THE ELECTRIC TELEGRAPH.
Needle instruments--Influence of current on the magnetic
needle--Method of reversing the current--Sounding
instruments--Telegraphic relays--Recording telegraphs--High-speed
telegraphy.
Take a small pocket compass and wind several turns of fine insulated
wire round the case, over the top and under the bottom. Now lay the
compass on a table, and turn it about until the coil is on a line with
the needle--in fact, covers it. Next touch the terminals of a battery
with the ends of the wire. The needle at once shifts either to right or
left, and remains in that position as long as the current flows. If you
change the wires over, so reversing the direction of the current, the
needle at once points in the other direction. It is to this conduct on
the part of a magnetic needle when in a "magnetic field" that we owe the
existence of the needle telegraph instrument.
NEEDLE INSTRUMENTS.
[Illustration: FIG. 54.--Sketch of the side elevation of a Wheatstone
needle instrument.]
Probably the best-known needle instrument is the Cooke-Wheatstone,
largely used in signal-boxes and in some post-offices. A vertical
section of it is shown in Fig. 54. It consists of a base, B, and an
upright front, A, to the back of which are attached two hollow coils on
either side of a magnetic needle mounted on the same shaft as a second
dial needle, N, outside the front. The wires W W are connected to the
telegraph line and to the commutator, a device which, when the operator
moves the handle H to right and left, keeps reversing the direction of
the current. The needles on both receiving and transmitting instruments
wag in accordance with the movements of the handle. One or more
movements form an alphabetical letter of the Morse code. Thus, if the
needle points first to left, and then to right, and comes to rest in a
normal position for a moment, the letter A is signified;
right-left-left-left in quick succession = B; right-left-right-left = C,
and so on. Where a marking instrument is used, a dot signifies a "left,"
and a dash a right; and if a "sounder" is employed, the operator judges
by the length of the intervals between the clicks.
INFLUENCE OF CURRENT ON A MAGNETIC NEEDLE.
[Illustration: FIGS. 55, 56.--The coils of a needle instrument. The
arrows show the direction taken by the current.]
Figs. 55 and 56 are two views of the coils and magnetic needle of the
Wheatstone instrument as they appear from behind. In Fig. 55 the current
enters the left-hand coil from the left, and travels round and round it
in a clockwise direction to the other end, whence it passes to the other
coil and away to the battery. Now, a coil through which a current passes
becomes a magnet. Its polarity depends on the direction in which the
current flows. Suppose that you are looking through the coil, and that
the current enters it from your end. If the wire is wound in a clockwise
direction, the S. pole will be nearest you; if in an anti-clockwise
direction, the N. pole. In Fig. 55 the N. poles are at the right end of
the coils, the S. poles at the left end; so the N. pole of the needle is
attracted to the right, and the S. pole to the left. When the current is
reversed, as in Fig. 56, the needle moves over. If no current passes, it
remains vertical.
METHOD OF REVERSING THE CURRENT.
[Illustration: FIG. 57.--General arrangement of needle-instrument
circuit. The shaded plates on the left (B and R) are in contact.]
A simple method of changing the direction of the current in a
two-instrument circuit is shown diagrammatically in Fig. 57. The
_principle_ is used in the Wheatstone needle instrument. The battery
terminals at each station are attached to two brass plates, A B, A^1
B^1. Crossing these at right angles (under A A^1 and over B B^1)
are the flat brass springs, L R, L^1 R^1, having buttons at their
lower ends, and fixed at their upper ends to baseboards. When at rest
they all press upwards against the plates A and A^1 respectively. R
and L^1 are connected with the line circuit, in which are the coils of
dials 1 and 2, one at each station. L and R^1 are connected with the
earth-plates E E^1. An operator at station 1 depresses R so as to
touch B. Current now flows from the battery to B, thence through R to
the line circuit, round the coils of both dials through L^1 A^1 and
R to earth-plate E^1, through the earth to E, and then back to the
battery through L and A. The needles assume the position shown. To
reverse the current the operator allows R to rise into contact with A,
and depresses L to touch B. The course can be traced out easily.
In the Wheatstone "drop-handle" instrument (Fig. 54) the commutator may
be described as an insulated core on which are two short lengths of
brass tubing. One of these has rubbing against it a spring connected
with the + terminal of the battery; the other has similar communication
with the - terminal. Projecting from each tube is a spike, and rising
from the baseboard are four upright brass strips not quite touching the
commutator. Those on one side lead to the line circuit, those on the
other to the earth-plate. When the handle is turned one way, the spikes
touch the forward line strip and the rear earth strip, and _vice versâ_
when moved in the opposite direction.
