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629 HANDBOOK OF PROJECTION FOR
the entire reel. Any tension in excess of this is not onlybad, but it is very bad, particularly if the old style V/z inch reel hub be used. A moment's study of this matter will convince you of its importance. Throughout the entire process of re-winding, the take-up friction will exert precisely the same amount of pull on the spindle which carries the takeup reel. When the film first begins to wind on the hub of the lower reel, the diameter of the film roll will be less than 2 inches, if an old type reel be used, therefore the pull on the film will be very heavy. As the diameter of the film roll increases, however, the pull on the film decreases, until when the reel is full it will be very slight.
In other words, since the pull of the take-up is constant, and must be sufficient to revolve the reel when it is full, the actual pull exerted on the film at the beginning of the process of re-wind, is very many times greater than it is at the end. This means that not only is the film wound entirely too tightly at the beginning, and too loosely at the end, but also that during the beginning of the process of rewinding the pull on the film is so heavy that if there is excess tension it is quite possible the sprocket holes may be strained, or even broken, since the pull is against the lower sprocket of the projector. It is even possible, and does often happen, that the film is pulled over the lower sprocket, and the lower loop is thus lost, though ordinarily this only takes place when a bad splice comes through.
Excess tension also is apt to pull weak splices in two. It is in every way detrimental, therefore the projectionist should use every precaution to have his take-up tension set exactly right, and "exactly right" is that tension which will no more than insure completion of the process of rewinding. Of late there have been some very excellent devices invented which equalize the take-up pull throughout the process of rewinding. Also the split pulley friction takeup has been to a very great extent improved by increasing its diameter.
GENERAL INSTRUCTION NO. 3— DIRTY SPROCKETS.
— It is of the utmost importance that the sprockets of the projector be kept perfectly clean. This is important for all sprockets, but particularly for the intermittent, because any dirt accumulating on the face of the intermittent sprocket will cause unsteadiness of the picture on the screen. The best method of cleaning sprockets is as follows: Procure a rather stiff bristle toothbrush, and either a wide-mouthed