THE MOST STRIKING INVENTION OF RECENT TIMES
Probably no invention has made such a sensation during recent years as
wireless telegraphy. And since it is the direct outcome of the most
abstruse, purely scientific investigations, there could be no more
appropriate subject for a place in this book.
For many years there has been a belief in the existence of a mysterious
something to which has been given the name of "The Ether." Totally
different, it should be noted, from the chemical of the same name, it is
entirely a creature of the intellect. None of our senses give us the
slightest direct indication of its existence. No one has either seen,
felt, heard, smelt or tasted it. Yet we feel that it must exist, for the
simple reason that some things which our senses do tell us of are
utterly inexplicable without it.
It was originally thought of in connection with light. Standing at night
upon the top of a hill, we see the lights of a town a mile away. How is
it that those distant gas or electric lamps affect our eyes? They are a
mile away; and the idea that one object can affect another _at a
distance_ is one which the human mind refuses to accept. We feel
compelled to believe that there is something in contact with the source
of light which is affected first, and through which the disturbance,
whatever it may be, is conveyed to our eyes, with which it must also be
in contact. We feel that there must be a something stretching from our
eyes to the distant objects, by which the light is carried. Of course
the air fills the space referred to, but that cannot be the carrier of
light, for if we look through a glass vessel from which the air has been
exhausted we see distant objects undimmed. We also have good reason to
believe that the air belongs specially to our globe, and does not extend
upwards for more than a few miles. Consequently it cannot be air which
brings sunlight and starlight. We are forced to fall back, therefore,
upon the belief in something, of which we have no other knowledge, which
must fill all the vacant spaces in the whole universe, passing, even,
between the particles of which ordinary matter is composed, reaching as
far as the remotest star, able to penetrate everything, and consequently
not excludable from the most perfect vacuum. It is something so
different from anything of which we have any direct knowledge that one
is tempted sometimes to doubt whether there must not be some other
explanation of light. In order to transmit light at the speed at which
we find that it does in fact travel, the ether must be more rigid than
the hardest substance we know of. Many, many thousand times more rigid,
indeed. Yet it seems to offer no resistance to the passage of the
planets through it. Still, there is no other alternative, so far as men
can conceive, and we are compelled, therefore, to believe in the
existence of the ether.
The first things discovered by the telescope were the larger satellites
of Jupiter. With that precision for which astronomers are noted, they
soon drew up time-tables, showing not only the past movements of these
bodies, but also their future ones. They were soon puzzled, however, by
the obvious fact that the moons of Jupiter were not working according to
schedule, to use a railway expression. They got later and later for a
time, and then gradually quickened up until they got too fast. Then they
slowed down again. This repeated itself, and is going on still, with
this difference, however, that the cause has been discovered and the
schedules amended accordingly. The solution of the puzzle was that when
the earth and the great planet are on the same side of the sun they are
some 186 millions of miles nearer together than when they are on
opposite sides of the sun. The evolutions of the satellites are quite
regular, according to the astronomers' calculations, but they seemed to
the earthly astronomers to vary, because of the time which light took to
traverse that 186 millions of miles. When the two bodies were nearest
together the occurrences seemed to happen about 1000 seconds (16
minutes) earlier than when they were farthest apart. Consequently it
became evident that light took 1000 seconds to travel 186 million miles,
or that, in other words, it moved at the prodigious speed of 186
thousand miles per second. That discovery was, of course, many years
ago, but experiments since have proved the figure mentioned to be about
right.
It put beyond question the fact that the action of a distant light upon
the eye was not an "action at a distance," for such action, were it
possible, would take effect at once. Seeing that light passed from the
distant satellites at a definite velocity, and took a certain time to
reach us, it was evident that it was, during that time, passing through
a medium of some sort, and that medium must be the ether, for no
alternative explanation will suffice.
So it became recognised that light really consists of waves or
undulations of some sort in the ether; that a distant, luminous body set
these waves going; that they travelled with a definite velocity, and
then, striking our eyes, produced the sensation known as light. Many
things were found out about light in the years which followed the
discovery of its velocity. The lengths of the waves were
ascertained--that is to say, the distance from the crest of one to the
crest of the next. The different lengths were sorted out and found to
give rise to different colours, while longer waves, which produced no
sensation of light, were found to carry heat, thereby explaining how the
heat reaches us from a distant fire, or from the sun.
Of the actual nature of the waves, however, little was known, although
there was a vague idea that they were connected in some way with
electricity, at which point in the story there comes in the famous name
of James Clerk Maxwell, a professor of Cambridge University, who in
1864 produced before the Royal Society the explanation of the nature of
the waves and their connection with electricity and magnetism. That in
itself was a wonderful achievement, but far more wonderful still is the
fact that he truly predicted the existence of longer waves than any then
known, which no one knew how to cause, or how to detect if caused. That
prediction has since been fulfilled. The long waves have been found; we
know how to make them and how to perceive their presence. They are the
messengers which carry our wireless messages.
The discovery of these, at that time unknown waves, on paper, by simply
calculating and reasoning about them, is more marvellous even than the
feat of Adams and Le Verrier in discovering a planet on paper before
anyone had seen it. It established Maxwell among the heroes of science
for all time.
