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410
HAYEK
Vol 49, No. 5
therefore, shows this additional film-trap drag. Experimental evidence confirms this value at least well enough to justify its use in a discussion of this kind. The lower curve shows the net pull on the film caused by its acceleration by the intermittent mechanism as shown in the upper curve and the film-trap drag as shown in the middle curve. During the first half of the pull-down the force on the film caused by acceleration and film-trap drag act in the same direction and must be added. During the last hah7 of the pull-down they act in opposite directions and must be subtracted. As previously indicated, the net
5,000 FT / SEC
5,000 FT /SEC
1
PULL DOWN
FILM ACCELERATION OF GENEVA
DOWN ANGLE
FIG. 3. Comparison of a constant filmacceleration curve with the film-acceleration curve of the Geneva.
pull on the film should never be negative if a steady picture is to be achieved. Inspection of the curve shows that the film, is subject to its greatest stress during the first half of the pull-down, that this point occurs at the point of maximum film acceleration, and that the net pull on the film is approximately one pound.
The thoughts which follow are developed on the assumption that if the.net pull on the film is not allowed to exceed this value of one pound, no more damage will be done to the film by other intermittent mechanisms than by the Geneva movement and this has been the controlling factor in the train of thought which follows.
1. C0«5/a«/ Film Acceleration and Deceleration. — Let us now take our first step toward the synthesis of an intermittent mechanism which will have an operating period of less than the 90-degree period of the Geneva movement. Let us see what would be the result if we could make an intermittent mechanism, which in association with the