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.
Having hundreds of postals of a single subject to print, I made a perfect timing apparatus for exposing the prints from an old metronome and an old gong magnet. A disk, B, 20 in. in diameter, was made of heavy tin with two apertures, C C, each cut 7 in. in diameter, on a line with the center of the disk, and 2 in. from the edge. A large spool, F, was used to serve as a hub and also as a reel on which strong twine was wound, with a weight, E, attached to the free end.
The disk was bolted to the partition P of the darkroom, the partition having a hole, G, to coincide with the holes in the disk as it revolved. Four catch pins were fastened on the rim of the disk to engage a catch pin on the armature of the magnet. The gong and commutator were removed and the magnet placed in the position shown in the sketch. A strip of wood was fastened across the face of the metronome H, about 1 in. above the pendulum shaft or axle. On the inside of the center of the strip a small piece of wood was projected, with copper wire on one side only, to form a contact with a piece of flexible copper on the pendulum. Wiring was made as shown in the sketch and a switch used to stop the disk from revolving.
An ordinary postal-card printing frame, D, with a hinged back was used and placed on the shelf A, as shown. A hinge was made from heavy elastic bands to allow for two dozen cards in the frame at one time. As each card was printed it was taken out and dropped into the developer. The reel and metronome should be wound up after printing two dozen cards. The stops can be varied for any length of time by regulating the weight on the metronome. The disk and all woodwork must be painted a dull black. The circuit is completed on the return stroke of the pendulum, causing the magnet to attract the armature, which releases the catch, allowing a quarter turn of the disk. - Contributed by Frank W. Preston, Paterson, N. J.
Ill: Details of Timing Apparatus