Wireless Telegraph Construction For Amateurs

by Alfred Powell Morgan

The resistance of metallic conductors to high frequency currents is several times their normal resistance to constant currents. The larger the diameter of the wire the greater is this ratio. This increase of resistance is due to the fact that the high frequency currents permeate wires only a very short distance. In the case of copper, the depth is only about one three-hundredth of an inch and with other metals much less. Therefore it is advisable to use as leads and conductors of large condensers, stranded wires or flat ribbons of sheet copper in order to present more surface and offer less resistance than solid conductors of an equal cross sectional area.

Iron must never be used, as its resistance to these currents is over fifty times that of copper.

After connections are once established between the jars or the condenser units, they cannot be altered nor the capacity changed without re tuning the circuits afterwards.

CHAPTER VIII. SPARK GAPS OR OSCILLATORS.

The oscillator or spark gap is one of the most important yet often the most poorly adjusted part of a wireless station.

To obtain a good oscillatory discharge with little damping it is necessary that the resistance of the circuit should be kept low, and since the greatest part of the resistance is in the spark gap it would appear as if this must be very short. While there are reasons for keeping it short there are on the other hand very good reasons why it should not be made too short, and the proper length should be a sort of compromise to be determined by experiment.

If the gap is too short, the discharge will form an arc and the only oscillations taking place will be those corresponding to the frequency of the charging current. Power consumption is also in favor of a long gap, since it causes the induction coil or transformer to draw less current from the line. Another argument in favor of a long gap is the fact that the condenser is charged to a higher voltage and more energy stored up, which makes a greater distance of transmission possible. But as stated above (and there always is a tendency for the amateur operator to open out his gap as long as the sparks will continue to jump steadily) the discharge will not oscillate but will merely set up a unidirectional current.

There exists a proper gap length for a given circuit which will cause that circuit to emit a maximum amount of energy and which may be determined accurately only by means of a hot-wire ammeter placed in the aerial circuit. The proper gap length is then indicated by the maximum deflection of the meter.

The correct adjustment of the gap may be approximated by the experienced operator. If too short, the spark will be hissing and flaming and is in extreme cases red or yellow colored. It should be lengthened out until it is thick and white and a slight increase in sound is noted. The spark should not be, as commonly thought, stringy and crackling.

Arcing is often caused by insufficient condenser capacity in the circuit. But if the capacity is increased to remedy the arcing, the gap length should also be increased or otherwise the potential of the condenser may not be sufficiently raised to permit a disruptive discharge to take place.