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655 HANDBOOK OF PROJECTION FOR
tent projectionist. He cannot get the best results except where, purely by chance, the shutter sent happens to fit the condition under which it must work.
We might add that :
Tendency to flicker increases with screen brilliancy and with the size of the picture. Conversely, it decreases with decrease in screen brilliancy and decrease in size of picture.
THE SHUTTER AND ALTERNATING CURRENT.—
Where 60 cycle A. C. is used at the arc the use of the threewing shutter is inadvisable for the following reasons : 60 cycle A. C. reverses its direction (alternates) 120 times per second, or 7,200 per minute. When a projector running at the rate of 60 feet of film per minute is equipped with a three-wing shutter, the light is cut 2880 times a minute. Half of the alternations per minute is 3,600, and if the cycle of the current happens to be not quite 60 (as often is the case) but 55 to 58 instead, it would require but just a little over-speeding of the projector to bring the wings of the shutter into synchronism with one side of the alternations. Under this condition, if the wings happen to cut the light at the point of its greatest brilliancy (See Fig. 4, Page 16), the brilliancy of the light on the screen would be diminished by probably one-half, or maybe even more. And this is what very often does take place where an attempt is made to use a threewing shutter in conjunction with a 60 cycle A. C. projection arc. The net result is that screen brilliancy will die down and come up time after time, and in a way which is very mysterious to the uninitiated. This is because of the fact that in order to dim the light the shutter must be precisely in synchronism with the alternations, and while this may occur, it is not at all likely the synchronism would be long maintained because an almost infinitesimal variation of the speed of the projector would throw the shutter out of synchronism with the alternations, with result that the screen brilliancy would come back to normal. Put in simple words the effect of this is that screen brilliancy will alternately diminish and increase; decreasing when the shutter blades happen to be in synchronism with the one side of the alternations, and increasing when they are not.
TWO-WING SHUTTER FOR 60 CYCLE.— For the foregoing reason it is advisable that a two-wing shutter be used with 60 cycle A. C, unless the speed of projection be such ?s will preclude the possibility of the shutter, blades and alternations getting into synchronism.