TELEGRAPHY AT HOME
A GALVANOMETER ADAPTED
LIKE most of the inventions that have revolutionized the world, the
electric telegraph is constructed upon the simplest principles.
As will be shown, an instrument for transmitting and receiving messages
can easily be made at a very small cost by any one prepared to devote a
little care to its construction; and when completed it will be found of
considerable use and convenience.
To make a model of the elaborate machines now employed, which not only
receive but print messages in legible Roman characters, would be far
beyond the scope of any amateur, necessitating the purchase of many
costly parts and requiring a considerable knowledge of engineering to
fit them satisfactorily together. But the instrument that can be made
according to the following instructions, is exactly similar to those in
use throughout many smaller offices the world over.
The telegraph is based upon the following principle. A current of
electricity, passed through a hoop of wire within which a needle is
suspended, will move that needle from left to right, or from right to
left, according as the wires are attached to one or other of the poles
of a battery. This simple apparatus is called a galvanometer. A
telegraph instrument is nothing but a galvanometer adapted to convenient
usage.
Before entering upon the method of construction, it would be as well to
inform whoever undertakes this work that the utmost care is necessary in
adjusting the various parts. The slightest fault will certainly spoil
the whole instrument, and the least carelessness render all the labor
expended utterly useless.
Now to describe the simplest way to make a receiver and transmitter.
Three pieces of wood will be required, a cigar-box furnishing the best
material for the purpose; if you have not one at home, any tobacconist
will let you have one for a few cents.
To commence with the base. Cut a piece of wood measuring 6 inches by 5
inches. In this two grooves must be made (D D and E) according to the
measurements shown in A, Fig. 1. The width of these grooves must, of
course, depend upon the thickness of the wood, for the dial and the
upright will have to be fitted into them.
[Illustration: A.--Base of instrument.
B.
C.--Upright.
FIG. 1.]
For the dial you will require a piece of wood similar to the base, but
measuring 5 inches by 4 inches. One end of this should be rounded as
shown in B, Fig. 1. Drill a hole (F) right through this dial, 3 inches
from the bottom and 2¹⁄₂ inches from either side. As a piece of wire
will have to revolve easily in this hole, take great care to cut it
cleanly. In the lower end of the dial cut out two notches (G H), each
according to the measurements in B, Fig. 1, _i.e._ ³⁄₄-inch deep and
¹⁄₂-inch wide. Each notch must be ³⁄₄-inch from its side of the dial.
The upright is very easily made, being simply a strip of wood 3¹⁄₂
inches by 1 inch (C, Fig. 1). In this drill a hole (K) 3 inches from the
end and ¹⁄₂-inch from either side, taking the same care as with the hole
in the dial.
Having thus prepared the several pieces of the framework, the dial
should be glued into its groove. A small square block of wood glued into
the angle formed by the base and the dial will insure the latter’s being
perfectly perpendicular. The upright will be fixed in its place later.
To proceed with the more delicate part of the work. Cut a piece of
cardboard, fairly thin in texture, 5 inches long and 1 inch wide, and
bend it into the shape of an oblong hoop. Upon the hoop or bobbin wind
about half an ounce of fine cotton-covered copper wire, size No. 36,
which can be obtained from any electrician. Wind this wire very
carefully and evenly, but not tightly, leaving about 4 inches free at
either end (A, Fig. 2).
The next thing to be made is a magnetized needle, which can be easily
done as follows. Take a piece of ordinary watch spring, an inch long,
and make it red hot. When in this condition drill a hole through the
center, and when it is cooler round the ends, as in B, Fig. 2. Heat it
once more to a red heat and then plunge into cold water to harden. The
needle must now be magnetized by being drawn several times firmly across
the pole of a strong magnet, care being taken that it is always drawn in
the same direction.
The pointer (C, Fig. 2) should be made in a similar manner, except that
it must not be magnetized, and should be at least twice as large as the
needle. Through the hole in the magnetized needle pass a couple of
inches of straight stout wire, and with a little solder, or even
sealing-wax, fasten the needle as shown in B, Fig. 2.
