The Tuning Coil is a device for accurately adjusting the oscillation circuits to receive the waves.
Its action may be illustrated to a certain extent by pressing down the loud pedal of a piano and at the same time whistling a note loudly and clearly. Listen carefully and some of the wires in the piano will be heard sounding the note whistled. At each vibration of the note of the whistle a wave of pressure went forth from the lips and reaching the wires gave them all a tiny impulse. The impulses followed each other rapidly at definite intervals giving each of the wires the same push each time. The wires which are tuned to produce the note on the piano corresponding to that of the whistle will vibrate energetically enough to produce a sound themselves. They are the wires to which the impulses are rightly tuned so that each one adds to the motion it has already acquired. We all know how a child sitting in a swing may be made to swing back and forth by giving a succession of little impulses properly timed. The small pushes are superimposed on one another, the result being a single large motion.
FIG. 76.—Analogy between swinging and tuning.
The "impulses" generated in the receiving aerial are exceedingly weak and in order to produce an effect must be timed so as to follow one another in proper succession. Tuning devices are for this purpose and by their means the receiving circuits and instruments may be carefully adjusted to the same wave length or "note" as the transmitter so that the high frequency currents in the aerial will arrive at the proper time to oscillate or surge back and forth to produce the maximum results.
In this way it is possible to convey intelligence over long distances by the repetition of small impulses without it being necessary to send any very energetic ones. By arranging the stations so that each one emits its own definite wave different in period or length from that of the others it is possible to operate several stations at the same time in the same neighborhood without interfering with one another. The apparatus is then said to be selective because the instruments can be adjusted in a few seconds to receive from any desired station and to exclude others.
FIG. 77.—Receiving a message in a Marconi transatlantic station.
The tuning coil consists of a cylinder wound with bare copper wire spaced so that the turns do not touch one another. Variable contacts called "sliders" are so arranged that connection can be made almost instantly to any desirable turn of wire. The tuning coil is connected to the aerial and receiving apparatus in the manner illustrated in Fig. 79. By moving the sliders back and forth the wave length of the system may be added to or detracted from and any desired "tune" quickly reached so that it is possible to listen to any station desirable and exclude the others. The cylinder over which the wire is wound usually consists of a thick cardboard tube treated so as to be moisture proof. Bare wire is preferable to all forms of insulated wire. The coil is usually three to four inches in diameter and eight to twelve inches long.
