In modern wireless telegraphy the messages are transmitted by electromagnetic waves which are horizontally polarized at the earth's surface, and which are developed by electrical oscillations whose axis is normal to the surface of the earth and which is connected to the earth at its lowest extremity.

The reason for employing waves of this type and so developed, is that such waves have, at the earth's surface, no component of electric force parallel to the surface and no component of the magnetic force normal to or cutting that surface, except in the immediate neighborhood of the base of the transmitting oscillator. As a result they do not tend to induce currents in the surface of the earth as they travel away from the transmitter except where the surface of the earth deviates from the plane of polarization of the waves at that surface. The energy which would otherwise be rapidly dissipated through the production of induced currents in the earth is therefore conserved in the waves. Moreover, when a deviation occurs between the plane of the earth's surface and the plane of polarization of the waves at the earth's surface, the currents induced in the surface of the earth are such as to bend the wave front into a position normal to the surface with the result that the waves travel over and around mountains and in fact follow the earth's surface whatever be its contour, instead of tending to travel in straight lines

Fig. 3.

like ordinary light waves and as would be the case with vertically polarized electromagnetic waves.

There is not space in this paper to discuss in detail the development of the waves described above, but their genesis and mode of propagation are clearly suggested by Figs. 3 to 8. Fig. 3 and Fig. 4 illustrate the development of a wave by free electrical oscillations in a straight wire AoB, the curve lines indicating a a line of electric force in its various phases from t = 0 to t=5/8T, where T is the time of a complete free electrical oscillation in the wire. In Figs. 5 and 6 the genesis of a wave from a vertical linear oscillator, AO, earthed at its lower extremity, is illustrated, and in Figs. 7 and 8 the effect of an elevation and depression in the surface of the earth upon the wave fronts at that surface is indicated.

We may next consider the earth connection of the transmitting oscillator and the nature of the supports for the oscillator.

Although, by the use of such waves as are described above, normally the current density in the surface of the earth at a distance from the transmitting oscillator is rendered excessively small, nevertheless at and in the immediate neighborhood of the base of the vertical oscillator, the current density is in general very great. Moreover, since the current is of high frequency, it tends to flow only on the surface of the earth. For this reason the usual specifications for obtaining a good earth connection which involve burying a conductor of large area so deep in the ground that it shall be in permanently moist earth are no longer advantageous, and in a properly constructed wireless telegraph station the conductivity of the surface of the earth in the immediate neighborhood of the base of oscillator is artificially increased by a superficial ground-plate composed of sheet metal, or of wire netting, extending radially from the base of the oscillator and covering as large an area about the base of the os-sillator as is available for the purpose and consistent with reasonable economy.

Fig. 4.

Since the waves are horizonally polarized, the supports for the vertical oscillator such as masts or guy ropes, should either be of insulating material or, if of metal, they should be divided by insulation into sections short compared to one-half the wave length of any of the waves to be employed. The reason for this is that the fundamental free period of vibration of a linear conductor is such as to respond most energetically to waves of twice its own length. For such wave-lengths, therefore, the linear conductor would absorb an undue amount of the energy of the waves and moreover because of the rise of potential which occurs at the ends of the conductor for waves of such length it would be difficult to insulate the conductor from the earth or adjacent portions of the support of the vertical oscillator.

We come now to the vertical oscillator itself. This in general consists of two parts, the elevated conductor per re and the devices through which it is connected with the earth.

We shall here consider the relative simple case in which the elevated conductor is a straight, cylindrical copper wire of length a and radius p.

Both theory and practice show that the electrical vibrations in such a wire connected directly to the earth at its lower extremity correspond very closely to the transverse vibrations of a heavily damped stretched string.

Fig. 5.

It such a wire be charged to a high potential and be then permitted to discharge to earth, the electrical oscillations developed in it, and therefore also the electromagnetic waves radiated by it, will not be simple harmonic type and will not be persistent. These waves will not, therefore, correspond to the sound waves given out by a tuning-fork, but will be of the character of those given out by a heavily damped violin string plucked at its center.

The fundamental of the waves given out by such an oscillator has a wave-length which very closely approximates four times the length of the wire, and this is accompanied by all the odd harmonics. The oscillations are so much damped by the energy drawn off by the energy drawn off from the oscillator by radiation as to make the resulting waves more nearly an equivalent of an impulse than of a sustained or persistent train of waves.