Wireless Telegraph Construction For Amateurs

by Alfred Powell Morgan

Fig. 6. Resonance Tube.

This may be made more clear by the comparison of an electrical circuit with a column of air. Fig. 6 represents a cross section of a glass tube, T, lying in a horizontal position and containing a cork, C, which can be slid to various positions. By adjusting the cork we are able to obtain various depths of air in the tube from its open end, M, to the cork, C.

When a vibrating tuning fork, F, is held opposite the open mouth and the cork slid back and forth it is found that the sound of the tuning fork is greatly increased in volume at a certain position of the cork. If the cork is then removed from this position the sound decreases in intensity. When the cork is in such a position that the sound of the fork is reenforced, we have secured resonance. When in this condition and the prong of the vibrating fork is moving toward the open mouth of the tube a "condensed" pulse of air travels down the tube and back again, having been reflected at the cork and reaching M just as the prong of the fork begins its excursion away from the open mouth of the tube. When the prong of the fork is moving away from M a "rarefied" pulse of air moves from M to C and back again by the time the prong is ready to begin its next vibration. When the tube is not in resonance, the successive condensations and rarefactions passing up and down the air column interfere with one another and decrease instead of increase the sound of the tuning fork.

If we substitute the sound waves emitted by the tuning fork for high frequency oscillations and the air column for the electrical circuit we may readily see that by adjusting its length, resonance can be produced. If the length of the air column is measured it will be found that the reenforcing of the sound of the fork reaches a maximum when the depth of the air column is one-fourth of the sound wave length given by the fork. Likewise resonance is produced in wireless telegraphy when the length of the circuit is approximately one-fourth the length of the waves. Vice versa, the wave emitted from an ordinary closed circuit transmitter is approximately four times the length of the aerial wire. For example, an aerial 25 meters long will emit waves having a length in the neighborhood of 100 meters.

As stated above, tuning is accomplished and resonance or syntony established by varying the inductance and capacity of the circuit. The capacity of a circuit may be defined as its relative ability to retain an electrical charge, while inductance is the property of an electric circuit by virtue of which lines of force are developed around it.

Capacity and inductance are opposite or reactive in their effects upon a circuit. If the value of one is decreased the influence of the other in increased. Fig. 7 and the following explanation will serve to illustrate this.

Fig. 7. Lag and Lead.

Alternating currents do not always keep step with the voltage impulses of a circuit. If there is inductance in the circuit, the current will lag behind the voltage, and if there is capacity, the impulses of the current will lead. Fig. 7 A illustrates the lag produced by inductance and B the lead produced by capacity. In A the impulses of the current, represented by the full line, occur a little later than those of the volts as represented by the dotted line. In B the effect is just the opposite and the current leads. These reactive effects of inductance and capacity are very pronounced with the high frequency currents of wireless telegraphy, and, as stated before, are the factors which determine the period of the circuit.