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XIII. PEOJECTION AND PROJECTORS
manner. The "jittery" speed modulation effect became especially serious when the very thin chromium plating of the gate began to wear off. Fortunately, such designs are rarely seen in the better grade machines now manufactured.
A definitely better system uses a film-driven drum that is rigidly coupled to a relatively large flywheel such as the arrangement used in the Ampro (Fig. 111). With this construction there is a relatively large distance — about 32 frames — between the idler roller with its cooperating pressure-pad roller and the overhanging film-driven drum. There is a free-running loop of film that forms between the bottom of the picture gate and the idler roller. In this free loop the major por
Fig. 112. Threading diagram of Bell and Howell projector in wide use.
tion of the flutter introduced by the intermittent claw is filtered out. Additional filtering occurs between the idler roller and the film-driven overhanging drum where the light beam translates the moving image on the film into light variations. Particularly bad flutter can be produced on this type of machine if the idler roller does not roll or if the pressurepad roller does not press firmly upon the film as it passes by the idler roller pad roller combination.
Still further refinements have been made by other manufacturers in various models of their machines. The oil-coupled flywheel first utilized in 35-mm RCA sound heads has been used in slightly different versions and in differing degrees of effectiveness in various models made by Eastman Kodak, Bell and Howell, and RCA. Merely because a particular machine has an oil-coupled flywheel ("fluid flywheel' ') is no assurance