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This movement, like the ordinary Geneva Movement, with radial slots shown in Fig. 1, starts the film slowly, without shock or strain on either the mechanism or the film perforations.
But in other characteristics the two movements are surprisingly different, and the eccentric star develops a number of marked projection advantages.
First : On account of the obliquity of the slot, the film starts
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Fig. 3.
more slowly than with the radial slot and its early acceleration of velocity is less. This is of great importance in prolonging the life of the film as the strain on the perforations is greatest in starting the film from a period of rest.
Second : The period of acceleration is prolonged to beyond the middle of the movement. At the middle of the cam action the film has been advanced only one-third of a picture length.
Third: The prolongation of the period of acceleration is accompanied by a corresponding shortening of the period of retardation. This is of advantage in that the friction of the pressure plate or tension springs in the film gate naturally assists the intermittent sprocket to bring the film, to a full stop.
It must now be evident to the observer that whereas other rapid intermittent movements wear out both themselves and the film, the eccentric star intermittent movement conserves the life of the film even beyond that of the slower radial star intermittent movement of 90° cam action. Actual life tests confirm this theoretical analysis.
Fourth : It has long been an accepted principle that, with any ordinary intermittent movement, the interrupting or "travel'* blade of the shutter must have approximately the same angular area as the action of the cam, plus the additional area necessary to cover the light ray completely before and after the actual movement of
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