A magnet acts upon a piece of iron some distance away. The pull must be
transmitted through some kind of ether. A current of electricity behaves
in the same way, acting precisely as a magnet, with power to affect
things at a distance. Again an ether is necessary. A dynamo works by
moving a magnet past a wire which it does not touch, thereby generating
current in it. There again an ether is necessary to transmit the effect
from the one to the other.
Taking, then, the known magnetic effects of an electric current and the
electrifying effects of magnets, he was able to show that the same ether
accounted for all, and for the transmission of light as well, that, in
fact, there was but one ether which performed all these various duties.
He proved from the known facts about electricity and magnetism that
waves such as he imagined would, in fact, move with the speed of light.
And once knowing the nature of the waves, he asserted that in all
probability there were others of which men had then no practical
knowledge.
Maxwell's theory soon set experimenters searching for the means of
producing the long waves which he had predicted would be found.
Several authorities had before then stated their belief that the current
derived from a Leyden jar was not simply a flow in one direction. They
suggested, and gave grounds for the belief, that the current surged to
and fro for some time before it settled down; that it swung to and fro,
indeed, like a pendulum.
There may be some of my readers who are unacquainted with this
interesting piece of electrical apparatus the Leyden jar. It is a
convenient form of what is called an electrostatic condenser. This is
two conductors, generally in the form of two plates with an insulator
between them. In the Leyden jar the insulator is a glass jar, while the
"plates" are coatings of tinfoil, one inside and the other outside. On
connecting one coating to one pole of a battery, and the other to the
other pole, they become charged, one positively and the other
negatively. One, that is, acquires an excess of electricity, while the
other becomes deficient to an exactly similar extent. When the two are
afterwards connected by a wire the surplus on one flashes through it to
make good the deficiency on the other.
Rushing first of all from positive coating to negative, electrical
inertia causes it to overshoot the mark and to recharge the jar with the
charges reversed. Then current begins to flow back again, doing the same
several times over, until at last equilibrium is established.
The power to absorb and hold a charge of electricity, which is the
characteristic of a condenser, is called "capacity."
What, then, is "electrical inertia"? I have already referred to the
effect which the creation of a magnetic field around a current has upon
neighbouring conductors. It also has an effect upon itself. As soon as
the current begins to flow it builds up the magnetic field, and in the
process some of its energy is exhausted. On the original current
ceasing, however, the magnetic field collapses back on to the conductor
once more and in so doing restores that energy. This occurs whenever
current flows, but it is specially noticeable in long conductors, like
submarine cables. In them the battery has to act for a considerable time
before any current reaches the farther end. It is in the meantime
employed in building up the magnetic field around the wire. Then when
the battery has ceased to act the current still comes flowing out at the
farther end--the magnetic field is giving back the energy expended upon
it. Thus a current is reluctant to start flowing through a conductor,
and, having started, is disinclined to stop. This is called
"inductance," and it has exactly the same effect upon the current that
inertia has upon a body. What inertia is to a material body inductance
is to an electric current.
And lastly, the resistance which the conductor offers to the passage of
the current is precisely analagous to the friction of the water in a
pipe.
So, we see, the "capacity" of the two coatings of the jar and the
inductance which occurs in the connecting wire cause the current to
oscillate to and fro for a while when the jar is discharged, which
surging or oscillation is ultimately stopped by the resistance of the
wire. The two coatings and the wire form what is called an oscillatory
circuit.
We can now resume our story.
After much experimenting Hertz, of Carlsruhe, discovered the fact that
when a discharge was taking place in an oscillatory circuit tiny sparks
passed between the ends of a curved wire held some distance away. His
apparatus is illustrated in Figs. 6 and 7. The former, which is termed
nowadays a "Hertz Oscillator," is simply two metal discs almost
connected by a thick wire. The wire is broken, however, at the centre,
and the two halves terminate in two metal balls. Each ball is connected
to one terminal of an induction coil. Now the current comes from an
induction coil in a series of spurts. It is not an alternating current
exactly (since every alternate current is so feeble as to be
negligible), but is practically an intermittent current always in the
same direction. Thus we may call one the positive end of the coil and
the other the negative. A short current comes along with every backward
movement of the little vibrating arm which forms a part of the
apparatus. This breaking of the "primary" circuit may take place perhaps
fifty times per second, so that the intermittent "secondary" currents
will succeed each other at intervals of a fiftieth of a second, or even
less. The brain reels at the attempt to think of a fiftieth of a second,
but it is really quite a long interval as these things go, and during
that interval quite a lot happens. For the current first of all charges
the two plates as a condenser.
[Illustration: FIG. 6.--The apparatus by which Hertz made his
discoveries, hence called the Hertz Oscillator. _a a_ are metal plates;
_d_ is the spark-gap between the two metal balls; _b_ is the battery,
and _c_ the induction coil.]
When they are as full as they will hold the current overflows, as it
were, across the gap between the two balls.
