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average projection machine doesn't have any arrangement for keeping the film centered on the sprockets while it is running through, and I wondered if there is possible danger of this occurring.
Mb. J. G. Jones: There is, if there is no provision for keeping the transverse position of the film in proper alignment with the sprocket teeth. It is a very difficult job to make a sprocket light and absolutely correct, the teeth on both sides being parallel with the axis. The sprocket, of course, should be properly set in line.
Me. Richardson: That is the job of the projectionist. If the projector sprockets are kept in correct alignment, the film will stay centered. I believe one or two of the old style professional projectors had an arrangement for holding the film central. It consisted of a film track with one stationary side and one movable side held against the film by a light spring. It was, however, discarded, though I do not remember for what reason. Possibly because of the opportunity it gave for projectionists to be careless in the matter of the alignment of the sprockets.
If the sprockets be kept in perfect alignment, there will be no reason why the film will not stay central over the aperture.
Mr. Renwick: I should like to ask Mr. Jones — this is purely an engineering paper and I am not an engineer — the limits of expansion and contraction in motion picture film for which it is possible to design satisfactory sprockets.
Mr. J. G. Jones: There is sometimes negative shrinkage of .2% and positive shrinkage of about 3% when film is old. Sprockets can be designed for running throughout this range.
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