THE STORY OF WIRELESS TELEGRAPHY
For ages people were puzzled as to the nature of light. Pythagoras, that
old Greek who invented what we now call the forty-seventh proposition of
Euclid, thought that the bright body shot off streams of tiny particles
which literally hit the observer in the eye. Sir Isaac Newton thought
the same, but for once "the greatest scientist of all time" was wrong.
For when the Danish astronomer, Romer, discovered that light travelled
at the rate of somewhere about 186,000 miles per second it dawned upon
people that it was scarcely believable that particles of any kind could
by any means be made to move so fast. So they set about searching for a
new explanation, and they found it in the idea that light was conveyed
from the bright body to the observer's eye by means of waves, and as
there cannot be waves of nothing they had to imagine a something to
exist in all the vacant spaces of the universe capable of forming the
waves of light. This something was called the luminiferous ether or
light-bearing ether. We can neither see, feel, taste nor hear it. Our
senses tell us nothing about it. Indeed, if it does really exist it
must be so very different from anything that we do know by our senses
that one is often tempted to doubt its existence. Still, it explains so
many things which are otherwise unexplainable and enables us so
correctly to reason from one phenomenon to another that our reason
forces us to accept it as a fact, at all events until something better
comes along.
This wave theory in regard to light was finally set at rest by the
curious discovery about a century ago by Dr. Thomas Young of London that
if two lots of light were brought together in a certain way they
produced darkness.
Now if a ray of light were a stream of particles, two such rays would
inevitably and always, if added together, produce a doubly brilliant
light, and under no conceivable circumstances could they do anything
else. But two lots of waves can, and do, under the proper conditions,
neutralize each other so as to produce rest.
This mutual action upon each other of two sets of waves can be very
simply exhibited by two violin strings tuned to _nearly but not quite
the same note_. If you have a violin handy, try it and you will find
that when either string is plucked separately it gives a steady
continuous sound, but if both be plucked at the same time they give a
throbbing sound. That is because, periodically, as one string is coming
up the other is going down, so that they neutralize each other, while at
other times, owing to the fact that one is vibrating faster than its
fellow, both are rising and falling together. When neutralizing each
other there is a momentary silence, while in between the silences come
the times when both are acting together and therefore producing a
specially loud sound. And so as the vibrations of the faster keep
gaining upon those of the slower string one hears a continual crescendo
and then diminuendo repeated over and over again. So two sets of sound
waves sometimes produce silence.
And in like manner two sets of light waves can be made so to "interfere"
(that is the technical term) that together they produce darkness.
So for a hundred years or more people have, generally speaking, accepted
the idea that light consists of waves in a medium called The Ether. Heat
also is brought to us from the sun and from any distant hot body by
similar means, the difference between light waves and heat waves being
simply in their wave length or the distance apart. The different colours
of light, too, are to be accounted for by different wave lengths.
You have of course seen how a magnet can act upon a piece of iron at a
distance. You may, too, have tried the experiment of jerking a magnet
past a piece of wire, thereby generating an electric current in the
wire. Both those things need, for explanation, that we assume the
existence of a something invisible and undetectable by our senses
between the magnet and the iron and between the magnet and the wire, by
which the action of one is conveyed to the other. So people imagined
another Ether capable of acting like a link between the magnet and the
iron and between the magnet and the wire.
Now just about half a century ago a celebrated professor of Cambridge
University brought all these facts about light, heat, magnetism and
electricity together and by skilful reasoning showed that but one Ether
sufficed to explain all these things. He showed how magnetic and
electric forces acting together could produce waves like those of light
and heat. And finally he demonstrated by figures that waves so formed
would necessarily travel at the very speed at which light and heat are
known to move.
This is known as the electro-magnetic theory of light. And not content
with showing the nature of things already known, Professor Clerk-Maxwell
added a prophecy that there were other waves in existence of longer wave
length, which no one then knew how to make or to detect if made.
Following up this prophecy many investigators sought these waves, and
the first to find them was Professor Hertz of Carlsruhe in Germany.
Fortunately for his position in the minds of English people he died
before the War, so that his name is not sullied by the stupidities of
which German professors in more recent days have been guilty. On the
contrary, his writings show him to have been a kindly, modest, genial
soul, and particularly gratifying is his generous assertion in one of
his books that had he not himself discovered these waves he is certain
Sir Oliver Lodge would have done so. He seemed quite anxious to share
the credit of his discovery with his "English colleague" as he called
him.