[Illustration: A.--Coil of wire.
B.--Magnetized needle.
C. Pointer.
FIG. 2.]
All these parts can now be fixed together. In the middle of the broad
side of the coil of wire you must push aside the strands in such a
manner that holes can be made through the cardboard hoop. Pass the two
ends of the thick wire, to which the needle is attached through these
holes, so that the needle is then suspended within the coil. Take care
to make these holes large enough to allow the axles of the needle to
revolve freely, as the whole success of your apparatus depends upon the
needle turning easily within the coil, which should now appear as in
Fig. 3.
Fasten the coil upon the back of the dial, one end of the protruding
axle passing through the hole you have already made in the wood. Then
glue the upright firmly in its groove, with the other end of the axle
through its corresponding hole. Take great care that the needle can
revolve freely within the coil. Fasten the pointer to the end of the
axle passing through the dial, and in this latter drive two pins or
wooden pegs (P P) on either side of the pointer, in order that it may
only move within a limited arc (B, Fig. 1).
[Illustration: FIG. 3.--Revolving needle in coil.]
TESTING
It would now be as well to test what you have already done. Connect the
loose ends of the coil to the poles of a battery, reverse them, and
repeat this two or three times. If the instrument has been properly
made, the needle should swing to the left when the wires are connected
in one way; to the right when they are reversed.
[Illustration: FIG. 4.--The keyboard.]
Nothing more remains to be done but to make the keyboard, which is
really the base of the instrument you have just been constructing. A
glance at Fig. 4 shows how this is to be done. C and D are two thin
brass springs, 5 inches long, and, as they have to pass through the
notches in the dial, less than half an inch in width, they must be bent
upwards, so that when fastened to the board at C and D the opposite
ends will spring up and press against the under side of the metal bridge
BF. They are then in what is called “metallic contact.”
The bridge should be made from a piece similar to the springs and bent
as shown in Fig. 5. Another strip of brass must be screwed flat to the
board at AE, parallel to BF, and over this a couple of wooden or ivory
knobs should be fastened to the springs, as they are used to press down
CD in contact with AE (Fig. 4). When released they should fly back to
BF. Be very careful to see that all these details are thoroughly in
order, as they form the most vital part of the keyboard.
[Illustration: FIG. 5.--The instruments ready for use.]
MESSAGE TRANSMITTING
At the ends CD of the springs fasten two binding screws, and do the same
at A and B. Fasten the two loose ends from the coil to C and D, and
attach the wires from the battery to A and B. Now is the time to see
whether your machine has been properly made. By pressing down one key
you will find that the current of electricity deflects the needle and
the pointer to one side, and by releasing this key and pressing the
other one, the pointer will swing down to the other side. If this is
done satisfactorily your telegraph instrument is complete, and with a
couple of them you will be able to send messages from one room, or one
house, to another.
The complete apparatus should appear as in Fig. 5, which also shows the
connections with the battery.
A couple of Daniels’ cells, or two ordinary dry batteries are sufficient
to work both instruments.
Two copies of the Morse alphabet should also be made, one to be kept by
the side of each machine. When the needle deflects to the left it
designates a dot. When it turns to the right it is a dash. A short space
separates the letters, and a longer one the words.
When the alphabet has been mastered both for transmitting and receiving,
the use of your telegraph will be a matter of the greatest simplicity,
and you will be able to send messages with considerable speed to your
friend at the other end of the line. Below, the Morse alphabet is given,
as well as a message, which it is hoped you will be able to send.
MORSE ALPHABET
A · —
B — · · ·
C — · — ·
D — · ·
E ·
F · · — —
G — — ·
H · · · ·
I · ·
J · — — —
K — · —
L · — · ·
M — —
N — ·
O — — —
P · — — ·
Q — — · —
R · — ·
S · · ·
T —
U · · —
V · · · —
W · — —
X — · · —
Y — · — —
Z — — · ·
MESSAGE
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