If it has seemed extraordinary to you that only one wire should be
necessary for sending a message by the electric telegraph, and that our
earth can be used instead of a second wire, how much more wonderful it
is to realize that in these days we can exchange telegraphic messages
with different points without any connecting wires at all between them,
even though the places be many hundred miles apart. Thus, two ships on
the ocean, entirely out of sight of each other, may intercommunicate,
or may telegraph to or receive despatches from a far-distant shore;
indeed, telegraphy without wires has been accomplished across the
Atlantic Ocean. In the language of the day, this is called "wireless
telegraphy," although it is more correct to think of it as aerial, or
space, telegraphy. As you will naturally want to know how this is
effected, we will try to explain the main principles in a simple manner.
If you drop a stone into a quiet pond, you will see the water form into
ring-like waves, or ripples, which travel on and on until they die away
in the far distance. These waves are caused, as we have seen, by a
disturbance of the body of water.
Probably you have already learned in school that all known space is
said to be filled with a medium called "ether," and that this medium is
so exceedingly thin that it penetrates, or permeates, everything, so
that it exists in the densest bodies as well as in free space. For the
sake of obtaining a clear idea of this theory we may imagine that the
ether envelops and permeates every thing in the entire universe. Hence
we can easily realize that, although we cannot see or feel the ether,
any disturbance of it will set it in wavelike motion.
Modern science accounts for light, radiant heat, and electrical
phenomena by reason of wavelike disturbances, vibrations, or pulsations
of this ether. Thus, if you should strike a light, the ether would be
disturbed, causing waves to form, which, like the waves in the water,
would travel in every direction. When these waves reached the eyes of
another person within seeing distance, that person's eyes would be so
acted upon by the waves that he would see the light which you had made,
and would see it instantly, for light waves travel about 186,000 miles
per second.
So, if you create an electrical disturbance, the same kind of an effect
will be produced; that is to say, waves in the ether will be created,
or propagated, and will travel on and on in every direction. Now, if
some form of electrical appliance can be made that will be of the right
kind to respond to them (as the eye responds to light rays), these
electric waves can be made practically useful for transmitting messages
through space. This is just what has been done, and we will now give
you a brief general description of one kind of apparatus used.
For "sending," or "transmitting," as it is usually termed, there
is used an induction-coil, having rather large brass balls on the
secondary terminals; suitable batteries, a condenser, a Morse telegraph
key, and an "aerial," or wire which is carried away up into the air
vertically, and is made fast to a pole or special tower. When these are
connected properly, the closing of the circuit with the key will cause
sparks to jump between the brass balls. This electrical discharge,
or oscillation, is carried by the aerial into the upper air and
causes intense pulsations in the ether, which set up waves as already
mentioned. If the circuit is opened again the disturbance ceases. So,
by alternately closing and opening the circuit, the Morse characters
can be imitated.
But how can these signals be received by the man for whom they are
intended, who may be a hundred miles or more away? He has a "receiving"
set, consisting of a sensitive relay, batteries, resistance-coils, a
Morse register, an aerial, and a special device called a "coherer."
This is the important part of the whole set, because it is sensitive
to the electrical waves. It consists of a little glass tube about as
large around as an ordinary lead-pencil, and perhaps two inches long.
In the tube are two metallic plugs, each having a wire attached so that
one wire projects from each end of the tube. The plugs are separated
inside the tube by a very small space, and in this space are some
metal filings. One wire from the coherer is connected to the aerial
and the other to the ground. When there are no electrical ether waves
to influence them, these filings, being loosely separated, are at
rest and offer high resistance; but when the ether is disturbed by
electrical vibrations and the waves arrive at the coherer (through the
aerial), these filings are drawn together, or cohere. This lowers their
resistance and they become a better conductor. Now, the coherer wires
are also connected through a battery to the relay, which in turn is
connected through another battery to a Morse register. Therefore, when
the filings become a conductor, the current flows through them and the
circuit to the relay is closed. That attracts an armature which closes
the circuit of the Morse register and thus marks the electrical impulse
on a strip of paper tape. In the mean time, a restoring device, called
a "decoherer," operated also by the relay circuit, has tapped upon the
coherer, thus shaking the filings loose again, so that they are ready
to cohere again and register another impulse, or character. Thus, by
pressing the key at the transmitting end for long or short periods, to
represent Morse characters, long and short waves are propagated in the
ether and are received and recorded at the receiving end through the
coherer and other parts of the receiving set. In this way telegraphic
messages are sent and received through space, between points separated
by hundreds or thousands of miles.
We have tried to describe to you the general principles underlying
the art of wireless telegraphy as plainly as possible, using for
illustration the simplest kind of apparatus employed for the practical
sending and receiving of messages. At the present day there are
several systems in actual practice, and with the growth of the art
there have been many elaborations of apparatus that have come into
use. For instance, the coherer is not as much used as formerly. In its
place there are employed several kinds of "wave-detectors" as they
are now termed, and in many of the systems the electrical pulsations
are generated by a dynamo-machine instead of batteries. Then, again,
instead of the messages being recorded by a Morse register at the
receiving end, the operator receives them by means of a telephone
receiver, through which he hears the Morse characters and writes them
down in words as he hears them. Generally the aerial, or "antennæ," as
it is sometimes named, consists of several wires, sometimes a large
number, carried to a considerable height.
There are a great many other details which might be written to explain
all the complicated apparatus which is used in some of the systems,
but it is not intended in this book to offer more than a general
explanation of main principles. We must leave it to you to study the
details elsewhere if you so desire after you have read these pages.