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PROCESSING EQUIPMENT 101
isolated within a separate tube of solution, again uses driven sprockets to each loop and must, of course, employ a lower roller within each tube to control the loop and to prevent it 'whipping' against the walls of the tube. The third system, known as the 'tendency-drive' is so vastly different from the others that it will be described in detail.
The machine maintains even film loop size throughout the entire process and at no time is the film driven by sprocket teeth engaging with the perforations. One may at first ask why driven sprockets should not be used to effect the traction through this machine. The answer is, of course, that quite apart from the advantages to normal 35-mm perforated film, many types of photographic recording processes employ 35-mm film which is not perforated and, therefore, could only be processed on a machine of this type or by individual rack-and-tank methods. The machine is fitted with racks, the general principles of which are similar to those described previously it will be seen from Figure 48 that the difference lies in the method by which these racks are driven.
The rollers in this machine which correspond to the top bank of rollers in other machines, and over which the film is carried, are mounted on shafts seen at 'A', and it must be remembered that all these rollers are exactly alike and none are fitted with sprocket teeth. The lower rollers, shown at 'F', are similar in design to those previously described. The drive to the top rollers is via a chain 'B\ from a sprocket wheel *C\ This sprocket wheel is mounted on a short spindle to the opposite end of which is attached a large flanged pulley wheel *D\ This pulley is driven by an endless belt shown between the pulley and the weighted roller 'E\ It is the adjustment of the contact between the belt and pulley 'D\ by pressure from roller *E', which controls the speed at which the film rollers rotate.
This adjustment can only be effected by the one variable in the design, namely, the distance between the top and bottom rollers in the racks. It will be helpful to remember that, in this design, the film will become loose in a rack not because the rack preceding it is feeding the film in at high speed, but because the rack following it is not taking film out fast enough. Similarly, film will become very tight in any rack because the following rack is taking film too rapidly, and not because the preceding rack is not supplying film at a sufficiently high speed.
The lower racks *F\ Figure 48, are each supplied with a thin vertical rod 'G', which passes through the top of the rack framework. The upper section of this rod is fitted with a compression