SOUNDING INSTRUMENTS.
Sometimes little brass strips are attached to the dial plate of a needle
instrument for the needle to strike against. As these give different
notes, the operator can comprehend the message by ear alone. But the
most widely used sounding instrument is the Morse sounder, named after
its inventor. For this a reversible current is not needed. The receiver
is merely an electro-magnet (connected with the line circuit and an
earth-plate) which, when a current passes, attracts a little iron bar
attached to the middle of a pivoted lever. The free end of the lever
works between two stops. Every time the circuit is closed by the
transmitting key at the sending station the lever flies down against the
lower stop, to rise again when the circuit is broken. The duration of
its stay decides whether a "long" or "short" is meant.
TELEGRAPHIC RELAYS.
[Illustration: FIG. 58.--Section of a telegraph wire insulator on its
arm. The shaded circle is the line wire, the two blank circles indicate
the wire which ties the line wire to the insulator.]
When an electric current has travelled for a long distance through a
wire its strength is much reduced on account of the resistance of the
wire, and may be insufficient to cause the electro-magnet of the sounder
to move the heavy lever. Instead, therefore, of the current acting
directly on the sounder magnet, it is used to energize a small magnet,
or _relay_, which pulls down a light bar and closes a second "local"
circuit--that is, one at the receiver end--worked by a separate battery,
which has sufficient power to operate the sounder.
RECORDING TELEGRAPHS.
By attaching a small wheel to the end of a Morse-sounder lever, by
arranging an ink-well for the wheel to dip into when the end falls, and
by moving a paper ribbon slowly along for the wheel to press against
when it rises, a self-recording Morse inker is produced. The
ribbon-feeding apparatus is set in motion automatically by the current,
and continues to pull the ribbon along until the message is completed.
The Hughes type-printer covers a sheet of paper with printed characters
in bold Roman type. The transmitter has a keyboard, on which are marked
letters, signs, and numbers; also a type-wheel, with the characters on
its circumference, rotated by electricity. The receiver contains
mechanisms for rotating another type-wheel synchronously--that is, in
time--with the first; for shifting the wheel across the paper; for
pressing the paper against the wheel; and for moving the paper when a
fresh line is needed. These are too complicated to be described here in
detail. By means of relays one transmitter may be made to work five
hundred receivers. In London a single operator, controlling a keyboard
in the central dispatching office, causes typewritten messages to spell
themselves out simultaneously in machines distributed all over the
metropolis.
The tape machine resembles that just described in many details. The main
difference is that it prints on a continuous ribbon instead of on
sheets.
Automatic electric printers of some kind or other are to be found in
the vestibules of all the principal hotels and clubs of our large
cities, and in the offices of bankers, stockbrokers, and newspaper
editors. In London alone over 500 million words are printed by the
receivers in a year.
HIGH-SPEED TELEGRAPHY.
At certain seasons, or when important political events are taking place,
the telegraph service would become congested with news were there not
some means of transmitting messages at a much greater speed than is
possible by hand signalling. Fifty words a minute is about the limit
speed that a good operator can maintain. By means of Wheatstone's
_automatic transmitter_ the rate can be increased to 400 words per
minute. Paper ribbons are punched in special machines by a number of
clerks with a series of holes which by their position indicate a dot or
a dash. The ribbons are passed through a special transmitter, over
little electric brushes, which make contact through the holes with
surfaces connected to the line circuit. At the receiver end the message
is printed by a Morse inker.
It has been found possible to send several messages simultaneously over
a single line. To effect this a _distributer_ is used to put a number of
transmitters at one end of the line in communication with an equal
number of receivers at the other end, fed by a second distributer
keeping perfect time with the first. Instead of a signal coming as a
whole to any one instrument it arrives in little bits, but these follow
one another so closely as to be practically continuous. By working a
number of automatic transmitters through a distributer, a thousand words
or more per minute are easily dispatched over a single wire.
The Pollak Virag system employs a punched ribbon, and the receiver
traces out the message in alphabetical characters on a moving strip of
sensitized photographic paper. A mirror attached to a vibrating
diaphragm reflects light from a lamp on to the strip, which is
automatically developed and fixed in chemical baths. The method of
moving the mirror so as to make the rays trace out words is extremely
ingenious. Messages have been transmitted by this system at the rate of
180,000 words per hour.