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.
Necessity may be the mother of invention, but it is also the grandmother of application, and application is the practical side of invention. Both the amateur and the professional photographer have been bothered by spotting and unequal development of negatives and prints in tray development, due to various causes, and sometimes by the presence of dirt particles or the unequal Or incomplete flowing of the developer over the surface of the sensitive emulsion.
Most professionals and many amateurs are familiar with the use of the camel's-hair brush to avoid failures of this character, and many of them use a brush for local development in certain cases where it is necessary or desirable. Usually the brush is kept in a small glass cup, somewhere close at hand, but it is often in the way when not wanted and misplaced when most needed. The brush can be kept within reach and handy for the operator by arranging a light counterweight and pulley with a string attached to the brush, so that, normally, the brush will hang from the ceiling directly over the developing tray and can be obtained for use when desired.
The detail of this brush-string and counterweight combination was deliberately appropriated from the old plan of suspending the piece of chalk over a billiard table, so that the players could easily reach it, when needed, while, when released, it would be pulled out of the way by the counterweight. The developing brush thus suspended is always ready, never misplaced, nor in the way for other operations. This arrangement is particularly convenient where a bathroom is used as a dark room, and the shelf space is limited.
This same manner of counterweight-ing chalk on the billiard table may be applied to a stove-lid lifter, to keep it within easy reach and always cool enough to handle. The simplest and most inexpensive way of making this apparatus is to cut off a small piece of lead pipe for a counterweight, and, in the absence of a suitable pulley, use an ordinary screweye fastened in the ceiling. The latter is really better than a pulley because the string cannot run off the screweye. The arrangement is better understood by referring to the sketch.