Let us see then how these "Hertzian waves" are produced. In the year
1748 a Dutch experimenter named Cuneus thought he would try to
electrify water. He got a glass flask and filled it with water into
which he let drop one end of a chain connected to an old-fashioned
frictional electrical machine. Thus he stood with the flask in his hand
while a friend worked the machine. After a short time the friend stopped
and Cuneus took hold of the chain to lift it out, when to his
astonishment he received a shock which knocked him over, broke his flask
and sent him to bed to recover.
Unwittingly Cuneus had invented what became known thereafter as a Leyden
jar, Leyden being the town in which he lived. It consisted, you will
notice, of two conductors, the water and his hand, with an insulator,
the glass, in between.
To understand or rather to give ourselves a useful working explanation
of how such an apparatus comes to be charged we must first imagine that
everything contains a certain normal amount of electricity which we can
by certain means add to or take away from at will. When we add some to
anything we say we have given it a positive charge: when we subtract
some we say that we have imparted a negative charge. Clearly, if we add
some to one thing we must first obtain it from something else, and if we
take some away from one thing we must do something with what we have
taken, and so we add it to something else. Therefore whenever we charge
anything positively we must charge something else negatively and vice
versa.
Now the ease with which we can thus charge two bodies seems to depend
upon their nearness to each other, so that the easiest things to charge
are two plates of metal separated by the thinnest possible insulator.
Modern Leyden jars are usually formed of a thin glass jar with a lining
inside and out of tinfoil.
The Leyden jar is, however, only one form of the piece of electrical
apparatus known as an electrical condenser, and many other forms exist.
For example, a flat sheet of glass with foil above and below, or several
such piled one on top of another. An eminent electrician whom I know has
recently made some of two tin patty pans put bottom to bottom, nearly
but not quite touching, the whole being enclosed in a solid block of
paraffin wax. And I might describe many other forms, but whatever they
may be every one is essentially two conductors with an insulator
between.
Now when a condenser has been charged its charges remain for a
considerable time unless they be given a chance to escape. Suppose you
have a charged condenser and that you take a wire and with it touch
simultaneously both the conductors, the surplus on one "plate" will rush
through the wire and make good the deficiency upon the other; it will
thus in an instant become discharged.
Now several scientific men had suggested, before Hertz's time, that when
that occurred something else happened too. They thought that the charge
did not simply rush from one plate to the other instantly, but that it
oscillated to and fro for a period; that the surplus rushing round
overshot the mark, so to speak, and not only made up the deficiency but
caused a surplus on the opposite plate, after which this new surplus
rushed back again through the wire, doing the same thing, though to a
less and less degree, several times over before a condition of perfect
rest was reached. To use a simple analogy, it was thought that the
surplus swung to and fro like the swinging of a pendulum. We know that a
pendulum swings because of its inertia, and electricity possesses a
property very like inertia which, it was thought, would cause it to
behave in the same way.
The Ether waves travel at the rate of 186,000 miles per second, so that
if, as was thought, a sudden current of electricity gives rise to a
wave, currents which succeed each other at the rate of one per second
would produce waves 186,000 miles apart. A hundred currents per second
would give a wave length of 1860 miles. A thousand per second would give
186 miles. But a thousand succeeding currents per second are difficult
to produce, and 186 miles is so very much greater than the tiny fraction
of an inch, which is the length of the light and heat waves, that Hertz
had to find some way of making currents succeed each other faster even
than a thousand times per second.
So he thought of these oscillating currents which were supposed to occur
when a condenser was discharged, and he rigged up a condenser with an
induction coil and a spark gap in a way which he thought would do what
he wanted.
There is not room here to explain the Induction Coil, indeed it is so
well known that it will be quite sufficient to state that it is an
apparatus which takes steady current from a battery and gives back
instead a lot of little spurts or splashes of current at a rate of,
say, fifty or one hundred splashes per second, according as we adjust
the little vibrating spring which forms a part of the coil. We can so
connect this to a condenser that each splash will charge it up; and we
can combine with it a spark-gap, that is to say, a gap between two
knobs, so that every time it is charged it immediately discharges again
through this gap. Thus we may have, say, one hundred splashes per
second, and each splash is followed by several oscillations across the
air-gap, the oscillations taking place at the rate of perhaps a million
per second. Each series of oscillations is called a "train."
Now a million per second gives a wave-length somewhere about what Hertz
wanted, so he arranged his apparatus as just described.
