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878 S. W. COCHRAN [j. s. M. p. E.
hardened, ground, and polished to prevent excessive wear and deformation of sprocket teeth. The shape of the sprocket teeth, the size of the sprockets, and the general alignment of the projector parts are such that the projector will pass a continuous film loop a minimum of 400 cycles without any indication of wear on the film. This film loop should indicate no need of repair at the completion of 800 cycles through the machine.
SOUND HEAD
The sound head, although a major unit, may well be classed as an accessory to the projector in that no adapters or machining operations are required to fit it to the projector.
It is composed of a main casting which contains the film handling mechanism and sound take-off devices, and a sheet metal phototube housing. The sound head is shown in Fig. 2. The main casting and film handling parts embody the same general features of construction as does the projector head, in that all parts resist corrosion and are designed to produce maximum film life.
The sound head derives its power from the picture head through a train of gears. The constant speed sprocket and the take-up sprocket are the driven members. Film enters the sound gate through a set of guide rollers by means of which the sound track is aligned with the image of the optical slit. The film is held snugly, though gently, against the gate shoe by means of adjustable tension springs on the gate. This particular feature is very important in handling film in the torrid temperatures of the tropics, where the wax protective coating on the film is very sensitive to abrasion and pressure and is very apt to pile up on the gate shoe. After leaving the sound gate the film wraps about the impedance roller and then passes to the constant speed sprocket and take-up sprocket in that order. The impedance roller is attached to a small flywheel, the inertia of which tends to keep it running at a constant speed, thus imparting a constant speed to the film. The take-up sprocket maintains a loop between itself and the constant speed sprocket to prevent changes in film speed introduced by the take-up mechanism from carrying through to the constant speed sprocket.
The exciter lamp is mounted in a socket which is removable from the projector and which permits prefocusing. It is possible to change exciter lamps with less than 30 seconds' interruption in a performance. The optical system is contained in a single barrel which is bolted to the