TELEGRAPH LINE.
The modern telegraph consists essentially of four things:
1. A battery which produces the electric current.
2. A wire to conduct the current from one point to another.
3. A key or switch for turning the current off and on.
4. An electro-magnetic receiving apparatus, which changes the
pulsations of the electric current into sounds.
[Illustration: FIG. 83.—Key Frame.]
Its operation is not, as many suppose, complicated and difficult to
understand, but is quite simple.
The key is really a form of switch and is simply a contrivance for
controlling the electric current. It consists of a steel lever, pivoted
in the centre, and provided with a rubber knob, which the operator
grasps lightly with the thumb and forefinger. When the lever is pressed
downwards, a platinum point on the under side is brought into contact
with another similar point set into a rubber bushing in the base of the
key, so that there is no electrical connection between the two points
unless the key is pressed down or "closed" as it is usually termed.
The receiving device, or "sounder," consists of two electro-magnets
mounted on a base under a flat strip or iron, called the armature, which
is attracted by the magnetism of the electro-magnets when a current
flows through them, and is withdrawn by a spring when no magnetism is
produced by the windings.
[Illustration: FIG. 84.—Sounder Frame.]
The armature is usually mounted on a strip of brass or aluminum called
the "lever". The lever strikes against an anvil and produces the clicks
which form the dots and dashes of the telegraph alphabet.
Every time the key is pressed, an electric current goes through the
line, causing the magnets to draw the armature downward and produce a
click. When the key lever is released, the current is shut off and the
lever flies up and clicks against the top of the anvil.
The time between the first and second clicks may be easily varied at
will, by changing the length of time that the key lever is held down.
Two clicks very close together, made by pressing the key and immediately
releasing it, are called a "dot". A "dash" is made by pressing the key
down and holding it down for a fraction of a second so that the interval
between the first and second clicks is about three times as long as that
between the clicks of a dot.
[Illustration: FIG. 85.—The Electro Magnets.]
Any boy can make a simple telegraph set quite easily. The only drawback
of such an apparatus, however, is that it is usually not practical for
long lines but may only be used for ticking messages from one room to
another.
It is equally possible for the young experimenter to make a perfectly
practical telegraph outfit which will operate over a line a mile long,
if he is willing to give the time and labor necessary to the work.
The telegraph set-described below is one which, because of its
simplicity, particularly lends itself to construction by the young
experimenter whose tools are rather limited.
In order to make a really practical telegraph outfit, it is almost
essential that the key and sounder frame be made out of cast iron so as
to give the instruments the requisite stiffness. It is easier to use
castings than to attempt to bend the parts out of metal.
In order to secure the proper castings it will be necessary to first
make a set of "patterns". This is not very difficult.
The key frame is shown in Figure 83 and the Sounder frame in Figure 84.
The patterns should be cut out of wood according to the exact shape and
dimensions shown in the illustrations. Omit all the holes, however,
because no holes should appear in the patterns but only in the finished
castings.
[Illustration: FIG. 86—The Sounder Armature.]
The patterns should be made perfectly smooth and be a good, clean job
throughout. After they are finished, give them each a coat of shellac
and they are ready for the foundry. Almost any iron foundry will make up
the castings if supplied with the patterns. If a number of experimenters
"chip in" and make up a set of patterns from which several sets of
castings are ordered at the same time, the cost will be very slight.
After the castings are secured, they should be cleaned up with a file
and drilled according to the illustration in Figures 83 and 84. Several
of the holes should be tapped to receive an 8-30 machine screw. Such
holes are marked in the illustrations.
The holes not marked "Tap" should be just large enough to allow an 8-32
screw to slip through.
[Illustration: FIG. 87.—Sounder Frame with Lever in Position.]
*The Electro-magnets* are illustrated in Figure 85. The magnet core is
made of a piece of 5-16 inch round iron rod an inch and a quarter long.
A fibre washer, three-quarters of an inch in diameter is fitted over
each end of the core. One end of the core is drilled and tapped to
receive an 8-32 screw.
The magnet bobbin, before winding is shown at the left of the
illustration and a finished magnet at the right. Two magnets are
required. They should both be wound in the same direction with No. 25
single cotton covered wire. The wire should be wound in smooth, even
layers and the terminals led out through two small holes in the fibre
heads.
