L. T. KNIGHT

Wireless telegraphy is the name given to the transmission of dots and dashes by means of waves set in the ether by powerful oscillating discharges.

Ethral waves conveying perceptible light and heat have long been recognized, and these waves are known to have a velocity of propagation of about 180,000 miles per second, differing from one another only in their wave length and frequency. Violet light waves have a frequency of 750 trillions, and the lowest perceptible heat wave is estimated at 160 trillions per second.

With the discovery that an electric spark or •discharge could produce disturbances at a distance without a conducting medium, came the general acceptance of what has been styled the Hertzian wave, a new and distinctive wave, possessing practically the same velocity as other etheral waves and likewise capable of reflection, deflection and refraction.

Early experiments consisted merely of devising apparatus for sending Hertzian waves into space and receiving the same at a distance. More recent experiments have been devoted to the adjustment of wave lengths, with the intention of perfecting methods whereby messages can be sent to predetermined points speedily and without fear of interference.

Since the velocity of all etheral waves is about the same it will be found that the length varies inversely with the frequency. In a recently published article on etheral waves, by a foreign scientist, was given such an admirable description of the relation of the aerial wire and the wave length, that the writer prefers to quote a paragraph or two, instead of setting forth the same matter in a less masterly way.

"If a rod of steel be held firmly by one end and struck a sharp blow, it will vibrate at a frequency depending entirely on the length and other physical properties of tine rod. The period of this vibration will be the same in all parts of the rod: - the amplitude will vary from nothing at the held end to a maximum at the free end. Similarly, if a wire or other conductor having one end insulated and thus free to vibrate electrically, while the other end is held at a constant potential by earthing or other means, be struck an electric blow, electric oscillations will be set up in the wire, the freguency of which will depend entirely on the length and other properties of the wire, and the amplitude of this vibration (alternating potential) will vary from nothing at the earth end to a maximum at the free end.

If the steel rod had been struck a number of light blows properly timed, the same amplitude of vibration could have been obtained as by the single heavy blow, but these light blows must be properly timed. From the oscillatary nature of the electric spark it is not practicable to strike one heavy blow and so get the vibratory or radiating effect. The frequency of the blows delivered by it must be suited to the natural frequency of the wire. This accord of spark frequency with aerial wire is known as resonance. When perfect resonance is obtained the aerial wire vibrates freely and sets up an electrical wave in the ether which can be shown to be proportional to the length of the wire. "

But in a simple installation, where one terminal of the spark gap is connected to one end of the aerial wire and the other terminal of the spark gap connected with the earth, the capacity of the aerial wire being small and likewise the quantity of oscillating electricity small, the oscillations are short lived and not likely to be effective at any distance.

The value, therefore, of a closed circuit installation presents itself, and this subject will be fully treated, together with diagrams, in future articles.