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PROCESSING EQUIPMENT 75
*B' in which the various processing solutions are maintained at a constant temperature by a water jacketing system. A countershaft, running the entire length of the machine provides, via gear boxes, a driving clutch at the pedestal mounting over each solution tray. A lever mechanism 'C, which is permanently attached to the film drum, is used to engage with transfer sockets *D' to enable the drum to be moved from one tray to the next. The gearing which rotates the drum whilst in position at each stage of the process is arranged to automatically disengage as the drum is lifted from one tray to the next. The transfer from tray to tray is carried out manually by the rocking motion of lever 'C against the sockets 'D\
This machine is designed to accommodate lengths of film up to 200-ft (60 • 96-metres) and is claimed to process such lengths very satisfactorily. Much experimental work was carried out to eliminate fogging, which was traced to the use of an open or hollow drum composed of rods arranged horizontally between the end flanges. Trouble was also experienced in the early stages of development with slack loops of film caused by swelling taking place during the processing operations. Although elastic bands accommodate the expansion at the ends of the film spiral, they do not pull sufficiently to remove the slackness over the centre turns of the spiral. To overcome this trouble a smooth drum was made with a spiral film guide channel formed on the surface and into which the film was threaded; specially made soft-bristle brushes were then held in contact with the film during the processing and served to urge the film to slide along the smooth drum surface and so permit the elastic tension at each end of the spiral to take up the slackness at the centre portions. After further experiments it was found that, by increasing the speed at which the drum rotated to 250-ft (76 • 2-metres) per minute, the centrifugal force created was sufficient to urge the film to slide in the guiding spiral without the aid of the brushes. This increased speed also caused sufficient developing solution to be carried round the film so that, in effect, it was always covered by a layer of solution and, in consequence, more even processing resulted.
The second main type of portable processing apparatus may be termed the 'rack and tank' method, the most simple form of which is seen in Figure 31. A simple rectangular frame, usually made of hard wood, carries a series of film locating pins at each end. These pins are so arranged that the film may be loaded in the form of a spiral around the rack. Central rods, shown at 'A', are often provided to permit the rack to be mounted on bearings and rotated during the loading operation. The loaded rack is then