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FILM PROCESSING APPARATUSES 273
It is seen from the above that, since the film is run through the various apparatuses only when dry, the shrinkage effect is more troublesome than that of swelling and stretching.
It is quite true that the changes of dimensions which result from shrinkage are small and that reasonable care can maintain them so. at least within the walls of the processing laboratory. In spite of their smallness, they are, however, of great importance because any error of registration due to these errors is greatly magnified during projection and cannot be tolerated but within extremely small limits.
The problems that the mechanical engineer confronts with those conditions would not present extreme difficulties of solution if film shrinkage could be predetermined and, therefore, controlled. But such is not the case. Although films taken from the same roll of celluloid base and handled under equal atmospheric conditions offer equal shrinkage characteristics, no two films taken from rolls of celluloid made at different times or exposed to different conditions of heat or cold or dryness or humidity, shrink to the rame extent. Therefore, the mechanical problems are greatly multiplied.
In fact, film shrinkage cannot be controlled but only accommodated and certain limits of error had to be determined beyond which accommodation is impossible in practice.
The problems resultant from film shrinkage became evident from the very inception of motion pictures and their complexity became more and more apparent with the rapid progress made in commercializing the new discovery.
It is quite interesting to note that these problems were attacked from two points of view in Europe and in America.
European manufacturers attempted to accommodate shrinkage by varying the dimensions and pitch of perforations for negative and positive films.
Since negative film shrinks considerably, during the drying after the wet processing, an average percentage of shrinkage was determined and positive film was perforated to accommodate both the longitudinal and the lateral changes of dimensions suffered by the negative.
Perforating machines manufactured in Europe were so devised that the punches and dies for the two dimensions were interchangeable.
When the shrunk negative and the unshrunk positive were run through the printing machine, this being of the step-printing type, the driving fingers would bring two sets of corresponding perforations into coincidence, while a pressure plate would assure contact and stability of positioning.
This method of registering, though fairly accurate, cannot be relied upon for critical register under shrinkage conditions varying from the predetermined average and, of course, the greater these variations, the greater would be the error in registration.
It was A. S. Howell of the Bell & Howell Company of Chicago, who first conceived and constructed a printer in which he replaced the intermittent motion of the films with a continuous motion, which