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212 WALTER H. CARSON [j. s. M. P. K.
negative or print gets the more this shrinkage is retarded until a final point of shrinkage is reached. This continued shrinkage is rather slow in the case of negative^because of the fact that negative is usually more carefully stored in sealed tin cans and under favorable atmospheric conditions. In the case of positive film, which is put through the projection machine many times and subjected to the intense heat of the projection lamps, the process of shrinkage will be comparatively rapid.
It is interesting to note the experience gained from 100 pieces of positive stock of various ages as they came into the exchanges. These were carefully measured, and it was found that the measurement between the perforations varied from 4.74 mm. to 4.69 mm., showing an average of 4.71 mm. which, of course, is still usable.
Let us compare these figures with the measurements established by the French standardization, which call for 16 teeth engaging on the transport sprockets. The diameter of the rolls at the base of the teeth is 23.85 mm. Adding to this diameter the thickness of normal film, which is approximately 0.15 mm., we obtain a total diameter of 24 mm. which, in turn, gives us a circumference of 75.3 mm. Dividing this by 16 because of the 16 tooth engagement, we find that the distance between the centers of the teeth on the circumference of the sprockets at the base of the teeth is 4.71 mm., or exactly the figure which we secured by the empiric experiment mentioned above.
It is very unusual to encounter shrinkage greater than this, but, in the case of very old film — which, in some instances, had been perforated prior to the setting of the present standard — we encounter shrinkage down to 4.68. Usually when film has reached this point it would be necessary to discard it in any case because of its excessive curl and brittleness.
From the foregoing it will be seen that it is possible with microscopically accurate instruments to determine the shrinkage of the film by measurement of the distance between corresponding edges of the perforation holes or by placing two pieces of film together, one of which is as new as possible, and thus test visually whether the old film has shrunk or not, and, if so, how much.
The difference of one perforation width in one-half meter would indicate, by the above test, that the difference in shrinkage between the two pieces of film amounted to 2 mm. in 500 cm.
There are three methods with which we are familiar by which the difference between perforations can be established.