at Edinburgh in 1893.
During the same year Nikola Tesla published his researches on high
frequency currents; on these much of the later work on wireless
telegraphy was based. In 1895-6 William Rutherford set up at the
Cavendish Laboratory apparatus by which he received signals in distant
parts of Cambridge up to a distance of half a mile from the oscillator.
Many other men of science, among whom was Captain H. B. Jackson, of the
Royal Navy, were at work on the problem, when in 1896 Signor Guglielmo
Marconi arrived in England with an apparatus of his own construction
which ultimately brought wireless telegraphy to the stage of practical
and commercial utility. By 1899 signals had been transmitted across the
English Channel.

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 th