For a condenser he used two metal plates a little distance apart, the
air between forming the insulating material. He set up his apparatus in
a large room, and having started the coil he moved about with a nearly
complete hoop of wire, the ends of which nearly touched. Working in
darkness he found after a while that sometimes he could see little
sparks, very small but just visible across the gap between the ends of
the bent wire. Those sparks only occurred when the coil was in action,
and so he knew that the one was the result of the other's work. By
careful painstaking experiment he found that the sparks were
unquestionably caused by waves, and that the waves moved with the same
speed as light, also that they could be reflected and refracted just on
precisely the same principles as those which control light. Moreover,
he measured the wave-length.
At first sight it seems incredible that anyone could measure the
distance apart of waves which travel at such a speed as 186,000 miles
per second, but fortunately, by a special application of "interference,"
it is possible to make the waves stand still and tamely submit to
measurement. An example of this can be seen by simply tapping a glass of
water, when the ripples being reflected off the sides interfere with
each other and become stationary. Stationary waves are half the
wave-length of the original waves, and by using this method Hertz was
able to make a measurement which at first sight seems beyond the bounds
of possibility.
Thus Hertz discovered how to make the waves which Clerk-Maxwell had
predicted and also how to detect them when made.
It was not long before the idea arose of using these waves for
signalling to a distance. Many experiments were made but with no very
striking success until 1896 when Marconi first came to England.
Hertz had noticed that the farther apart he placed the plates of his
condenser the farther could he get his tell-tale spark, so Marconi saw
that the plates of his condenser, too, must be far apart. He also found
that the earth could be used as one of the plates, that in fact there
was a great advantage in so using it. So, one plate having to be the
earth itself and the other removed as far as possible from it, the tall
masts of the wireless antenna came into being.
[Illustration: LISTENING FOR THE ENEMY.
Special sensitive cylinders are sunk into the ground to which the usual
telephonic apparatus is fixed. This enables the sappers to detect any
underground operations by the enemy.]
When Marconi came to England he was taken under the kindly wing of Sir
William Preece, the veteran engineer of the Post Office, and the
facilities which Sir William was able to give no doubt helped largely in
his subsequent rapid progress. After a few experiments in London he got
to work across the Channel, sending messages from the North Foreland
Lighthouse to Wimereux on the coast of France, including congratulatory
messages between the French authorities and good Queen Victoria.
A little later he was signalling from Niton in the Isle of Wight to the
mainland and to the far west at the Lizard. The first wireless telegram
which was actually paid for was sent by Lord Kelvin, the father of cable
telegraphy, from Niton to the mainland, whence it was transmitted by
land wires to Sir George Stokes. This incident, so interesting because
of its marking a stage in the history of this great invention, also
because of the persons concerned, occurred in 1898.
But Marconi was quickly increasing the range of his apparatus far beyond
anything already mentioned. He journeyed in the Italian warship _Carlo
Alberto_ as far north as Cronstadt and as far east as Italy, keeping in
communication with England all the time. Then he crossed the Atlantic,
again keeping up communication with England the greater part of the
journey.
Raising his wires to a great height by means of kites he was soon able
to signal from Nova Scotia to the great station just previously built at
Poldhu in Cornwall, and then wireless telegraphy from land to land
across the great ocean became an accomplished fact.
We all know how things have progressed since then. A telegram by Marconi
is as commonplace to-day as a telegram by cable. The British Government
is now engaged upon a series of stations dotted about the globe in such
a way that every part of the widely separated British Empire shall be in
constant touch with every other part by wireless telegraphy. In other
words, the range of the system has now become such that nothing further
is needed.
The British Admiralty has a few wires slung to posts on the top of the
offices in London, and those few wires enable touch to be maintained
with ships. As almost every intelligent newspaper reader in Great
Britain knows, the Germans were in the habit, during the war, of sending
news to the United States by wireless telegraphy, which news was always
picked up by the Admiralty installation and circulated to the British
newspapers, often to the amusement of their British readers.
The famous _Emden_, too, which had such a run of success until it
encountered the Australian cruiser _Sydney_, met its end entirely
through the intervention of wireless telegraphy.
These incidents give us a good idea of the usefulness of wireless in
naval warfare. In military work it is used chiefly in connection with
air-craft, but of that more will be said in another chapter.
[Illustration: TRANSMITTER. RECEIVER.
DIAGRAM SHOWING THE PRINCIPLE BY WHICH THE AERIALS ARE CONNECTED TO THE
APPARATUS.]