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Sept., 1930] WIDE FILM SHRINKAGE 311
film, a 20 tooth take-up sprocket, Blt presents considerable advantage over the 16 teeth illustrated at^l1, though both can accommodate film shrunk within the accepted limits without danger of interference. The take-up sprocket dimension can therefore be established as follows:
Base diameter of sprocket 1 . 1679 in.
Circular pitch 0 . 1844 in.
Tooth thickness 0 . 0500 in.
The shape of the tooth to be within the involute curve generated on the base circle.
It remains now to calculate the transversal dimensions of the sprocket and sprocket teeth.
In reference to Fig. 1, it shall be assumed that the over-all width of the film is 70 mm. and that in consideration of the over-all width of picture record, sound record, spacings between them, and spacings between outer edges of perforation and edges of film, the transverse gauge from center to center of perforations is to be 2.426 in. for unshrunk film.
The width of the perforation is the first dimension which must be established.
It is quite evident that with the present knowledge of film behavior in regard to its resistance to stress, the conclusions which may be arrived at are based more upon practical experience than upon academic principles.
It may be opportune to mention here the advisability of undertaking a thorough scientific investigation in order to verify or contradict the theories advanced in this paper before definite conclusions are derived from them.
From this investigation, which would of necessity be rather laborious, the exact width of the perforation could be scientifically and therefore accurately derived for the film width which may be chosen as standard.
It is, however, quite within reason to consider: First, that it is advisable to secure film registration from the sound track side of the film surface. Second, that the width of the perforation must be calculated proportionally to the over-all width of the film so as to permit the accommodation of the shrunken film without reducing to a harmful extent the transverse width of the tooth, which engages the perforations at the side opposite to the sound record. Third,