WIRELESS TELEGRAPHY.
Broadly speaking, "Wireless Telegraphy" is any method of transmitting
intelligible signals to a distance without wires; and this includes the
old Semaphore systems of visual signals, such as flags and long arms of
wood by day, and lights by night; also the Heliograph (an apparatus for
flashing sunlight), and Sound Signals, made either through the air or
water. Electrical conduction, either through rarefied air or the earth,
also comes under this heading.
The name "Wireless Telegraphy," however, is specifically applied to a
system of signaling by means of ether-waves induced by electrical
discharges of very high voltage. Ether-waves of a greater or less degree
are always set up whenever there are sudden electrical disturbances,
however slight. Ether-waves, electrically induced, are probably as old
as the universe. When "there were thunders and lightnings" from the
cloud that hovered over Mount Sinai in the time of Moses, ether-waves of
great power were sent out through the camp of Israel. But the people of
those days had no "coherer" or telephone or any other means of
converting these waves into visual or audible signals. Thousands of
years had to elapse before the intellect of man could grasp the meaning
of these natural phenomena sufficiently to harness them and make them
subservient to his will.
Many people have been powerfully "shocked"--some even killed--by the
impact of ether-waves set up by powerful discharges of lightning between
the clouds and the earth--when they were not in the direct path of the
lightning-stroke.
The history of Electro-Wireless Telegraphy, like that of all inventions,
is one of successive stages, and all the work was not done by one man.
The one who gets the most credit is usually the one who puts on the
finishing touches and brings it out before the public. He may have done
much toward its development or he may have done but little.
In the year 1842 Morse transmitted a battery current through the water
of a canal eighty feet wide so as to affect a galvanometer on the
opposite side from the battery. This was wireless telegraphy by
_conduction_ through water.
In 1835 Joseph Henry produced an effect on a galvanometer by ether-waves
through a distance of twenty feet by an arrangement of batteries and
circuits like that shown in Fig. 1, Chapter VI. This was called
_induction_, and is still so called when electrical effects are produced
from one wire to another through the ether for short distances. All
induction-coils and transformers (see Chapter XXIV) are operated by
effects produced through the ether from the primary to the secondary
coil--but through very short distances.
In 1880 Professor Trowbridge transmitted an electrical current through
the earth for one mile so as to produce signals in a telephone. In
1881-2 Professor Dolbear used for a short distance (fifty feet)
substantially the same arrangement as Marconi now uses, except that the
former used a telephone as a receiver. He used an induction-coil having
one end of the secondary wire connected with the earth, while the other
was attached to a wire running up into the air. At the receiving-end a
wire starting from the earth extended into the air, passing through a
telephone, which acted as a receiver. In 1886 he used a kite to elevate
the wire, through which electrical discharges of high voltage were made
into the air to produce ether-waves--the receiver being 2000 feet away.
Dolbear's experiments were public fourteen years ago, but at that time
there was no interest in such matters, so that his work received little
or no attention. In 1887 Dr. Hertz of Germany made some experiments in
producing and detecting ether-waves, and he did a great deal to awaken
an interest in the subject, so that others began investigations that
have led to its present use as a means of telegraphing to a distance of
many miles.
In 1891 Professor Branly of Paris invented the coherer. In 1894 it was
improved by Lodge and by him used as a detector of ether-waves. In 1896,
ten years after Dolbear had used it with the kite at the
transmitting-end and telephone at the receiving-end, Marconi, an
Italian, substituted the coherer of Branly for the telephone of Dolbear.
This coherer is constructed and operated as follows:
It consists of a glass tube, of comparatively small diameter, loosely
filled with metal filings of a certain grade. This body of metal-dust is
made a part of a local battery circuit in which is placed an ordinary
electric bell or telegraphic sounder. The resistance of this body of
filings is so great that current enough will not pass through it to ring
the bell or actuate the sounder until an ether-wave strikes it and the
wire attached to it, when the metal particles are made to cohere to such
an extent that the conductivity of the mass is greatly increased; so
that a current of sufficient volume will now pass through the
bell-magnet to ring it. Before the next signal comes the filings must be
made to de-cohere; and to accomplish this a little "tapper," that works
automatically between the signals, strikes the glass tube with a
succession of light blows.
Briefly stated, the wireless system of Marconi, in its essentials,
consists of a powerful induction-coil with one end of the secondary wire
connected with the earth, while the other extends into the air a greater
or less distance according to the distance it is desired to send
signals. The greater the distance the higher the wire should extend into
the air. At the receiving-end a wire of corresponding height is erected,
also connected with the earth. In this wire--as a part of its
circuit--is placed the coherer. In a local circuit that is connected to
the upright wire in parallel with the coherer is placed a battery, a
sounder, or a bell, that is rung when the filings cohere.
