone would furnish. This resulted in an unsteady motion of the film past the recording light beam.
A new recorder, the PR-3, was developed, in which was employed a mechanical device to compensate for the shrinkage of the film. In the PR-3, a synchronous motor was used to drive a flywheel through a pinion and gear, or a worm gear reduction. On the same shaft with the flywheel, within the recorder head, a sprocket was mounted, which pulled the film from the film magazine. A cone on this shaft was arranged to drive another cone on the recording drum shaft through the medium of an idler, the position of which was