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 proper time to expose a printing paper under a negative should be determined and the negative marked for future printing. When this time has been found some means should be provided for making the exposure exactly the same, then the prints will be perfect and of a good tone at all times. For this purpose the instrument shown in the illustration was designed and used with entire satisfaction.
The device consists of an ordinary cheap watch, a standard, or support, for an adjusting screw, a small coil, a movable armature, a knife switch, and a trip arrangement. A neat box or case, about 5 in. square and 3 in. high, is first constructed. A round recess, 1/8 in-deep, is cut in the center of the top, to admit the watch. The standard A is made of brass, 1/8s in. thick and 1/2in. wide, bent as shown, and a 3/16-in. hole is drilled in the end of the long arm where it will exactly center over the pivot holding the watch hands. A 3/16-in. rod, B, is closely fitted in the hole and supplied with a knurled wheel, C, on the upper end, and an L-shaped arm. D, is fastened to the lower end. The end of this arm should be filed to a point, or a very thin piece of brass soldered to it. so that the end will just touch the minute hand of the watch. The tip end of the point should be bent slightly from the perpendicular toward the direction in which the watch hands are moving, so that, when it is set, the moving hand will easily break the contact.
The magnetic arrangement consists of a 3-ohm coil, E, mounted, as shown, to one side of the case, where it operates the trip levers. The armature parts consist of an L-shaped piece of brass, F, pivoted at G, to which a square piece of soft iron, H, is attached. Two small parts, K, are bent and attached as shown, to furnish a limit stop for the piece F and a support for a spiral spring which holds the armature H away from the coil.
Ill: Time Switch for Operating an Electric Light in Printing Photographic Developing Papers
The knife switch L is fastened to the bottom of the case so that the handle will project through a slot in one side of the box. A trip piece, M, and a small eye for attaching a spiral spring, N, are soldered to the knife switch. These two attachments for the switch are insulated from the other parts.
Two binding posts are mounted on top, one being connected to one terminal of the coil E and the other to the watch case. The other terminal of the coil is connected to the standard A. The two binding posts are connected in series with one or two dry cells, and the switch L is connected in series with the lamp used for printing.
The operation is as follows : The arm D, being set for a certain time, the lever of the switch L is set and the light remains lit until the minute hand strikes the point on the arm D, when the battery circuit is closed causing the coil to draw the armature H and allowing the spring N to open the switch L. The lamp is then extinguished. - Contributed by James P. Lewis, Golden. Colorado.