*The Sounder Armature* is a piece of steel or iron rod, five-sixteenths
of an inch square and three and three-eighths inches long. It serves
both as the armature and the sounder lever. The exact location of the
holes is shown in Figure 86. The holes "h" and "i" should be tapped to
receive an 8-32 screw. The hole "j" should pass a screw of the same
size. This screw serves as the pivot for the armature. Figure 87 shows
how the sounder frame should appear when the lever is in position. The
screw A controls the distance which the armature moves on its upward
stroke. The screw B strikes the "anvil" on the downward stroke and
controls the distance between the armature and the electro-magnets.
[Illustration: FIG. 88.—Top View of Completed Instrument]
The armature is pushed upwards and held away from the magnets against
the screw A by a small spiral spring. The lower end of the spring sets
in the hole "C" in the sounder frame and the upper end rests against the
end of the screw C so that the tension of the spring can be regulated by
moving the screw up and down. The armature should be free to move up and
down on the pivot screw but should not move sideways.
*The Base* of the instrument is a rectangular piece of hardwood, four
and one-half inches long, three and three-quarter inches wide and
one-half inch thick. The sounder is mounted on the base in the position
shown in Figure 88, which is a top view of the complete instrument. Two
long 8-32 machine screws pass upwards through the wooden base, through
the holes "e" and "f" in the sounder frame and into the bottom of the
magnets, thus serving not only to hold the magnets in position but also
the sounder frame to the base.
[Illustration: FIG. 89.—Side View of Key.]
Figure 89 is a side view of the completed key. The key lever and the
circuit closing lever are illustrated in Figure 90. The lever is four
and three-eighths inches long and five-sixteenths of an inch wide. It
should be made from spring brass. The circuit closing lever is a strip
of brass of the same width. All the holes in both levers, with the
exception of the centre hole in the key lever should be large enough to
pass an 8-32 screw. The centre hole should be tapped to receive an 8-32
screw.
The key lever is fastened to the frame by means of a round headed 8-32
machine screw. The screw should be tightened so that the lever cannot
move from side to side. The circuit closing lever is pivoted to the back
end so that it can swing. The key lever should be fitted with a regular
key knob and the circuit closing lever with a small knob such as is
generally used for the purpose. Both of these styles of knobs can be
purchased from almost any electrical supply house.
The contact points are best made of silver. Silver makes a better
contact metal than almost anything else, with the exception of platinum.
If silver cannot be obtained a piece of large german-silver wire may be
used.
[Illustration: FIG. 90.—Key and Circuit Closing Levers.]
One contact is soldered to the under side of the key lever. The other
contact should be mounted on the head of a screw and pass through a hole
in the iron key frame and through the wooden base underneath. This lower
contact should be directly underneath the upper contact and be
thoroughly insulated from the base. Several thicknesses of well
shellaced paper will form a first class insulating medium. The length of
the contacts should be such that there is a distance of about 3-32 of an
inch between them when the lever is horizontal.
The circuit closing lever should make contact with a small strip of
spring brass under the lower contact point so that the contact is
connected to the frame of the key through the circuit closing lever when
the latter is "closed".
The adjustment screw on the key frame is used to regulate the upward
stroke of the key lever.
The adjustment screw on the key lever is used to regulate the spring
tension of the lever. The lower end rests against the top of a small
spiral spring made of brass wire.
[Illustration: FIG. 91.—American Morse Code.]
Connecting the outfit is a very simple mater. The two inside terminals
of the sounder magnets should be connected together. One of the outside
terminals is led to a binding post mounted on the back of the base. The
other terminal is connected to the lower key contact. The frame of the
key is connected to a second binding post at the back of the base. The
connecting wires should pass through small holes and along the under
side of the base.
The Morse Telegraph Code is shown in Figure 91.
A single dry cell will provide sufficient current for practising the
Code. The cell should be connected to the two binding posts and the
circuit closing lever should be kept open at all times or the battery
will become exhausted.
[Illustration: FIG. 92.—Circuit for Two Instruments.]
The number of batteries required on a long line will depend upon the
resistance of the line and cannot easily be pre-determined.
Figure 92 shows a circuit for connecting two instruments on a line. When
the double contact switch is on point A the batteries are out of
circuit. Throwing the switch on B puts the batteries into circuit. They
should be kept out of circuit when the stations are not in use. The
circuit closer at Station 1 must be kept closed when Station 2 is
telegraphing and vice versa.