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 tank method of photographic development is acknowledged as the best, yet there are many who, for vari- ous reasons, still use the old-style tray method. For those who use the tray, a splendid and simple method that combines the good qualities of both the tank and tray is the tray-rocking device shown in the illustration.
Ill: Developing-Tray Rocker to Keep the Liquid in Motion over the Plate Automatically
The rocker consists of a wood box, 13 in. long, 9 in. wide and 1 1/2 in. deep, made of 3/8-in. material, together with a similar box 1 1/2 in. deep, that fits over the other as a light-proof cover. Both are given a coat of black paint.
At the center on the under side of the tray part, a right angle made of strap iron is fastened with screws. On the part projecting down, a hole is drilled to receive a sleeve made of a brass tube which is soldered in place. An ordinary shelf bracket is procured, one end of which is filed and fitted with a strip of metal having both ends turned up slightly. Small-pointed pins are fastened in holes drilled near the turned-up part. The points of the pins serve as a knife-edge for the rocker. The extending end of the strap iron is fitted with a pendulum rod having a weight at the bottom.
The rocker is attached to the wall in a convenient place in the dark room. The tray with the developer and plate is placed in the box, which is light-tight, and the pendulum is started swinging. - Contributed by T. B. Lambert, Chicago.