Now an air-gap--a gap that is filled with air, between two
conductors--is a very strong insulator. But when current has once broken
through it it becomes a fairly good conductor. Hence as soon as the
first spark has passed between the two knobs the plates become connected
almost as if a wire were passed from one to the other. And there we have
quite a good oscillatory circuit. There is capacity at each end and a
fairly long length of wire to provide the inductance. Consequently that
breakdown of the insulation of the air in the spark-gap is followed by
electrical oscillations which take place with inconceivable rapidity.
Yet because of the resistance of the spark-gap, which is considerable
even after it has been broken through, the oscillations do not continue
for long. They have died away long before the lapse of a fiftieth of a
second, when the next impulse comes along from the coil. In the meantime
the air-gap regains its insulating properties, and so, on the arrival of
the next impulse, the whole thing occurs once more.
Thus a little train of oscillations is produced for every impulse from
the coil. Every train causes a corresponding disturbance in the ether,
and sends off a train of electro-magnetic waves, and these, falling upon
the distant wire, generate in it a train similar to that which brought
them into being. These trains, in Hertz' simple apparatus, manifested
themselves in the form of minute sparks leaping across the small gap
between the ends of the curved wire (Fig. 7).
[Illustration: FIG. 7.--Hertz "Detector." It was with this simple
apparatus that Hertz discovered how to detect the "wireless waves."]
It was in 1888 that Hertz made this discovery of a way to detect long
electric waves. He subjected the matter to many more experiments and
found that the waves have many points in common with light rays. He
found that they were reflected from certain surfaces, just as light is
reflected from the surface of a mirror. He made prisms which were able
to bend them as light waves are bent by a prism of glass. Some things
appeared to be transparent to them, as clear glass is to light, while
others are opaque. It does not follow that the same things which reflect
light waves reflect electric waves, and so on. The latter can pass
through a brick wall, for example. But the same divergence is to be
observed between light and radiant heat, of which the action of glass is
a familiar example. Clear glass will let light through almost undimmed,
yet we use it for fire-screens to shield us from too much radiant heat.
The important fact is that all three--light, radiant heat and Hertzian
waves--in addition to travelling at the same speed, are reflected,
absorbed or refracted, according to precisely the same principles. This
is almost perfect testimony to their essential identity.
The difference between them, as has been said already, is the distance
from crest to crest of the waves--the "wave-length," that is. And the
reader will wonder by what manner of means this mysterious dimension can
be ascertained. In spite of its seeming mystery the method is very
simple.
It is based upon the fact that two sets of similar waves travelling at
the same speed in opposite directions interfere with one another in a
peculiar way. Suppose that one set of waves travel along to a reflector
and strike it vertically; then another set will travel back from the
reflector exactly similar to the first, except that their direction will
be opposite. And the result will be that at certain intervals they will
exactly neutralise each other, so that at those points there will be no
wave-action appreciable at all. Those points where no action is to be
perceived are called "nodes," and they are exactly half a wave-length
apart.
This will be quite easily understood from the accompanying diagrams. In
each of these diagrams the set of waves marked _a_ are supposed to be
moving from left to right, while those denoted by _b_ are reflected back
and are moving from right to left. It will be noticed that each wavy
line has a straight line drawn through it, dividing it into alternate
crests and hollows, which line is known as the axis of the waves.
Now notice that in Fig. 8 there are points marked x, where
the _a_ waves are just as much above the axis as the _b_ waves are below
it, and vice versa. Hence at those points the two sets of waves will
neutralise each other.
Now turn to the next figure, which, be it remembered, shows the same
waves a moment later, when they have moved a little farther on in their
respective journeys, and it will be seen that there, too, are places
marked x where the two sets of waves neutralise each other. And the same
with the third diagram.
And finally observe that the places marked x are always
the same in all the diagrams--that is to say, they are always the same
distance from the line on the right-hand side, which denotes the
reflector. It will be clear, too, that each node is half a wave-length
from the next.
Thus it can be shown that at every moment, and not merely at the three
indicated in the diagrams, the two sets neutralise each other at the
nodes, that the nodes are always in the same places and half a
wave-length apart.
[Illustration: FIGS. 8, 9 and 10.--These diagrams help us to see how the
"wireless waves" are measured. The _a_ waves are supposed to be moving
from left to right and the _b_ waves from right to left. At the points
marked x they neutralise each other. It is then easy to
discover those points and the distance apart of any two adjacent ones is
half the "wave-length."
_N.B._--In Fig. 10 the _b_ waves fall exactly on top of the _a_ waves.]
Everywhere else, except at the nodes, there is action more or less
energetic, but _there_ is perpetual calm.
But how can we tell where the nodes are? When we recollect that they are
points at which no wave-motion at all takes place it is easy to see that
we shall at those points get no spark in our detector. So what Hertz did
was to set his oscillator going so that it threw waves upon a reflecting
surface and then move his detector to and fro in the neighbourhood until
he found the nodes. Between the nodes, as will be seen by an inspection
of the curves once more, there are other points at which the wave-action
will be twice as great as with the single wave, and so at those points
the response of the detector would be especially energetic.
