W. C. Hutchinson

To obtain the best results with wireless telegraphy apparatus it is necessary to have the sendirig-receiving stations tuned as near as possible to the same wave length. As considerable difference will always exist in the workings of coils of the same rated capacity, caused by variations in the rate of vibrations, strength of current, etc., some quick method of adjusting the receiving apparatus must be used, by means of which the wave detector can be made as sensitive as possible to the incoming waves. This is done in part by what is known as a tuning coil, and the one here described has the advantage of being shaped so that it can be fastened to the wall and, while convenient for use, is out of the way of the rest of the instruments which of necessity occupy considerable room and cannot be thus disposed of.

The materials required are: A base of well seasoned wood about 40 in. long, 15 in. wide and 7/8 in. thick. Whitewood is excellent for the purpose, as it is easily obtained of the required width. Two cleats about 3 in. wide are fastened near each end at the back with 1 1/2 in- screws to prevent warping.

Several maple or birch dowel rods 1 in. diameter are then sawed into 12 pieces each 3 in. long. Shallow grooves are turned about 1/2 in. from one end of each piece, and the ends smoothed off. Holes are then bored near the ends of the base, spaced 2 1/2 in. apart, to receive the pins, which are put in with glue.

Wireless Tuning Coil 295

The whole frame is then given two coats of shellac, and an additional coat in the grooves of the pins will be an advantage, as it improves the insulation. The wire is then strung back and forth over the pins, beginning at the top, where connection is made with the aerial wire, ending with one of the bottom pins, around which a full turn is taken and the end twisted around the main wire. The first pin to be used is either on the right or left side of the board, according as the coil is placed on one side or the other of the aerial, so as to make as direct a connection as possible. The length of wire in the coil is about 33 ft., which should be sufficient for an aerial not over 100 to 125 ft. high. The wire used should be the same gauge as that used for the aeiial.

A clip for adjusting is made from two pieces of spring brass about 1/2 in. wide and 1-32 in. thick, which are riveted or soldered to another piece of flat brass a trifle less in thickness than the diameter of the wire. The corners are then rounded off and a cover for one end is then made from a piece of hard rubber tubing, so that the clip may be adjusted without the possibility of shock. The wire connecting the clip to the wave detector may be a piece of flexible electric light wire, one strand only being used. It is best connected to the clip by means of a round head brass screw, which necessitates drilling and tapping a hole in the clip. If this is not convenient, the wire may be wound around the clip near the handle and then fastened by winding a strong thread over it, but the latter is not as good a joint, as the wire oxidizes and the wave impulses, which at best are quite feeble, will not pass as freely as if the screw fastening be used.