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SOUND REDUCTION PRINTING 321
the accuracy with which any given film will exactly mesh with such sprockets, is lower than desirable for adequate reduction printing. Whilst this is no longer true in some cases, it certainly does hold true unless the highest quality workmanship and design are employed to produce sprockets within very close tolerances and, since a large number of printers do rely on this method of film traction, it is necessary to consider very briefly the main factors controlling sprocket design.
Diagram 'A', Figure 141, shows the relationship between a section of film and a sprocket wheel specifically designed to mesh with that film. Here it is seen that, over the area of complete contact between the film and sprocket, and excluding those sections which are partially engaged, every sprocket tooth is in contact with the leading edge of its respective perforation and, in consequence, the driving load is equally distributed amongst all the engaged teeth. If this condition could be maintained in practice the film would move at speeds exactly equal to those of the sprockets themselves that is, the only variations in film speed would be due to variations in the gear train and no 'flutter' would be introduced as one perforation ceased to be engaged or a new perforation was brought into contact with the teeth.
As is well known, both nitrate and acetate film bases are continually shrinking (if only by a very minute amount) throughout their useful life although, super-imposed upon this very gradual contraction, may be other and larger variations in length solely dependent upon the atmospherics conditions. These larger variations may either increase or decrease the apparent film length but, unless exceptionally severe, will not cause a permanent deformation and films subjected to changes in humidity may easily be reconditioned to bring their dimensions back almost to those which obtained before the changes took place. In a paper published in the September 1945 issue of the Journal of the Society of Motion Picture Engineers, R. H. Talbot, of the Eastman Kodak Company gives the following figures for the shrinkage of nitrate film: (a) that due to processing may be expected to be 01 % along the film length and 0-15% across its width, (b) that due to changing humidity may be 0 • 2 % along the length and 0-25% across the width, (c) the shrinkage which may be expected after the film has been kept for one year is approximately 0-3 % along the length and 0-4% across the film width.
Diagram *B', Figure 141 indicates the condition which exists when film having a short perforation pitch is meshed with a sprocket made to the correct dimensions. It is important to notice that,
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