This mutual action between an incident wave and a reflected wave is
termed "interference," and by it the wave-lengths of all the ethereal
waves have been measured. The plan used in the case of light waves,
although the same in principle, is somewhat different because of the
extreme shortness of the waves.
So the experiments of Hertz not only showed that long electric waves
existed, but that they were in all essentials similar to light, and
their wave-lengths were ascertained. On that basis has been built up
modern wireless telegraphy.
It may be interesting to mention at this point a very curious, and in a
sense pathetic, incident. Professor Hughes, whose name is associated
with certain well-known instruments for ordinary telegraphy, nine years
before Hertz' discovery noticed that a microphone was affected by the
action of an induction coil some distance away. He himself attached some
importance to the matter, but he allowed himself to be dissuaded from
following up the discovery by other scientists, more eminent than
himself at the time, who thought that it was not a promising field for
investigation. But for the influence of these friends he would possibly
be the hero of this story in place of Hertz.
Professor Silvanus Thomson has said that he too noticed the sparks
produced at a distance when a Leyden jar was discharged, but he makes no
claim to precedence over Hertz, since, seeing the phenomenon, he did not
perceive its real meaning, while Hertz, though a little later in time,
realised the profound significance of it.
Hertz himself in his account of his experiments is generous enough to
assert that, had he not discovered the waves when he did, he is quite
certain that Sir Oliver Lodge would have done so.
Before proceeding to describe the principal apparatus used in the
wireless station I should like to devote a little space to the
explanation of a term which will come up again and again, and which
represents that which is responsible, in the main, for the marvellous
advances which the art of sending wireless messages has achieved in the
last few years. I refer to "resonance."
It will be a great help if the reader will try for himself a simple,
inexpensive little experiment. Stretch a string horizontally across a
room and on to it tie two other strings so that they hang down
vertically a little distance apart. To the ends of the two strings tie
some small objects--a cotton reel on each will answer admirably. They
will thus form two pendulums, and, to commence with, they should be just
the same length. Having rigged all this up, give one pendulum a good
swing. It will impart motion of a to-and-fro variety to the supporting
string, which in its turn will pass that motion on to the other
pendulum. In a very short time, then, the second pendulum will be
vibrating like the first. Indeed the _whole_ motion of the first will
shortly become transferred to the second, so that the second will be
swinging and the first still. Then the second will re-transfer its
energy back to the first, and so they will go on until the original
energy given to the first pendulum is exhausted. The point to be
observed is the quickness with which one pendulum responds to the
impulses given it by the other, and the ease with which the energy of
the one passes to the other.
Now reduce the length of one pendulum. On setting the first in motion a
certain irregular spasmodic action is to be observed in the second, but
it is very different from the "whole-hearted" response in the previous
instance. In the former case the second one responded naturally and
readily to the first. Now its response is reluctant in the extreme. It
moves somewhat because it is forced to, but it is apparently unwilling.
Energy has to be _impressed_ upon it. There is no readiness, because
there is no sympathy between them.
That sympathy between the two equal pendulums is "resonance." The same
occurs between two violin or piano strings when they are "in tune."
The explanation is that a pendulum has a certain natural frequency which
depends upon its length. Another pendulum of the same length, arranged
as just described, therefore imparts impulses to it at just the
frequency which is natural to it. Consequently the effect is a
cumulative one, and it responds quickly. Impulses at any other frequency
tend more or less to neutralise each other. In the same way a string, of
a certain length and a certain tension, has a frequency peculiarly its
own, and it will respond to another similar string because the other
gives its impulses at its own natural frequency.
It is on record that an engine in a factory happened to run at precisely
the same speed as the natural frequency of the building, with the result
that after a little time the structure shook so much that it collapsed.
Now electrical circuits in which currents oscillate have a natural
frequency of their own. That frequency depends upon the two electrical
properties of the circuit: capacity and inductance. And if you want to
set up an electrical oscillation in any circuit you can best do it by
giving it impulses at intervals which agree with its natural frequency.
Sir Oliver Lodge seems to have been the first to appreciate fully the
effects of resonance in wireless telegraphy. It is strange that in
England the work of this eminent man in "wireless" matters is not more
fully recognised. When wireless telegraphy reached the point at which
the public became interested, Marconi was just coming to the front and
so, for ever, will his name be foremost in the public estimation. Indeed
more than foremost, for in the minds of many he monopolises the credit
for this invention. Many people are under the impression that he is the
one and only, or at any rate the original, inventor of wireless
telegraphy.
Now Marconi has done exceedingly valuable work in this field. Moreover,
he has been the means of placing the affair on a good commercial
footing. But all the same he is by no means the original or only
inventor. While admitting that he is a remarkable man, who has done
wonders, it is only common justice to refer to the others whose
contributions to the solution of the problem are possibly of equal
value. And, of these, few can compare with Sir Oliver Lodge.
But to return to the question of resonance. At first the distances over
which messages could be sent were but small. Now a marconigram can be
flung across a hemisphere. At first little could be done by day, work
had to be done mainly at night. Now communication passes by day and
night alike. Yet in principle, and in many details, the instruments are
unaltered from what they were several years ago. The main source of all
this improvement is the use of resonance.