When an ether-wave is set up by a discharge of electricity into the air
it strikes the perpendicular wire of the receiver, and that portion of
the wave that strikes is converted into electricity, which is called an
induced current. It is this current, as it discharges through the
coherer to the earth, that causes the filings to unite so as to close
the local circuit and operate the sounder. To send a message it is only
necessary to make the discharges into the air, at the sending-end,
correspond to the Morse alphabet.
While Marconi has done more than any other man to improve and popularize
wireless telegraphy, history shows that he invented none of the
essential elements so far as the system has been made public.
What he seems to have really done was to substitute the coherer of
Branly and Lodge, with its adjuncts, for the telephone of Dolbear. There
is no doubt but that Marconi has done much to improve and enlarge the
capacity of the apparatus and to demonstrate to the world some of its
possibilities. He has been an indefatigable worker and deserves great
credit; but without the work of those who preceded him he could not have
succeeded: the honors should be divided.
This system has been used at various times for reporting yacht-races,
and between ships. It is said also to have been used to some extent in
the South African War. There is much to be done yet, however, before it
can be made entirely reliable for defensive work in time of war. As it
is now, all an enemy would have to do to destroy its usefulness would be
to set an ether-wave-producer to work automatically anywhere within the
"sphere of influence" of the system--to speak diplomatically--when it
would render unintelligible any message that should be sent. To make the
system of the greatest value some sort of selective receiver must be
invented that will select signals sent from a transmitter that is
designed to work with it.
There is no doubt but that wireless telegraphy will some time play an
important part in many spheres of usefulness.
There is another mode (already referred to) for transmitting signals
electrically without wires through the earth instead of through the air,
but in this case it is not through the medium of induction, but
conduction. It has been explained in former chapters that earth-currents
are constantly flowing from one point to another where the potentials
are unequal. Sometimes these inequalities of potential are caused by
heat and sometimes by electricity, as in the case of a thunder-storm. If
a cloud is heavily charged with positive electricity, say, the earth
underneath will have an equal charge of negative electricity. Let us
illustrate it by the tides. As the moon passes over the ocean it
attracts the water toward it and tends to pile up, as it were, at the
nearest point between the earth and the moon. Suppose that (while the
water is thus piled up at a point under the moon) we could suddenly
suspend the attraction between the earth and the moon--the water would
begin immediately to flow off by the force of gravitation until it had
found a common level. Suppose in the place of the moon we have a cloud
containing a static charge of positive electricity--it attracts a
negative charge to a point on the earth nearest the cloud. If now a
discharge takes place between the earth and cloud the potential between
the two will suddenly become equalized and the static charge that was
accumulated in the earth is released and it dissipates in every
direction, seeking an equilibrium, following the analogy of the water;
the difference being that in one case the movement is very slow, while
in the other it is as "quick as lightning."
About eighteen years ago I had a short telephone-line between my house
and that of one of my neighbors. This line was equipped with what was
known in those days as magneto-transmitters, such as we have described
in a previous chapter on the subject of telephony. When a line is
equipped in this way no batteries are needed, as the voice generates the
current, on the principle employed in the dynamo-electric machine. Often
on summer evenings, when the sky appears to be cloudless, we can see
faint flashes of lightning on the horizon, an appearance which is
commonly called "heat-lightning." As a matter of fact, I do not suppose
there is any such thing as heat-lightning, but what we see is the effect
of very distant storm-clouds. Often at such times I have held the
telephone receiver to my ear and could hear simultaneously with each
flash a slight sound in the telephone. This effect could be produced in
the earth by a simple discharge between two or more clouds, which would
distribute the electrical discharge over a greater area. And because my
line had connection with the earth it could have been disturbed
electrically by conduction instead of induction; or it may have been the
effect of ether-waves set up by the lightning discharges. There is no
doubt in my mind but that both of these effects (ether-waves and
conduction through earth) may be felt when a discharge takes place
between a cloud and the earth.
If we could, by operating an ordinary telegraphic key, cause the
lightning to discharge from cloud to earth, and some one was listening
at a telephone in a circuit that was grounded at both ends 100 miles or
more distant from the cloud, the man who controlled the discharges by
the key could transmit the Morse code through the earth to the man who
was listening at the telephone. Thousands of people might be listening
at telephones in every direction from the transmitting-station, and they
would all get the same message. If the receiving-station is near to the
point where there is a heavy discharge from the clouds to the earth the
earth-current is very strong--flowing out in every direction. For some
years I had an underground line between my house and laboratory, and no
part of the line between the two stations was above ground. Many and
many times during the prevalence of a thunder-storm have the
telephone-bells been made to ring at both ends of the line by a
discharge from the cloud to the earth, and in some cases the discharge
was several miles away. The wires could not have been affected so
powerfully in any other way than through the earth.
It will be seen by the foregoing statements that it is possible to
transmit messages through the earth for long distances, but the
difficulty in the way of its becoming a general system is twofold.
First, we cannot always have a thunder-cloud at hand from which to
transmit our signals, and, secondly, the signals would be received alike
at every station simultaneously.