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to follow in this particular, and for their guidance there must be a standard size and form of perforations.
The standard film perforating herein described was adopted only after long experience in building of motion picture machinery and the handling of film. As a suggested standard of measurements it departs from earlier standards only where it is necessary to do so in order to take into account the physical changes to which the film is subjected.
It is accepted as settled that the maximum shrinkage of motion picture film is .0937 inches per foot. Painstaking experiment warranted the conclusion — fully established by later experience — that a gauge length of 11.968 inches for 64 holes would insure the accuracy of perforation necessary to perfect results and at the same time make due allowance for shrinkage of the film.
The following computation shows why we adopt 11.968 inches instead of 12 inches as the standard for a perforation gauge measuring 64 holes :
Assuming the outside diameter of the sprocket wheel in all standard projecting machines to be 15/16 inches or .9375 inches, then;
The diameter of the sprocket being .9375 inches
The circumference of the sprocket is 2.94525 inches.
As standard motion picture film has an average thickness of .0065 inches the pitch diameter of the sprocket will be found to be .9375 inches plus .0065 inches, or .944 inches.
Pitch circumference is 3.1416 x .944 or 2.965704 inches. Circular pitch equal 2.965704 divided by 16 (the number of teeth to be found on the sprocket) or .1853 — plus inches.
The standard perforating gauge being 11.968 inches for 64 holes and the maximum allowance for shrinkage of film being 3/32 inches or .0937 inches for 64 holes, therefore, 11.968 inches less .0937 inches or 11.8743 inches is the average length of shrunken film measuring 64 holes.
The pitch of the film or length per hole is 11.8743 inches divided by 64, or .18553 inches.
Pitch of sprocket, .1853 plus inches Pitch of film, .1855 plus inches
The drawing shows the measurements by which the gauge is established ; width of film over all 1.375 inches ; width of film over all on perforation 1.219 inches ; distance (width) center to center of perforations 1.109 inches; distance (width) inside to inside of perforations .999 inch; width of perforation .110 inch; distance (width) center of perforation to margin of film .133 inch; distance (width) outside of perforation to margin of film .078 inch ; length of perforation .073 inch; distance (length) between perforations .114 inch; distance (length) center to center of perforations .187 inch.