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FILM PROCESSING APPARATUSES
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on the general mechanical characteristics of the film, their particular resistance to stress at both their faces and corners and many other, more or less, abstruse factors which are of the greatest importance. Perforating machines have always been rightly considered as the point of departure for mechanical perfection in motion pictures.
The Bell ft Howell perforator, which is here briefly described, represents an engineering achievement which has been recognized throughout the world, and which has standardized the mechanical process of the motion picture industry.
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Fig. 5. Dimensions of standard film perforation gauge.
In determining the size and shape of perforation best suited for the efficient operation of the film with consideration to its maximum strength and with a view to secure accurate control throughout the various film movement mechanisms of the cameras, printers and projectors, a problem of considerable magnitude was presented, particularly when investigation revealed the fact that many perforation holes of different shapes, sizes and gauges were in use by the earlier manufacturers of film, no two of these perforations being identical and none of them proving to be theoretically or practically correct.
In computing the standard film perforating gauge, A. S. Howell gave due consideration to film shrinkage and having determined that the maximum permissible shrinkage of motion picture film is .0937 inches per foot, he concluded that a gauge length of 11.968 inches for 64 perforations would insure the necessary accuracy of perforation and at the same time make due allowance for the shrinkage of the film.
The following computation indicates the basis of the conclusion.
Assuming the outside diameter of the sprocket wheel in all standard projection machines to be 15/16 of an inch or .9375 inch, then:
The circumference of the sprocket is 2.94525 inches.
As standard motion picture film has an average thickness of .0055 inch, the pitch diameter of the sprocket will be found to be .9375" plus .0055" or .943". Pitch circumference is 3.1416 X .943 = 2.9625288". Circular pitch equals 2.9625288" divided by 16 (the number of teeth on the sprocket) or .18516 + inch.
The standard perforation gauge being 1 1.968 inches for 64 perforations and the maximum allowance for shrinkage being 3/32 inch or .09375 inch for this length of film, therefore, 11.968 inches less .09375 inch or 1 1.8743 inches is the average length of shrunken film measuring 64 perforations in length.