Seventy-five years ago this month, the February 1942 issue of Radio News contained one in a series of articles on the subject of homemade parts for radio construction. While the article appears to have been written before Pearl Harbor, it acknowledged that the present emergency could sharpen its teeth still further, in which case radio men might need to make their own parts. This article focused on variable capacitors, and offered a number of ideas, as well as specific details and even formulas for computing capacity.
The first idea given is shown here, a variable capacitor consisting of two cans. The inner can would be about 1/8 to 1/2 inch smaller than the outer one. A vertical support made of wood would allow the inner can to move up and down, varying the capacitance. Since adjustment was not particularly convenient, this scheme was recommended for things such as neutralization, where the adjustment only needed to be made once.
For tuning, two ideas were offered. The sliding plate condenser shown below allowed tuning by pulling one set of plates in and out.
The “book” or “barn-door” capacitor is shown at left. It consists of two hinged plates. The article notes that this idea was used commercially until about 1927. In fact, it allowed adjustment with a rotary knob, by using the scheme with a cam shown below. According to the article, this system was used by Crosley in 1926.
In most cases, the insulated portions of these condensers were made of wood, and the author offers pointers on selecting wood. Other insulators are also discussed, for use in capacitors and other applications. Cardboard was offered as a good base for coils, and the article explains how to treat the cardboard with beeswax, parafin, or other substances. For coil bases, the article recommends burnt out tubes, which it notes are discarded by most shops by the bushel.