TELEGRAPHING WITHOUT WIRES
THE telegraph pole is one of the signs of civilisation. Along
every railway the telegraph poles, with their insulating cups and
strands of wires, stand like a military guard, connecting each station
with all the rest, and preserving the necessary communication with
headquarters. Along the high roads also, and sometimes over hill and
dale, these outposts of civilisation stretch their long arms. They
resemble human beings in this respect that they suffer from heat and
cold and exposure to the weather. In hot weather the wires expand,
in cold weather they contract, and in stormy weather they groan and
howl. Sometimes they are blown down, and then they are as useless for
purposes of telegraphy as a fallen soldier is useless for war. When
protection from storm is specially necessary, the wires are sometimes
buried in the ground. They are insulated properly and twisted together
into a cable, which looks very much like a wire rope; and the cable is
laid in the ground surrounded by a leaden tube. Such cables are used
when we want to connect two stations across the sea, and when that sea
is the Atlantic Ocean, the cables have to be very stout and strong.
[Illustration: Fig. 70.—Section of Transatlantic Cable. C, core; G,
gutta-percha; J, jute fibres; F, steel wires embedded in hemp.]
All this trouble would be saved if there were some way of telegraphing
without wires. We do not use wires in communicating with each other
by speech or sign, and there is no reason why a similar system
should not be invented for great distances. This has really been
accomplished, after long-sustained efforts, and now we have systems
of wireless telegraphy by which ships in mid-ocean can keep in
constant communication with the land, and even print a daily newspaper
containing all the latest news.
We have seen that when an electric current is sent through a coil of
wire wound round a piece of soft iron we get an electromagnet. The
electromagnet is capable of turning a magnetic needle at some distance.
If we could make the needle turn at a distance of a mile, we should
have a simple system of wireless telegraphy. It would be sufficient
to send the current into the electric magnet now in one direction and
now in another. The needle would then turn towards the West or the
East, and we could arrange with our friend who observes the magnet as
to what we mean by the various signals. The action of the magnet is
felt all over the world, but it is felt much less at a great distance
from us than within a few feet or inches. In fact, when the distance
becomes more than a few yards, the effect is so small that it is quite
invisible. That is why this simple system cannot succeed. We want
a system the power of which is not confined to close quarters. Our
success will depend upon the rate at which the power of our instrument
diminishes when the resistance is increased.
If we drop a stone into water, it sends out circular waves which become
gradually feebler and feebler as the distance increases. At a distance
of twenty feet they are twice as feeble as they are at a distance of
ten feet. Their crests only reach half the height. Bad as this is, it
is much worse in the case of light, and very much worse in the case
of magnetism. In the case of light only a quarter of the strength
remains when we double the distance, and in the case of magnetism
only one-eighth remains. If, however, we could have some system of
telegraphy resembling water waves it would be a great deal better than
any other kind. And this is precisely the way in which the inventors
of wireless telegraphy attained their triumph.
The first man to produce waves of electricity, and to prove that they
were waves of electricity, was a German professor of the name of Hertz.
He lived at Bonn on the Rhine, and spent a great deal of time trying
to prove that electric force does not spread out into space instantly,
but takes time to reach a distant point. He found that this time was
very short indeed. He found, in fact, that the speed of electricity is
the same as the speed of light. He proved that if electricity is made
to surge up and down an electric wire, a wave of electric force travels
out in all directions. If such a wave falls upon a sheet of metal, it
is reflected just as light is reflected from a mirror. A similarity
to light also appeared in many other properties of the electric waves
or radiations. These could be concentrated by a curved mirror, and
refracted by a large prism of pitch. These waves of electric force are
indeed a kind of light waves. But whereas light waves are so short
that some fifty thousand of them go to the inch, the waves which Hertz
discovered were each several yards long.
The first practical system of wireless telegraphy was invented and
carried out by a young Italian engineer of the name of Marconi.
Strictly speaking, it may be said that Marconi discovered nothing.
What he did was to apply the scientific discoveries of his predecessors
and make them practically useful. Marconi made the electric discharge
pass between two round knobs, and to these two knobs he attached two
wires. One of the wires went high into the air, and the other went down
into the ground. The consequence was that when the discharge passed
between the two knobs, electricity was in turn thrown up into the air
and thrown down into the ground. The effect was very much like that
produced by throwing a stone into water. A wave of electric force
started from the wire and spread out all around it. As soon as any
electricity was thrown into the earth, it spread out along the surface
in great waves, which travelled over the ground at an incredible
speed. As soon as electricity was thrust up along the wire, a want of
electricity was produced in the earth, and this electric vacuum, as we
might call it, spread out along the ground in the same way as the first
pulse. It was indeed a process resembling the crest and the valley of a
wave of water.
This was, therefore, the state of things likely to produce the best
results. Any other vertical wire at a distance along the surface of the
earth would experience the effect of such waves. The electricity would
surge up into the wire when the crest of the wave readied it, and would
rush down again as the wave subsided.
[Illustration: Fig. 71.--Guglielmo Marconi.]
Now, every such motion of electricity constitutes an electric current.
It remained to discover this current, and to devise a delicate
instrument for discovering it even when it is very weak.
The instrument which Marconi used for this purpose was called a
“coherer.” It was a little glass tube filled with some metallic powder.
When an electric wave passes through such powder, the particles
cohere or stick together, and then the powder conducts the current
easily. That being the case it was only necessary to have a battery in
readiness to take advantage of the new state of affairs. That battery
was connected to both the end wires of the coherer, and as soon as the
wave reached the wire and the coherer began to conduct it, a current
from the battery passed through and worked an ordinary telegraph
apparatus. When the coherer was shaken or tapped with a little
hammer, it lost its conductivity and was ready to receive the next
wave. Marconi had a little hammer tapping the coherer at very short
intervals, so that it was always ready to receive any wave that might
be sent out from the sending station as a signal.
That is, in a few words, the principle of Marconi’s system of wireless
telegraphy. It was first tried at a regatta on the Thames, being fixed
up on board a steam launch which followed the racing boats and sent
news of the exact position of the boats to the receiving station on
the shore. It was next tried on the open sea during a regatta off
Kingstown, in Ireland. Later it was used to send messages a distance
of forty miles over Salisbury Plain. For this purpose the wire had to
be taken very high into the air, and this was done by means of kites.
Wireless telegraphy was also tried on board an Italian war vessel
which sailed from Italy to Gibraltar, and round the coasts of France,
Denmark, and Germany as far as Russia. The war-ship remained all the
time in telegraphic communication with a wireless telegraphic station
in Cornwall. But the greatest triumph of all was when Marconi succeeded
in establishing communication between Cornwall, and Connemara, on this
side of the Atlantic, and Canada on the other side. To cover this great
distance large sending stations had to be erected and several thousand
horse-power had to be used. But this system had the enormous advantage
over the cable system of being free from any dangers threatened by the
Atlantic Ocean.
Wireless telegraphy has been the means of preventing great disasters at
sea by enabling vessels in distress to summon the aid of other vessels,
notably during the sinking of the _Republic_ off Nantucket and
stranding of the _Slavonia_ on Flores Island, Azores.