To enumerate broadly the apparatus used for the dispatch and receipt of
messages the following list will be useful:--
_Transmitting End_
(1) An Antenna, consisting of a number of wires raised to a
considerable height above the ground.
(2) A Spark-gap, consisting of a series of metal balls with gaps
between them, the outer ones being connected to the antenna and to
the induction coil.
(3) A powerful Induction Coil with batteries or other source of
current to work it.
(4) A Telegraph Key, by which the induction coil can be started and
stopped at will.
_Receiving End_
(1) An Antenna precisely similar to the other.
(2) A Coherer or other "oscillation detector."
(3) A Receiving Instrument which may be a writing telegraph
instrument, a telephone, any of a number of ordinary telegraph
instruments, or a galvanometer.
Transmitting and sending instruments are, of course, installed at both
ends and either of them can be connected to the antenna at will by the
simple movement of a switch.
The antenna plays the part of one of the metal plates in the Hertz
oscillator. Early experiments were made with Hertz apparatus, but the
range of such a contrivance is very limited. For one thing, it neglects
to take advantage of the earth. It is little realised what an important
part the earth plays in the carrying of wireless messages. A very great
step was taken when Marconi dispensed with one of the plates of Hertz,
and used the earth instead; while the other plate gave place to the
elevated wires, the most familiar part of the apparatus to most people.
The condenser is thus formed by the earth as one plate, the elevated
wires as the other, and the intervening air as the insulator. The
"capacity" must be exceedingly small in such an apparatus, but it is
sufficient; while the long lines of electrical force stretching from the
high antenna to the earth produce waves of great carrying power. Lastly,
when the earth forms a part of the condenser the waves cling to it, so
that instead of being largely dissipated into space, they move along the
surface of the earth. The advantage of this is obvious.
At first it was customary to place the spark-gap in the wire leading
from the antenna to the earth, as in the accompanying sketch. Later,
however, it was found better to place the coil and spark-gap in a local
circuit in which the oscillations are first produced. These oscillations
pass through a coil which is interwound with another one connected to
the antenna and to earth, and thus the local oscillations, as we might
call them, induce similar oscillations in the antenna, just as the
fluctuations in one part of an induction coil induce fluctuations in the
other. Indeed the coil in the local circuit and the one in the antenna
circuit actually constitute an induction coil.
The advantage of this is that by introducing condensers the capacity of
which can be varied, and coils the inductance of which can be varied,
into the oscillation circuit it becomes possible to "tune" the circuits
effectively. Thus resonance comes into play and the power expended can
be made to produce the maximum effect.
Some attempts have been made to displace the induction coil in wireless
telegraphy altogether by a specially made dynamo. These machines can
produce either alternating or continuous currents, in fact the
alternating current dynamo is really simpler than the more familiar
continuous-current machine. The difficulty is, however, to run it
sufficiently fast to produce sufficiently rapid alternations. Nicola
Tesla made an alternator (to give the alternating current dynamo its
short title) which could produce 1500 alternations per second, while Mr
W. Duddell made one which produced 120,000, but neither was satisfactory
for the work in question. Could such a machine be made, it would be
invaluable, for it will be apparent that a continuous succession of
waves would be formed by it and not a succession of short trains of
waves such as is produced by the induction coil and spark-gap. The
difficulties are not electrical, but mechanical. It seems doubtful if a
machine will ever be made to run with sufficient rapidity which would
not knock itself to pieces in a very short time.
[Illustration: FIG. 11.--The simplest form of wireless antenna.]
Small alternators are used sometimes, however, to supply alternating
current to the primary of an induction coil, or transformer, as it is
more often called in its larger sizes. The interrupter is only needed
when the primary current is continuous--from batteries, for example.
Alternating current needs no interrupter, and so that bother is removed.
The alternations of a hundred or so per second, which are quite the
common thing with alternators, are just what is needed to excite an
induction coil. Consequently small machines of this kind are to be found
in many stations.
A Danish inventor, Valdemar Poulsen, has adopted an altogether different
method of producing electrical oscillations, which method is the
distinctive feature of his mode of telegraphy. He takes advantage of a
curious effect of passing current between two rods, one of which is
carbon, so as to form an arc such as we see in arc lamps.
My readers are already familiar with the term "shunt" in connection with
electrical matters, and so will perceive at once what is meant when a
second circuit is said to be arranged as a shunt to the arc. The
accompanying diagram will in any case make the matter clear.
The current comes along from the battery or continuous-current dynamo to
a hollow rod of copper which, to prevent it being melted, has cold water
continually circulating inside it. Thence the current jumps across to a
carbon rod, forming an arc between the two rods, and returns whence it
came. In its journey it traverses the coils of an electro-magnet, the
poles of which are one each side of the arc. This tends to blow the arc
out, as a puff of wind blows out a candle, an effect which a magnet
always has upon an electric arc.
The shunt consists of a wire leading from the copper to the carbon rod
with a condenser and an inductance coil inserted in it. The latter coil
also forms one part of that coil by which the oscillations in the local
circuit are transferred to the antenna.
