<h2><SPAN name="CHAPTER_XXI" id="CHAPTER_XXI"></SPAN>CHAPTER XXI.</h2>
<h3>WIRELESS TELEGRAPHY.</h3>
<p>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.</p>
<p>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<span class="pagenum"><SPAN name="Page_177" id="Page_177"></SPAN></span>
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.</p>
<p>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.</p>
<p>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.</p>
<p>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 <i>conduction</i> through
water.</p>
<p>In 1835 Joseph Henry produced an effect on
a galvanometer by ether-waves through a distance
of twenty feet by an arrangement of<span class="pagenum"><SPAN name="Page_178" id="Page_178"></SPAN></span>
batteries and circuits like that shown in <SPAN href="#fig1">Fig. 1, Chapter VI</SPAN>. This was called <i>induction</i>, 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.</p>
<p>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<span class="pagenum"><SPAN name="Page_179" id="Page_179"></SPAN></span>
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.</p>
<p>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:</p>
<p>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<span class="pagenum"><SPAN name="Page_180" id="Page_180"></SPAN></span>
between the signals, strikes the glass tube with
a succession of light blows.</p>
<p>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.</p>
<p>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.</p>
<p>While Marconi has done more than any
other man to improve and popularize wireless<span class="pagenum"><SPAN name="Page_181" id="Page_181"></SPAN></span>
telegraphy, history shows that he invented
none of the essential elements so far as the
system has been made public.</p>
<p>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.</p>
<p>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.<span class="pagenum"><SPAN name="Page_182" id="Page_182"></SPAN></span></p>
<p>There is no doubt but that wireless telegraphy
will some time play an important part
in many spheres of usefulness.</p>
<p>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<span class="pagenum"><SPAN name="Page_183" id="Page_183"></SPAN></span>
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."</p>
<p>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<span class="pagenum"><SPAN name="Page_184" id="Page_184"></SPAN></span>
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.</p>
<p>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<span class="pagenum"><SPAN name="Page_185" id="Page_185"></SPAN></span>
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.</p>
<p>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.</p>
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