THE TELEGRAPH
The telegraph is a very simple instrument. The key is nothing more or
less than a switch which turns the current on and off alternately.
The signals sent over the wires are simply the audible sounds made by
the armature, as it moves to and from the magnets.
MECHANISM IN TELEGRAPH CIRCUITS.--A telegraph circuit requires three
pieces of mechanism at each station, namely, a key used by the sender, a
sounder for the receiver, and a battery.
THE SENDING KEY.--The base of the sending instrument is six inches long,
four inches wide, and three-quarters of an inch thick, made of wood, or
any suitable non-conducting material. The key (A) is a piece of brass
three-eighths by one-half inch in thickness and six inches long. Midway
between its ends is a cross hole, to receive the pivot pin (B), which
also passes through a pair of metal brackets (C, D), the bracket C
having a screw to hold one of the line wires, and the other bracket
having a metal switch (E) hinged thereto. This switch bar, like the
brackets, is made of brass, one-half inch wide by one-sixteenth of an
inch thick.
Below the forward end of the key (A) is a cross bar of brass (F),
screwed to the base by a screw at one end, to receive the other line
wire. Directly below the key (A) is a screw (G), so that the key will
strike it when moved downwardly. The other end of the bar (F) contacts
with the forward end of the switch bar (E) when the latter is moved
inwardly.
[Illustration: _Fig. 67._ TELEGRAPH SENDING KEY]
The forward end of the key (A) has a knob (H) for the fingers, and the
rear end has an elastic (I) attached thereto which is secured to the end
of the base, so that, normally, the rear end is held against the base
and away from the screw head (G). The head (J) of a screw projects from
the base at its rear end. Key A contacts with it.
When the key A contacts with the screw heads G, J, a click is produced,
one when the key is pressed down and the other when the key is released.
You will notice that the two plates C, F are connected up in circuit
with the battery, so that, as the switch E is thrown, so as to be out of
contact, the circuit is open, and may be closed either by the key A or
the switch E. The use of the switch will be illustrated in connection
with the sounder.
[Illustration: _Fig. 68._ TELEGRAPH SOUNDER]
When the key A is depressed, the circuit of course goes through plate C,
key A and plate F to the station signalled.
THE SOUNDER.--The sounder is the instrument which carries the
electro-magnet.
In Fig. 68 this is shown in perspective. The base is six inches long and
four inches wide, being made, preferably, of wood. Near the forward end
is mounted a pair of electro-magnets (A, A), with their terminal wires
connected up with plates B, B', to which the line wires are attached.
Midway between the magnets and the rear end of the base is a pair of
upwardly projecting brackets (C). Between these are pivoted a bar (D),
the forward end of which rests between the magnets and carries, thereon,
a cross bar (E) which is directly above the magnets, and serves as the
armature.
The rear end of the base has a screw (F) directly beneath the bar D of
such height that when the rear end of the bar D is in contact therewith
the armature E will be out of contact with the magnet cores (A, A). A
spiral spring (G) secured to the rear ends of the arm and to the base,
respectively, serves to keep the rear end of the key normally in contact
with the screw F.
CONNECTING UP THE KEY AND SOUNDER.--Having made these two instruments,
we must next connect them up in the circuit, or circuits, formed for
them, as there must be a battery, a key, and a sounder at each end of
the line.
In Fig. 69 you will note two groups of those instruments. Now observe
how the wires connect them together. There are two line wires, one (A)
which connects up the two batteries, the wire being attached so that
one end connects with the positive terminal of the battery, and the
other end with the negative terminal.
[Illustration: _Fig. 69._ A TELEGRAPH CIRCUIT]
The other line wire (B), between the two stations, has its opposite ends
connected with the terminals of the electro-magnet C of the sounders.
The other terminals of each electro-magnet are connected up with one
terminal of each key by a wire (D), and to complete the circuit at each
station, the other terminal of the key has a wire (E) to its own
battery.
TWO STATIONS IN CIRCUIT.--The illustration shows station 2 telegraphing
to station 1. This is indicated by the fact that the switch F' of that
instrument is open, and the switch F of station 1 closed. When,
therefore, the key of station 2 is depressed, a complete circuit is
formed which transmits the current through wire E' and battery, through
line A, then through the battery of station 1, through wire E to the
key, and from the key, through wire D, to the sounder, and finally from
the sounder over line wire B back to the sounder of station 2,
completing the circuit at the key through wire D'.
When the operator at station 2 closes the switch F', and the operator at
station 1 opens the switch F, the reverse operation takes place. In both
cases, however, the sounder is in at both ends of the line, and only the
circuit through the key is cut out by the switch F, or F'.
THE DOUBLE CLICK.--The importance of the double click of the sounder
will be understood when it is realized that the receiving operator must
have some means of determining if the sounder has transmitted a dot or a
dash. Whether he depresses the key for a dot or a dash, there must be
one click when the key is pressed down on the screw head G (Fig. 62),
and also another click, of a different kind, when the key is raised up
so that its rear end strikes the screw head J. This action of the key is
instantly duplicated by the bar D (Fig. 68) of the sounder, so that the
sounder as well as the receiver knows the time between the first and the
second click, and by that means he learns that a dot or a dash is made.
ILLUSTRATING THE DOT AND THE DASH.--To illustrate: Let us suppose, for
convenience, that the downward movement of the lever in the key, and the
bar in the sounder, make a sharp click, and the return of the lever and
bar make a dull click. In this case the ear, after a little practice,
can learn readily how to distinguish the number of downward impulses
that have been given to the key.
_The Morse Telegraph Code_
A . - N - . & . ...
B - ... O .. 1 . - - .
C .. . P ..... 2 .. - ..
D - . . Q .. - . 3 ... - .
E . R . .. 4 .... -
F . - . S ... 5 - - -
G - - . T - 6 ......
H .... U .. - 7 - - ..
I .. V ... - 8 - ....
J - . - . W . - - 9 - .. -
K - . - X . - .. 0 ---- ------
L -- Y .. ..
M - - Z ... .
EXAMPLE IN USE.--Let us take an example in the word "electrical."
E L E C T R I C A L
. -- . .. . - . .. .. .. . . - --
The operator first makes a dot, which means a sharp and a dull click
close together; there is then a brief interval, then a lapse, after
which there is a sharp click, followed, after a comparatively longer
interval, with the dull click. Now a dash by itself may be an L, a T, or
the figure 0, dependent upon its length. The short dash is T, and the
longest dash the figure 0. The operator will soon learn whether it is
either of these or the letter L, which is intermediate in length.
In time the sender as well as receiver will give a uniform length to the
dash impulse, so that it may be readily distinguished. In the same way,
we find that R, which is indicated by a dot, is followed, after a short
interval, by two dots. This might readily be mistaken for the single dot
for E and the two dots for I, were it not that the time element in R is
not as long between the first and second dots, as it ordinarily is
between the single dot of E when followed by the two dots of I.