The electrical explanation of what happens when the current is turned on
to an arrangement like this is rather too complex to set out here. It
depends upon a curious behaviour of the arc. It is really a conductor,
yet it does not behave as ordinary conductors do, and the result is that
the continuous current flowing through the arc is accompanied by an
oscillating current in the shunt circuit. And the important feature of
the arrangement is that these oscillations are continuous, in one long
train, not in a succession of trains. The advantage of this has already
been referred to.
One other feature of the apparatus just described should be mentioned,
since it will seem curious to the general reader. For it to work
properly it is necessary that the arc should be enclosed in a chamber
filled with hydrogen or a hydro-carbon gas. Coal-gas is generally used.
Hertz' original discovery was that small sparks could be seen to pass
between the ends of a curved wire when the electric waves fell upon it.
Such "spark detectors," as they are called, are useful in the
laboratory, but not for practical telegraphy.
[Illustration: FIG. 12.--Diagram (simplified) showing how Poulsen
generates oscillations. Current from a dynamo flows through the arc,
whereupon currents oscillate through the condenser and coil (as
described in the text).]
Several people seem to have noticed in years gone by that a mass of
loose metal filings, normally a very bad conductor of electricity,
became a much better conductor when an electrical discharge of some sort
occurred near by. The demand for a wireless receiver had not then
arisen, however, and so the discoveries were not followed up.
Consequently it remained to be rediscovered by Branly, of Paris, in
1890. He placed some metal filings in a glass tube, the ends of which
he closed with metal plugs. Lying loosely together the filings would not
conduct the current of a small battery from one plug to the other, but
when a spark occurred not far away they suddenly became conductive and
allowed it to pass. Several years after this Sir Oliver Lodge took up
the idea as a receiver for wireless messages, and believing that its
action was due to the waves causing the filings to cling together, he
christened it "Coherer."
Marconi succeeded in making a very delicate form of this, although
working on strictly the same lines.
The trouble with a coherer is that when once it becomes conductive it
remains so unless the filings be shaken apart. Lodge therefore arranged
for the tube to be continually struck by clockwork or by a mechanism
like that of an electric bell. Marconi effected a further improvement by
making the current passing through the coherer control the striking
mechanism, so that the latter is normally quiet but administers one or
two taps at just the right moment.
Sir Oliver Lodge and Dr Muirhead devised another detector which, though
quite different in form, is really much the same in principle. A steel
disc with a sharp knife-like edge is made to rotate above a vessel of
mercury. The edge just touches the mercury but no more. On the top of
the mercury there floats a thin layer of oil, a bad conductor. Now as
the disc revolves it picks up on its edge a film of oil, which it
carries down into the mercury. The film adheres so tightly that it
prevents the moving disc from actually touching the liquid metal. Thus,
under normal conditions, the two are electrically insulated from each
other by the film of oil and no current can pass from mercury to disc.
Oscillations, however, caused by incoming electric waves, are able to
break through the oil film and so bring disc and mercury into contact,
whereupon the current flows. The constant movement of the disc restores
the oil-film as soon as the oscillations cease.
The reason why these detectors act as they do is not quite understood.
One suggested explanation is that the oscillating currents heat the
particles and so partially weld them together. Another is that adjacent
particles become charged as the plates of a minute condenser, and so are
drawn tightly together as the plates in an electrostatic voltmeter are
drawn towards each other. Supposing that the original non-conductivity
of the loose filings be due to the film of air which may surround them,
either of these things would account for the film being broken or
squeezed out, resulting in better contact and improved conducting power.
But both suggestions seem to be contradicted by the fact that if the
pieces in contact be of certain substances the coherer works the
opposite way. Under those conditions the conductivity is normally good,
but the influence of the incoming waves causes it to become bad.
In 1896 Professor Rutherford, now of Manchester, described some
discoveries which he had made as to the magnetic effects of
oscillations. A simple little contrivance which he had constructed was
operated by the discharge of a coil half-a-mile away, at that time a
great performance. This detector was simply an electro-magnet with a
steel core instead of the usual soft iron core. The reason the latter is
used in the ordinary magnet is that it loses its magnetism the moment
the current ceases to pass through the coil with which it is surrounded,
while a steel core retains its magnetism. For most purposes a steel core
would render an electro-magnet useless, but in this case it was desired
that the core should be permanently magnetised. So a current was first
passed through the coil to magnetise the core, and then the coil was
connected to a simple form of antenna while a swinging magnet was
brought near so that the magnetic power of the core would be indicated
and any change made apparent. The effect of the discharge half-a-mile
away was to _de_magnetise the core slightly. This was shown by the
movement of the swinging magnet, and so the first "magnetic detector"
was found.
But here, perhaps, I ought to explain the use of the antenna at the
receiving station--its function at the sending end has already been
made clear. The electro-magnetic waves, coming from the distant
transmitter, strike the receiving antenna and in so doing _set up in it
oscillations such as those which set them in motion_. For every
oscillation in the sending antenna there will be another, similar in
every respect except that it will be feebler, in the receiving antenna.
