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April, 1937] THE QUEST FOR CONSTANT SPEED 357
propel film by means of a smooth roller having a fixed peripheral speed. Fundamentally it is not necessary to stretch or compress the film throughout its entire cross-section, for the film is driven solely by contact between one surface and roller 4. It is therefore sufficient to stretch or compress the material at this surface, and it is well known that the simple act of bending a flexible strip serves to compress one surface and stretch the other. It will be readily seen from the drawing that if a sufficient amount of film accumulates in the loop between the sprocket and the driving roller, the film will bend around the roller in such a way that the latter works on the concave side of the bent film. On the other hand, with a short loop, as indicated at 19, the driving roller will operate on the convex side of the bent film. When the driving is applied to the convex side, the mean film speed is less than that of the driven surface. In other words, when the loop is short the film does not pass so rapidly through the driving point between rollers 4 and 5. Whatever the degree of shrinkage of the film, an equilibrium is soon reached with the loop length such that the rate of passage between the rollers is exactly equal to the rate at which it is fed through by the sprocket. Engineers dealing with problems of belt drive are well acquainted with the necessity of making a distinction between pulley surface speed and mean belt speed, or the speed of the middle of the belt, and the Bedford drive depends upon exactly the same principle.
THE MAGNETIC DRIVE
The several systems just described, in which the drum is positively driven and compensation provided, possess one advantage over the system in which the drum is driven by the film only; namely, more rapid acceleration may be permitted, since the strength of the film is not a limiting factor. In most equipment, however, extremely rapid acceleration has not been essential, and in the case of the magnetic drive used in RCA recorders, the film is required to perform only a small part of the work of accelerating the drum flywheel.
During the period when the numerous fundamental problems of sound-on-film were being worked out, there were, of course, many machines both for recording and reproducing that fell seriously short of an adequate standard of constant speed. The recorder rather than the reproducer appeared to be the logical place to apply serious efforts to build a machine that would be less subject to speed fluctuations. In the machines, of which a number were built, that depended