The transmitting helix or tuning coil supplies the greater part of the inductance which is so necessary for the production of electrical oscillations in the transmitting circuit. It consists merely of a few turns of heavy copper or brass wire wound in a helix around a form. Two or more movable contacts permit various amounts of the inductance to be inserted in the open or closed circuits.

The tuning helixes described in this chapter are offered principally to serve as guides. They have been designed as carefully as possible to suit the transmitters for which they are recommended.

There are factors, such as the aerial, etc., which vary greatly in stations of the same rated power and make it best to determine the length of the helix by actual experiment.

A certain amount of inductance is necessary in the closed circuit for the production of electrical oscillations and to transfer energy to the open circuit. Inductance beyond that necessary to receive energy from the closed circuit lessens the radiation and makes it necessary to increase the period of the open circuit by adding wires to the aerial.

The open and closed circuits of a tuning helix constitute an oscillation transformer, and the two circuits if they are very close together or intertwined are said to be closely coupled. When separated or far apart they are loosely coupled.

Closely Coupled Helix.—Fig. 61 illustrates a closely coupled tuning helix, suitable for ordinary induction coils giving sparks up to 3 or 4 inches in length.

The heads of the helix are circular pieces of hard wood 1 inch thick and 12 inches in diameter. Six rectangular notches are made at equal distances along the edges of the heads. Six uprights 3/4 x 1/2 inch and 8 3/4 inches long are fastened in the notches with small round headed brass screws so that the heads are separated a distance of 6 3/4 inches. Grooves are cut in the outside face of each of the uprights at a distance of 7/8 inch apart. The wire forming the helix is 1/4 inch brass. Brass wire is springy and retains its shape better than copper. About twenty-two feet will be required. The wire is wound in the grooves in the uprights and held in place by a few double pointed tacks placed judiciously. The adjacent turns of wire will then have a separation of about 5/8 inch.

The helix is raised above the level of the operating bench or table by three small feet fastened to the under head, 120 degrees apart.

If it is desired to use this helix with the one quarter K.W. transformer, the diameter of the wire should be increased to 5/16 inch. The two K.W. transformer will require seven turns of 1/2-inch hard drawn brass wire two feet in diameter.

When setting up a transmitting station, it is a good plan to coil up a long piece of the wire to be used on the helix around a rough form. The adjacent turns should be the same distance apart as they are to be on the finished instrument. Tune up the station with this improvised helix and ascertain the amount of inductance required. It is then easy to design a helix containing the proper amount of wire. Four or five feet extra should be included to allow plenty of range in case it is ever necessary to make any changes in the aerial or condenser.

Plate III illustrates the transmitting circuits of the various commercial systems. The circuit used will determine the number of binding posts and movable clips necessary. Fig. 63 gives the diagram of a very good circuit.

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.

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.

Tuning a Transmitter.—Fig. 62 shows a complete wiring diagram of a transmitter with a hot wire ammeter inserted in the aerial circuit.

To tune such a transmitter place both contacts A and B together on a turn of wire near the center of the helix. Set the transmitter in operation by pressing the key and move both contacts together along the various turns of wire until the meter shows a maximum reading. Then vary B alone until the reading is still higher. Shifting A varies the inductance in the closed circuit and B that of the open circuit.

Fig. 65 illustrates the circuit of a loosely coupled transmitter. To tune such a circuit it is necessary to connect a small needle spark gap between the aerial and the ground.

A suitable spark gap for this purpose may be made of two darning needles mounted in double binding posts.

Set the clips, A and B, at the center turns of their respective coils. Press the key to operate the transmitter and adjust the clip, A, until the best spark is obtained in the small needle gap. Then adjust B until the spark is still better.

If several wires seem to give the same results, move the secondary a little further away from the primary and try again. After securing the best spark it is possible to obtain, remove the needle gap, which is only used in testing. The hot wire ammeter should then be placed in series with the aerial. By slowly and carefully adjusting the clips and varying the coupling, the hot wire ammeter will indicate the proper "tune."

The secondary of the loosely coupled tuning coil produces a "kick back" effect on the primary, due to induction, and unless the two coils are somewhat separated, the wave emitted from the transmitter will have more than one "hump" and a person receiving the wave will be able to tune it in on two places on his tuning coil.

A loosely coupled tuning coil does not radiate so much energy as a closely coupled helix, but will, when properly tuned, emit a wave which is not so highly damped. Therefore as far as distance is concerned the loosely coupled type is perhaps the most efficient.