Oscar N. Dame
Among my possessions was a bell ringer coil, about 3 in. long, with outside diameter of one inch and core diameter | of an inch. Originally the coil, or electromagnet, as it may be better called, had been a part of a 1000 ohm telephone bell, making the resistance of this single winding just 500 ohms. At one end the core was drilled and tapped for an 8-32 machine screw. It was a very simple matter to likewise drill into th other end and tap for a similar screw. This being done, two pieces of soft steel rod, 1 inch long, 3/4 wide and | thick were formed by drilling and filing into the shape as shown in Fig. 1. A third piece of similar stock was next fashioned into an armature, as in Fig. 2, with small holes drilled at each end near the bottom, into which pieces of polished steel wire were with about 1/2 in. projecting. These wire tip8 fitted freely into the slots in the heads of two heavy brass wood screws which were very appropriate for the purpose, as they could be set into a base board without difficulty.
Exactly in the middle, and close to the top edge of this armature, was affixed a small silver tip or contact point soldered to a strip of stout brass, shaped like Fig. 4 and inserted between the armature and the coil, half way between the two magnet poles. In assembling these parts a brass peg was driven into the base-board, to which was fastened, by a silk thread, a small closed spring of the type generally used on relays, to keep the armature clear of the magnets when the relay is not energized.
The sounder circuit was completed by soldering a wire to one of the wood screws supporting the armature, and a second wire to the contact strip between the pole pieces. The two terminals of the magnet itself were brought to binding posts at one end of the baseboard.
In testing with this relay it was found to be fully as sensitive as many of the two coil type which the writer has tried in wire telegraphy, and the apparent sensitivity of the device seems to warrant its use in such simple wireless experiments as the amateurmight care to indulge in at small cost. In fact, the actual cost of the entire outfit was but a few pennies, not counting, of couree, the bell ringer coil, which would have cost approximately $1.25 at the supply house. Even with this expense, the results make the building worth while. Following the design here given, the amateur could construct his own magnet out of 3/8 stock and wind on as little or as much wire as he chooses, according to the uses to which the relay will be put. Altogether it is the simplest practical relay the writer has ever used.