Lessons in Wireless Telegraphy

by Alfred Powell Morgan

The efficiency of an undamped wave transmitter is far greater in almost every respect. The selectivity at a receiving station listening to undamped wave signals is very marked in comparison to that when spark signals are received.

The problem of producing undamped oscillations by means of an arc was first solved by Poulsen and is known as the Poulsen arc or generator. This type of generator is used for radio work in this country, by the Federal Telegraph Co., in the stations at Sayville and Tuckerton, in many U. S. Naval stations and on board all the U. S. first line battleships.

FIG. 62. The Poulsen Arc for generating Undamped Oscillations.

An arrangement by which such oscillatory currents may be produced is shown in its simplest form in Fig. 62. It consists of an arc, around which is shunted a condenser in series with an inductance. The arc is connected to a source of direct current, preferably having an E. M. F. of 500 volts or more.

The positive electrode of the arc is copper, kept cool by circulating water through a hollow interior or a water jacket. The gap in itself is enclosed in a chamber filled with hydrogen gas or a gas containing hydrogen. The arcs in practical use for generating undamped oscillations are arranged so that they operate in a strong magnetic field. The carbon electrode is constructed so that it is slowly revolved by a small electric motor.

The hydrogen gas atmosphere in which the arc is enclosed is produced by a small feed cup, similar to the ordinary lubricating oil cup, located over the case and filled with alcohol which continuously drips into the flame chamber, where it is vaporized by the heat.

Arc transmitters of large capacity are not as expensive or as bulky as spark transmitters of equal power. The difficulties of handling and controlling a large amount of power in connection with a transmitter of this sort are also not as great as in the case of a spark transmitter. The condenser used with an arc is not nearly so large as that required for a spark transmitter of equal capacity and the voltage of the current is much lower. Condenser breakdown, leakage and insulation problems are therefore not as great.

For telegraphing with damped transmitters, a key which alternately makes and breaks the primary circuit is sufficient. This is not possible, however, with an arc. The distance between the arc electrodes is usually greater than the gap length which the dynamo voltage would jump and form an arc whenever the key should be closed. It is therefore usual to arrange a key or relay so that it short circuits a portion of the aerial inductance or helix when closed. This short circuit is sufficient to throw the circuit out of tune so that it cannot be heard at the receiving station without readjusting the instruments.

The Poulsen arc may be used for radio telephony. A telephone receiver is arranged so as to vary the currents and impress the vibrations of the voice upon the oscillations set up by the arc.