The part consisting of the two black pillars is an electromagnet,
and across the top of these pillars is a piece of iron called the
"armature," which is held up by a spring.
[Illustration: Fig. 7]
Now let us see how the battery and wire are placed in connection with
these instruments. You have seen that we usually have two wires for
the electricity to travel in, one wire for it to leave the battery,
and the other to return on. But you will easily see that if two wires
had to be used in telegraphing it would be a very expensive matter,
especially when they had to be carried thousands of miles. So, instead
of using a second wire, we use the earth to carry back the electricity
to the battery, because the earth is a better conductor even than wire.
Although a quantity of ground equal in size to the wire would offer
thousands of times greater resistance than the wire, yet, owing to the
great body of our earth, its total resistance is even less than any
telegraph wire used.
When two electric wires are run from a battery and connected together
through some instrument, this is called a "circuit," because the
electricity has a path in which it can travel back to the battery. This
would be a "metallic" circuit; _but when one wire only_ is used, and
the other side of the battery is connected with the earth, it is called
a "ground" or "earth" circuit, because the electricity returns through
the earth.
[Illustration: Fig. 8]
If you look at this sketch (Fig. 8) you will see how the telegraph
instruments are connected and will then be able to understand how a
message can be sent.
Here we have two sets of telegraph apparatus, one of which, let us
say, is in New York and the other in Philadelphia.
You will see that one wire from the battery is connected with the
earth, and the other wire with the sounder. Another wire goes from the
sounder to one leg of the key so as to make the brass base of the key
part of the circuit. The other leg of the key is "insulated" from the
brass base by being separated therefrom with some substance which will
not carry electricity, such, for instance, as hard rubber.
We will suppose that there is already a wire strung up on poles between
New York and Philadelphia, and that the key, sounder, and battery in
the latter city are connected in the same way as those in New York.
Now, to enable us to send a message from one city to the other we must
connect the ends of the wires to the instruments in each city; so we
connect one end to the insulated leg of the key in New York, and the
other end to the insulated leg of the key in Philadelphia.
Everything is now completed, and, as soon as we find out what is the
use of that part of the key that has a little round, black handle, we
shall be ready to start. This is called the "switch."
If you will look once more at the picture of the key you will see
under the long handle (or lever) a little point which the lever will
touch when it is pressed down. Now this little point is part of that
insulated leg, and, therefore, this point is also insulated from the
base. If a current of electricity were sent along the wire it could not
get any farther than this point unless we put in some arrangement to
complete the path, or circuit, for it to travel in. We therefore put in
the switch.
One end of the switch (which is made of brass with a rubber handle)
is fastened on the base of the key, so that it may be moved to the
right or left. The other end, when the switch is moved to the left
(or "closed"), touches a piece of brass fastened to the little point
we have mentioned, and so makes a free path for the electricity to go
through the base of the key and through the wire to the sounder, and
from there to the battery, and so back to the earth. This switch must
be opened before the sounder near it will respond to its neighboring
key.
Now we are ready to send a message. Suppose we want to send a telegram
from New York to Philadelphia. The operator in New York opens his
switch and presses down his key several times. The switch on the
Philadelphia key being closed, the electricity goes through to the
sounder, and, this being made an electromagnet by the current passing
through the wire, the iron armature is attracted by the magnetism and
drawn down to the magnet with a snap. It will stay there as long as the
New York operator keeps his lever pressed down, but, when he allows
it to spring up, there is no current passing through the Philadelphia
sounder and there is no magnetism, consequently the armature springs up
again with a click.
As often as the operator presses down his key lever and lets it spring
up again, the same action takes place in the sounder, and it makes that
click, click, which you have heard if you have ever seen telegraph
instruments in operation.
Let us continue, however, to send our message. The New York operator,
having pressed down his key several times to signal the Philadelphia
operator, closes his switch to receive the answer from Philadelphia.
The operator in the latter city then opens his switch and presses down
his key several times, which makes the New York sounder click, in the
same way, to let the operator there know that he is ready to receive
the message. He then closes his switch and receives the telegram which
the New York operator sends after opening _his_ key.
Telegraphic messages are sent and received in this way and are read by
the sound of the clicks.
These sounds may be represented on paper by dots, dashes, and spaces.
For instance, if you press down the key and let it spring back quickly,
that would represent a dot. If you press down the key and hold it a
little longer before letting it spring up again, it would represent a
dash. A space would be represented by waiting a little while before
pressing down the key again.
We show you below the alphabet in these dots, dashes, and spaces, and
these are the ones now used in sending all telegraphic messages.
[Illustration]
Thus, you see, if you were telegraphing the word "and" you would press
down your key and let it return quickly, then press down again and
return after a longer pause, which would give the letter A; then slowly
and quickly, which would be N; then slowly and twice quickly, which
would be D.
Any persevering boy can learn to operate a telegraph instrument by a
little study and regular practice; and, as complete learner's sets
can be purchased very cheaply, this affords a pleasant and useful
recreation for boys.
There are many cases where two boys living near each other have a set
of telegraph instruments in their homes and run a wire from one house
to the other, thus affording many hours of pleasant and profitable
amusement.
In giving the above explanation of telegraphing we have described only
the simple and elementary form. In large telegraph lines, such as
those of the Western Union, there are many more additional instruments
used, which are very complicated and difficult to understand; such,
for instance, as the quadruplex, by which four distinct messages can
be sent over the same wire at the same time. We have, therefore,
described only the simplest form in order to give the general idea
of the working of the telegraph by electromagnetism, which is the
principle of all telegraphing.
When you study electricity more deeply you will find this subject and
the many different instruments very interesting and wonderful.
V