And the oscillations are here led to the detector, of whatever form it
may be, and in it they make their presence felt.
In some few cases a Duddell thermo-galvanometer has been employed as the
detector, in which the oscillating currents report themselves directly.
In coherers the detector works by causing the oscillating currents to
control a continuous current from a battery and it is the latter which
actually gives the signal, but there are a number of extremely
interesting means which have been invented to detect the oscillating
currents by their heating effect.
R. A. Fessenden, for instance, has perfected one which is a marvel of
delicate workmanship. He depends upon the heating of a wire by the
currents passing through it. Such heating is the result of the
electrical force acting against resistance, and the difficulty is that
if the resistance be great it will almost entirely kill the faint
oscillating forces in the receiving antenna, while if, on the other
hand, it be small, the rise in temperature will be inappreciable. So he
encloses a fine thread of platinum in a glass bulb from which the air is
exhausted. The platinum wire is first of all embedded in a wire of
silver: the silver wire is given a core of platinum, in fact. Then the
compound wire is drawn down until it is so thin that the platinum core
is only one and a half thousandths of an inch in diameter. A short
length of this compound wire is then bent into a U-shaped
loop and its ends connected to thicker wires. Finally the bottom of the
loop is immersed in nitric acid, which eats away the silver at that
point and leaves the bare platinum. Thus is produced a very short length
(a few millimetres) of exceedingly thin platinum wire supported at its
ends by comparatively thick wires.
Being so short, this wire does not offer much resistance, and
consequently does not materially check the oscillations. At the same
time, since it is so fine, it does offer some resistance, and finally,
since what heat is generated will be in an exceedingly small space, it
will be appreciable there. A telephone is arranged so that its current
also passes through the fine wire, and every slight variation in the
temperature of the platinum wire, by varying its resistance, varies the
current through the telephone. And exceedingly slight variations can be
detected by sound in the telephone. Thus the oscillations generated in
the antenna affect the heat in the wire; that affects its resistance;
and that again affects the telephone, which, finally, affects the ear of
anyone who is listening to it. It must be understood, however, that this
is not a wireless telephone, for the sounds heard are not articulate but
merely long and short sounds, representing the dots and dashes of the
"Morse Code."
Electrolysis provides us with another form of detector. An exceedingly
small platinum wire forms one electrode and a large lead plate the
other, and both are immersed in dilute acid. The passage of current from
a local battery sets up electrolysis, and so stops itself by forming a
film of oxygen on the small electrode. This film, however, is broken by
the oscillating currents from the antenna, so that as long as they are
coming the battery current can flow, but as soon as they cease the
battery current stops itself again. Thus the flowing and stopping of the
oscillating currents is exactly copied by the current from the battery,
which current is led through a telephone or a sensitive galvanometer.
It may occur to readers to inquire why the oscillating currents are not
passed direct to a galvanometer. The answer is that because they are
oscillating a very sensitive galvanometer is not possible.
True, the Duddell thermo-galvanometer has been mentioned in this
connection, but although it is a beautiful instrument it cannot compare
for delicacy with the direct-current galvanometers. The latter are
easily a _hundred thousand times_ more sensitive. But the trouble can be
overcome by "rectifying" the oscillating currents, by passing them
through a "unidirectional" conductor--one, that is, which passes current
one way only. These remind one of a turnstile as installed at certain
public places, which let you out but will not let you in unless you pay.
In fact they will not let you _in_ at all. In like manner "rectifiers"
will only allow those currents to pass which are flowing in one
direction, and so they cut out every alternate oscillation, thus
producing something very like continuous current, which can be detected
by the very delicate galvanometers which are usable where continuous
currents are concerned, or more often by a telephone receiver. The
rectifying conductors are in many cases crystals, hence these detectors
are called "Crystal Detectors." Carborundum is a favourite for this
purpose.
And that brings us to the important question of the secrecy of wireless
communication, and the measures taken to prevent confusion from the
number of independent messages flying through the air at the same time.
This can be largely achieved by the aid of resonance. Trains of waves
flung out by one antenna may strike several other antennae, but unless
the latter are in tune with the sending apparatus they will probably not
be affected appreciably. Let one of them, however, be in tune, and it
will pick up easily the message which is not noticed by the others. It
is as if three people watching a distant lamp were affected by a form of
colour-blindness which rendered them practically blind to all colours
except one. Suppose one could see red only, the other blue and the third
yellow. A light sent through a blue glass being robbed of all rays
except the blue ones would be visible only to the man who could see
blue. The man who could see blue would, in like manner, be quite blind
to light sent through red or yellow glass. Each of them, in fact, could
be signalled to quite independently of the others by simply sending him
rays of the colour to which his eyes were sensitive. In precisely the
same way each wireless receiver is or can be made most sensitive to
waves of a particular length and practically blind to all others. The
operator can adjust his apparatus for certain prearranged wave-lengths,
and so he can communicate with secrecy to stations whose wave-length he
knows. The change, of course, is made by altering the capacity, or
inductance, or both. The instruments can be so calibrated that it is
quite easy to make the alteration.
