Though one or more means of transmitting messages by electricity have
been known now for a great many years, the mechanisms by which they are
accomplished are understood only by those who take a general interest in
physical science, and the few to whom electrical communication is a
profession. So far as theory and details of working are concerned, there
are a good many people still in the same shadowy frame of mind as the
old Aberdeen postmaster, of whom the story is told. When asked to
explain the working of a telegraph instrument he said, “Look at that
sheep-dog. Suppose we hold his hind-quarters here and stretch him out
until his head reaches Glasgow. Then if we tread on his tail here he
will bark in Glasgow. As it is not convenient to stretch a dog, we
stretch a wire, and that serves the purpose.”
As the name implies, “stretching a wire” is unnecessary in wireless
telegraphy, though in order to understand the finer points of theory one
needs to stretch the imagination a little. That, however, is not so
much, because there is any inherent obscurity or difficulty in the
underlying principles, as because the mechanism of all electrical
effects is more or less intangible. Electricity and magnetism operate
across apparently empty space, and the links which connect cause and
effect have to be guessed at.
Three different methods have been made use of in wireless telegraphy,
which may be classed as conduction, induction and wave methods. In the
first method currents are sent through the earth from an electrode to
another at the sending station. In induction, use is made of the
property which alternating currents possess of exciting similar currents
in neighboring conductors, the aim being to get as intense a current as
possible in the secondary circuit. Mr. W. H. Preece, of England, by
combining the two, signaled in this way as far as forty miles. The third
and the only method which has proved practically available is by the use
of electro-magnetic waves.
Guglielmo Marconi, an Italian, after long experiment, patented in 1897 a
method entirely independent of wires, the electric waves being sent,
presumably, through the ether, by the aid of a transmitting apparatus,
and being detected by a coherer, a glass tube filled with metallic
filings, into the end of which the terminals of a relay circuit enter.
The wave falls on conducting material and, the spark gap being replaced
by a coherer, the metallic filings magnetically cling together, closing
the relay circuit, so that a signal is made. On breaking the current, a
slight tap on the coherer or other means breaks the cohesion of the
filings and the relay circuit is broken. In this way a rapid succession
of signals can be sent.
In 1899 Marconi conducted in England an exhaustive series of successful
experiments, sending messages across the English Channel from the South
Foreland to the French coast near Boulogne, and extending his results
until much longer distances were covered. The process of development was
continued until, to the world’s astonishment, signals were sent across
the Atlantic and, finally, commercial messages were transmitted over
this distance.
Marconi’s system is based on the property supposed to be exerted by the
vibrations or waves of electric currents passing through a wire of
setting up similar vibrations in the ether of space. These waves extend
in every direction from the point of departure, and by ingenious and
very delicate receiving instruments their presence in space is indicated
and they are taken up in sufficient strength to repeat their pulsations
and in this way reproduce the signals sent from the transmitter. One
difficulty hitherto has been that a message may be received by hundreds
of receiving instruments in all directions, thus preventing secrecy.
Many efforts have been made to overcome this defect, but as yet with
only partial success.
The distance to which messages can be sent has so far depended largely
on the height to which the wires extend above the earth’s surface, lofty
poles being erected at the stations. The height of these has been
gradually increased until the Eiffel Tower at Paris has been utilized as
a sending station. The strength of the electric waves has been similarly
increased to add to their space-penetrating capacity. The record of
wireless telegraphy has been in this way improved until now it has come
into daily competition with other means of news sending. Methods of
tuning the instruments have been adopted which limit the influence of
the currents to properly tuned receivers and in this way some degree of
secrecy is attained.
[Illustration: MARCONI WIRELESS STATION]
Though the honor of inventing the art of wireless telegraphy is
generally ascribed to Marconi, this is to give him more credit than he
deserves. The principles involved were discovered by others and the
utmost done by him was to invent a practical method of applying them.
There are other systems of wireless telegraphy of later invention than
that of Marconi, through a different application of the same principles.
Messages have been sent to enormous distances, far surpassing the width
of the Atlantic, as from Nova Scotia and Ireland to Argentina, a
distance of 5,600 miles. Under exceptional conditions a distance of
6,500 miles has been attained, but the daily effective range of the best
equipped stations is little over 3,000 miles. For overland messages the
limit of distance is about 1,000 miles.
There are a number of kinds of interference which arise from electrical
disturbances in the earth’s atmosphere. A flash of lightning is liable
to give rise to a wave of enormous power which will set half the aerials
on the earth vibrating in spite of the differences of pitch to which
they are tuned. Thunderstorms are at their worst in the summer in
temperate latitudes, but they occur to some extent all the year round,
and those in the tropics are of extreme violence. As a consequence it is
frequently almost impossible to decipher earthly messages owing to the
imperious signals from the clouds. Of the various methods adopted for
choking off the “atmospherics,” as the disturbances are called, one is
to use receiving circuits which respond only to a narrow range of
oscillations very different from those produced by a lightning flash.
The employment of a high-pitched musical note in the telephone is also
an advantage because its extreme regularity distinguishes it from the
marked irregularity of the stray waves.
[Illustration: WIRELESS STATION ON A STEAMSHIP]
On the palatial passenger steamers that plow the Atlantic the Marconi
apparatus enables the travelers to keep in touch with their friends, to
transact important business on either side of the water, and to secure a
continuity of life which was formerly divided by a sea voyage. All the
larger vessels now publish a daily paper on board, the news in which has
been supplied by the same agencies who feed the newspaper on land.
Information is flashed to meet or overtake the vessel and caught up by
her aerial, as she pursues her way at twenty-five or thirty miles an
hour.
In the case of cargo vessels, the owners are able to get into touch with
them at any point of their voyage. They can advise the captain where to
call for coal or cargo, while he on his part can get into communication
with the authorities or his firm’s agents at the port of call, and have
every necessary or desirable preparation made for his arrival. Should an
accident happen, he can call assistance, inform the owners or relieve
anxiety and suspense. At no time is he isolated from the world. The
fortitude, courage and daring of those “who go down to the sea in ships”
has never been called into question, but it has if anything been
emphasized by the receipt of messages from an operator at his post, to
whom the bonds of duty were as bonds of steel, and who calmly operated
the key until the waves entered his cabin and brought him honorable
release.
[Illustration: U. S. BATTLESHIPS “NEVADA,” “OKLAHOMA” AND “ARIZONA”
The latest type of super-dreadnaught for the United States Navy, with a
displacement of 27,500 tons and engines of 28,000 horse-power.