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PROCESSING EQUIPMENT 91
decreased to any desired size. The take-up alarm is also a part of the film feeding mechanism. The last top roller on the take-up elevator is fitted with sprocket teeth, and a ratchet wheel and pawl are attached to one side of the roller. This ratchet and pawl are a safeguard to ensure that the film end is not lost when full rolls of film are removed from the machine. Since this pawl causes a considerable noise in striking continuously against the ratchet, this noise serves to indicate that the take-up roll is operating. The last roller on the take-up elevator is driven only by the take-up spindle and, therefore, would stop rotating immediately the takeup roll ceased to pull the film from the elevator.
The Developer Circulating Pump
In this particular machine the purpose of this pump is threefold: firstly, to circulate the developing solution sufficiently to keep the temperature even throughout the bath and to guard against any small pocket of highly concentrated solution forming at the inlet from the replenishing system and also to assist in the general evenness of development throughout the bath. To do this the pump removes solution from the bottom of the tank and forces it in again just below the surface of the liquid. Secondly, the pump is required to draw freshly made developer from the chemical mixing room and into the processing tanks. Lastly, it may be required to draw developer replenishing solution from the chemical mixing room into the replenisher tank, situated above the processing machine.
Any of the above functions may be performed by merely adjusting the position of two three-way valves, situated between the pump and the processing tanks. In order to do this work efficiently the pump must be capable of circulating the total 36 gallons of developer completely at least once every half-hour.
The Solution Replenishment System
A constant-head device, rather like the carburettor fitted in motor cars, is located above the developing tank and connected with the replenishing tank, thus ensuring a steady feed from this tank into the main processing tank. The rate of replenishment must be varied with the rate of travel of the film through the machine, since a given footage of film requires a given quantity of replenishment (this quantity will also vary according to the type of film which is being processed). Developing power is lost by two causes, firstly by the developing solution becoming exhausted and, secondly,