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210 WALTER H. CARSON [j. s. M. P. E.
allow a tolerance in the film due to the factor of shrinkage. The old empiric method of placing a piece of negative on a new piece of raw stock to prove that the pitch of the perforation hole is incorrect, is obviously not a safe method on which to form a judgment, since it is always practically impossible for the mechanical man to know the age of the film which he is using as a gauge.
It is hardly to be expected that newcomers in the industry, such as cameramen, operators in the printing rooms, and projectionists, will be entirely familiar with the long discussions which have taken place in the past and are still going on in some foreign countries to establish an international standardization. Neither can we expect them to know that the best results are secured from films which do not have an exact pitch in perforation or measurement between centers of perforation holes of 4.75 mm., but that there is a maximum tolerance between 4.68 and 4.76 mm. which will allow of free movement of the film on all of the conveying equipment without risk. By far the larger proportion of persons handling films, who have little interest in these mechanical and theoretical problems, merely ask, when difficulty is encountered: "What is the matter? Is the film or the equipment at fault?" In such cases it is very essential, when trouble is encountered, to be able to convince oneself whether the film perforation is correct or not, but, before attempting an examination or measurement of any kind, it is desirable to know definitely what is right and wrong. Theoretically, the answer to this is contained in the terms of standardization which indicate how the film must be manufactured or perforated to avoid trouble on standardized equipment through which the film must be transported.
In order to refresh your memory, we will state that the standard establishes the following measurements:
First: That the distance of perforations, or pitch, immediately after perforation, must be between 4.75 and 4.76 mm. for positive film, and between 4.75 and 4.77 mm. for negative film. Second: That the shrinkage under unfavorable conditions, such as 720 hours in air of approximately 140 °F. and of a 70 per cent relative humidity, should not exceed 37.5 inches in 400 feet, or 0.78 per cent.
In the matter of the first clause of standardization, the film user must, of necessity, take the manufacturer's word for the correctness of the measurement at the time of perforation, because it is very seldom that the film reaches the consumer immediately after being