Man has no sense organs which record the impact of electrical waves, but he has succeeded in devising instruments which register that impact, and which make it perceptible to the organs of sight or of hearing. The operation of the electrical waves may be best explained, perhaps, by the analogy of sound. When the string of a piano is struck by its hammer it vibrates, and communicates its vibrations to the surrounding air; these vibrations, travelling outwards in waves, produce corresponding vibrations in the ear-drum of a listener. The string is tuned, by its tension and its weight, to a single note; the ear can adapt itself to receive and transmit to the brain only a limited range of notes. There are many vibrations in the air which are too rapid or too slow for reception by the human ear. The sound-waves of the piano-string produce their effect on any neighbouring body which is capable of vibrating at the same rate as the incoming waves, as, for instance, another string tuned to the same note, or a volume of air enclosed in a vessel which vibrates in correspondence. These are in 'resonance' with the vibrating string; they repeat the original disturbance and reinforce its effect. So it is with electricity. If the electricity with which any conducting body is charged be suddenly disturbed, electrical waves are generated which travel outwards in all directions with the velocity of light. The problem of wireless telegraphy is the problem of producing these waves by means of an instrument called a transmitter, and of recording their impact at a distance by means of an instrument called a receiver. In its simplest form the transmitting instrument consists of two conducting bodies, or plates, charged the one with positive the other with negative electricity, separated from each other by air or some other insulating material, and connected by a coil of wire called an inductance coil. To explain the how and why, so far as these questions can be explained, would involve a whole treatise on electricity; for the present purpose it is enough to say that when the two plates are connected through the coil, the electrical discharge is oscillatory in character, as the current runs to and fro between the one plate and the other, and that these oscillations are radiated into space in the form of waves. The frequency of the waves, the rapidity, that is, with which wave follows wave, depends on the size and proximity of the plates and on the length and form of the coil which connects them. The receiving instrument is similarly constructed, and can be so adjusted that the waves which it would generate if it were a transmitter would have the same frequency as those it is to receive. It is thus in resonance with the transmitter, and the effect of the incoming impulses is greatly enhanced.