This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
The simple homemade developing machine, shown in the illustration, can be easily made with three film spools,
Developing a Roll Film in a Tray with a Machine That Drives the Film around Rollers and through the Developing Liquid by Turning a Crank
some strong wire, and odd pieces of wood. It consists of an open frame, having two side pieces provided with slots down the center, sufficiently wide to allow an ordinary wood screw, of suitable size, to slide up or down freely. The two end-connecting pieces act as supports for the developing tray and should be made of sufficient length so the tray can pass freely between the sliding upright frame, made to fit in between the side pieces of the base. This frame can be adjusted to suit the length of film and is clamped in place at the desired position by wood screws, fitting in the long notches and screwed into the uprights. The two bottom rollers consist of film spools which are fastened in place by being slipped over a suitable wire, bent so the spool can enter the developing tray and the wire pass over the sides. Another bend at the outer end provides for the adjustment of the spools and for securing the wire in place by staples. The top spool is secured to a wire fitted with a crank at the outer end, so that in turning the wire, the spool will also turn, thereby driving the film. When placing the film on the machine, the sensitive side should face outward so it will not rub against the spools. The ends of the film may be connected with pins or ordinary paper fasteners.-Contributed by H. R. F. Richardson, Ottawa, Ont.