Loosely Coupled Helix. The heads of the primary coil of the loosely coupled helix illustrated in Fig. 64 are circular pieces of hard wood 1 inch thick and 18 inches in diameter. Six uprights 3/4 x 1/2 inch and 11 inches long are set into notches in the upper head and fastened to the base so that the space between the heads for winding the wire is 10 inches.
Fig. 64. Loosely Coupled Transmitting Helix and Contact Clip.
A square wooden pillar 1 1/2 x 1 1/2 inches and 32 inches long passes through a square hole 1 1/2 x 1 1/2 inches in the center of the heads and projects 20 inches above the upper one. The heads of the secondary coil are each 12 inches in diameter. The distance between them is also 12 inches. A square hole 1 1/2 inches x 1 1/2 inches in the center of each of the heads permits the whole secondary coil to slide up and down on the pillar. Several 1/4 inch holes bored 1 inch apart in the pillar admit a small peg of the same diameter. The coupling between the two coils is varied by changing the height of S above P and inserting the peg to hold the upper coil in position.
The primary winding of the transformer should consist of about 20 turns 15 inches in diameter, and may range in size of wire from No. 2 to No. 8 B. S. gauge.
The secondary is 10 inches in diameter and will require about 40 turns of No. 10 to No. 16 B. S. gauge.
The largest sizes of wire are for the one-quarter and one-half K.W. transformers while the smaller sizes are best suited to small induction coils.
Fig. 64 also illustrates a clip for making connections to the turns of the helix. The handle is a piece of hard rubber rod 2 1/2 inches long and 5/8 inch in diameter. A saw slot 1 inch deep is cut down the center of one end. Two strips of spring brass 2 inches long, 5/8 inch wide and 3/64 inch thick are bent as shown in the illustration. The straight ends are slipped in the slot in the handle and clamped together by boring a hole and passing a machine screw through. The upper end may be fitted with a nut or a binding post to facilitate connection. Flexible copper ribbons or stranded wire should be the only conductor used for the tuning leads.
Fig. 65. Loosely Coupled Transmitting Circuit.