Fig. 55.—Venturi tube on side of plane.
Static discharges are produced by the friction of the celluloid against the pressure back or other surfaces with which it comes into contact. They show in the developed film as branching tree-like streaks (Fig. [56]) and in cold dry weather may be numerous enough to ruin a picture. The discharges are noticeably less frequent with film coated on the back with gelatine (“N.C.”), but the extra gelatine surface is extremely undesirable. When handled by developing machines, as large rolls must be, this back gelatine surface becomes scratched and bruised in a serious manner. Plain unbacked film is much to be preferred if the static can be obviated.
To avoid static, it is necessary to provide for the immediate dissipation of all acquired electrical charges. Experiments made by the United States Air Service have shown that nothing is so good as rather rough cloth, thoroughly impregnated with graphite, held in close contact with the celluloid during as great a portion of its travel as possible. In the United States Air Service film camera which uses suction through a perforated back plate, the plate has been covered with thin graphited cloth, and similar cloth sheets are pressed against the film rolls by sheets of spring metal (Fig. [65]). In cameras with this equipment no trouble has been experienced with static.
Fig. 56.—Print from film camera negative, showing static discharge, and (upper left-hand corner) record of altitude and compass direction made by Williamson film camera auxiliary lens (Fig. [58]).
Representative Film Cameras.—The English F type (Williamson). This is one of the earliest cameras designed for film, as is indicated by the nature of the power drive, which presupposes that the camera is to be carried on the outside of the fuselage. Its essential features are shown in Figs. [57] and [58]. It consists of a rectangular box with a cone at the front on which is mounted a propeller, intended to be rotated by the wind made by the motion of the plane. This drives, through a governor controlled friction clutch, a train of gears which draws the (5 × 4 inch) film across the focal plane, sets and exposes the shutter at regular intervals.
Fig. 57.—English type “F” (Williamson) automatic film camera.
Above the camera, supported on a tripod, are a compass and altimeter, both recording on a single dial, illuminated from below by the light reflected from a circular white disc painted on top of the camera. An image of the dial is thrown on a corner of the film by a lens, whose shutter is actuated in synchronism with the main focal-plane shutter. No special means are provided for holding the film flat. Special film with perforated edges is used.