A Tuning Coil is a very simple arrangement making it possible to receive messages from greater distances, and also somewhat to eliminate any messages not desirable and to listen without confusion to the one wanted.
A tuning coil consists of a single layer of wire wound upon a cylinder and arranged so that connection may be had with any part of it by means of sliding contacts.
The cylinder upon which the wire is wound is a cardboard tube six and three-quarters inches long and two and seven-eighths inches in diameter outside. It should be given two or three coats of shellac both inside and out so that it is thoroughly impregnated, and then laid away until dry. This treatment will prevent the wire from becoming loose after the tube is wound, due to shrinkage of the cardboard.
Fig. 201.—Details of the Tuning Coil.
After having become dry, the tube is wound with a single layer of No. 25 B. & S. gauge green silk or cotton-covered magnet wire. The wire must be wound on very smoothly and tightly, stopping and starting one-quarter of an inch back from each end. The ends of the wire are fastened by weaving back and forth through two small holes punched in the cardboard tube with a pin.
The winding should be given a single coat of clear varnish or white shellac and allowed to dry.
The coil heads or end pieces are cut from one-half-inch wood according to the plan and dimensions shown in the accompanying illustration.
The top corners are beveled and notched to receive the slider-rods. A circular piece of wood two and five-eighths inches in diameter and three-eighths of an inch thick is nailed to the inside of each of the coil heads to support the ends of the cylinder.
The wooden parts should be stained mahogany or some other dark color and finished with a coat of shellac or varnish.