Then, antennae can be so constructed that messages can be received with
most readiness from one particular direction. In others, they can be
received from any direction, but the direction can be discovered. This,
it will be easy to see, is of great value to ships in a fog.
Antennae made with a short vertical part and a long horizontal part
radiate best in the direction away from which their horizontal part
points. This is of great advantage in stations which are built specially
to communicate with other particular stations. In such cases the antenna
is carefully built, so as to point in the required direction. Such
antennae also receive more readily those signals which come from the
direction away from which they are pointing.
Reference has been made already to the interesting fact that wireless
communication is easier at night than in the daytime. That is probably
because of the "ionisation" of the atmosphere by the action of sunlight.
Along with the visible sunlight there comes to us from the sun a
quantity of light known as "ultra-violet," since it makes its effect
known in the spectrum of sunlight beyond the violet, which is the limit
of visibility at one end of the spectrum. We cannot see it but it
affects photographic plates powerfully. It has energetic chemical
powers, and it has the ability to make the air more conductive than it
is ordinarily. Comparatively little of it penetrates our atmosphere, but
it must exercise a good deal of influence a little higher up. Now
readers will remember that the process by which electro-magnetic waves
are propagated is checked when the waves strike a conductor. The energy
in the waves is then employed in causing currents in the conductor
instead of forming more waves. And so partially conductive air forms a
partial barrier to the waves. The effect is not appreciable in the case
of the tiny waves of light and heat, but it is in the case of the long
"wireless waves." Everyone has seen the waves of an advancing tide
coming up a sandy beach, and has noticed how the dry sand (a good
conductor of water) sucks up and destroys the foremost ripples. In like
manner are the wireless waves "sucked up" by the partially conductive
atmosphere. But the effect of the ultra-violet light does not last long,
and so, at night-time, it disappears. Therefore messages can be sent
better at night than by day.
For wireless _telephony_ what is wanted is a continuous uninterrupted
train of waves, such as those from the "Poulsen arc," and a receiver of
the magnetic type. The coherer is no good for this purpose, since it
either stops the current entirely or lets it flow copiously. The
magnetic detectors, however, respond to the variations in the strength
of the incoming waves. As the latter increase or decrease in strength so
does the magnetic detector give out stronger or weaker signals. So a
telephone transmitter of the ordinary type is made to vary the strength
of the oscillations at the sending end, while an ordinary telephone
receiver is placed in series with the detector at the receiving end.
Thus every slight variation corresponding to sound waves spoken into the
transmitter is reproduced in the receiver.
It is strange that wireless telephony has not made greater progress, for
it may be said, on the word of one of the greatest authorities, that
wireless telephony is simpler and easier than telephony through a
submarine cable. In the latter there are almost insuperable obstacles
caused by the capacity and inductance of the circuit, while in the
wireless method there is very little difficulty.
There are, of course, several so-called "systems" of wireless telegraphy
in use. There is the Marconi in Great Britain; the secret Admiralty
system in the British Navy; the De Forest in the United States; the
Telefunken in Germany, not to mention the promising Poulsen system. And
there are still others. But it would be futile to attempt to explain
how they differ from one another in a work like this. In principle they
are alike. The precise forms of instrument used may vary, but even there
there is much in common between them. As time goes on there will
inevitably be a tendency to more and more uniformity. That is always the
case, for some things are inherently better than others, and rival
systems, although each is working along its own lines, always come to
very much the same result in the end. Without making any comparisons, it
is safe to say that if the Telefunken system, for example, has any
points of superiority over the Marconi, the latter will sooner or later
find out the fact, and will modify their apparatus accordingly. In all
probability this will operate both ways, and some things which the
German system is now using will give place to those which the British
have in operation.
In another very modern industry this is very apparent. Having attended
and carefully studied several annual exhibitions of flying machines, I
have noticed with great interest how the varying types of a few years
ago are merging into the more or less uniform types of to-day. And it
has been the same with wireless telegraphy, and will be still more so in
the future.
The best means of generating the waves and the best means of detecting
them at a distance--that is the whole problem, and all the workers in it
will sooner or later come to much the same conclusions as to which are
the best ways.
Patents may do a little to delay this, but not much. For one thing,
patents only last a few years. For another, a patent only covers a
particular way of doing a particular thing. A machine that is termed
"patent" is often the subject of a hundred patents, each covering a
particular little point. It is well-nigh impossible to patent a whole
machine. A general principle cannot be patented, only a particular
application of that principle, and so there are in a great many cases
little variations of a patented method which are quite as good as the
patented one, and which can be used freely. So even patents will not
have much effect, in all probability, upon this unification process.
But, however that may be, there is no doubt that the whole world owes a
deep debt of gratitude to the men who have worked out this most
beneficent of inventions. It is difficult to think of a single one which
has ever brought such a load of benefits to poor, struggling humanity as
this has. The ship in distress, the lighthouse man on his lonely islet,
the explorer in the Polar regions, the pioneer settler in the new
lands--in fact, just those who most need some connecting link with their
fellows--are the people to whom the wireless telegraph brings aid and
comfort. All honour to the men who have done it.