MILITARY TELEGRAPHY
Telegraphy plays a very important part in warfare. The commander of even
a small unit cannot see all that his men are doing or suffering, but is
kept posted by telegraph or telephone, while communication between units
depends very largely indeed upon such means. Wireless telegraphy, in
land warfare, is largely devoted to communication between aircraft and
the artillery batteries with which they are working, and to avoid
interference with that important work telegraphy _by wire_ is employed
for most other purposes.
Right at the front this communication is kept up by means of that type
of instrument which the soldiers call a "buzzer," for the good and
sufficient reason that that is really what it does.
In view of the fact that soldiers speak of their home-land, for which
they are enduring all manner of risk and hardship, and to which they are
longing to return, by the contemptuous-sounding name of "Blighty," we
might expect that what they call a buzzer has nothing whatever to do
with making sound, but in this case the name describes the thing very
aptly. Its sole purpose and intent is to make buzzing sounds of either
long or short duration.
Perhaps the simplest way in which I can describe this useful and
interesting invention is by telling you how you can make one for
yourself. It is nothing more than an electric-bell mechanism connected
up in a certain way.
As most people know, an electric bell contains a magnet made of two
round pieces of iron placed parallel and yoked together at one end by
means of a third piece of iron, generally flat, while on to each round
piece is threaded a bobbin of insulated wire. The iron becomes a magnet
when, and only when, current flows through the wire.
Near the free ends of the round pieces, or the poles of the magnet, to
use the orthodox term, is placed another little piece of iron called the
armature, carried upon a light spring. When the current flows in the
wire the armature is pulled towards the poles against the force of the
spring, but when the current ceases the magnet lets go and the armature,
urged by the spring, swings back again.
Behind the armature is a little post through which passes a screw tipped
with platinum, and in operation this screw is advanced until its point
touches a small plate of platinum carried by the armature. Connection
for the current is made to this "contact screw" whence it passes to the
armature, through the spring to the wire upon the magnet, through that
and away. On completing the circuit, then, as when you push the button
at the front door, current flows and energizes the magnet. A moment
later, however, the armature moves, breaks the contact with the screw
and stops the current. Then the magnet lets go and the armature springs
back, making contact once more and setting the current flowing again.
These actions repeat themselves over and over again quite automatically,
and the hammer which is attached to the armature vibrates accordingly.
That is the ordinary familiar electric bell. Cut off the hammer and you
have a buzzer with which excellent telegraph signals can be sent.
So much for the sending apparatus. The receiving device is simply an
ordinary telephone receiver. There is sometimes a little confusion in
people's minds because of this. A telephone is used, but it is used as a
telegraph instrument. The sounds heard in it are not speech but long and
short buzzing sounds which, being interpreted according to the code of
Morse, deliver up their message.
Now the telephone, by which term is always meant the receiver (the
sending part of the telephone apparatus being a "microphone"), is one of
the most remarkable pieces of electrical apparatus which the mind of man
has ever conceived. It is astonishingly robust. With ordinary care you
cannot damage it. There is no need whatever to keep it wrapped in cotton
wool or even to keep it in a case. Without harm you can put it loose in
your pocket. Within reason you may even drop it a few times without
harm. Its cost is only a few shillings. Yet its sensitiveness is simply
astounding. It will detect the existence of currents so small that any
other type of instrument to deal with them has to be extremely delicate
and costly.
It consists of a magnet fitted into a little brass case with a little
piece of soft iron fixed on each pole, while each of these "pole-pieces"
is surrounded by a tiny coil of wire. The lid of the box is a disc of
thin sheet-iron, and things are so proportioned that the pole pieces
nearly but not quite touch this sheet-iron "diaphragm."
An outer cover, generally of ebonite, serves to catch the sound-waves
caused by any movement of the diaphragm and convey them to the ear.
The action of the permanent magnet tends to pull the diaphragm
inwards--to bulge it in slightly--so that it is in a state of very
unstable equilibrium. Because of this instability a very tiny current
flowing through the coils and either adding to or subtracting from the
strength of the magnet is sufficient either to draw it still closer or
to let it recede a little. Whether it approaches or recedes depends upon
the direction of the current through the coils and makes no difference
to the sound. The movement of the diaphragm is great or small according
as the current is strong or weak: any variation in the current causes a
perfectly corresponding movement in the diaphragm. Even those very small
and very complex changes in air-pressure which give us the sensation of
sound are very faithfully followed by this simple bit of sheet iron, so
that the sounds are faithfully reproduced for our benefit. At the
moment, however, we are not dealing with speech but with buzzing sounds,
which are very simple, being merely a rapid succession of "ticks."
The telephone, it must be remembered, takes no notice of a steady
current, except when it starts and stops. But each time that occurs it
gives a tick. Hence, if we start and stop a current very rapidly, or to
use another term, make it rapidly intermittent, we get a rapid
succession of ticks, and if rapid enough they form a humming, buzzing,
or singing sound. If very fast you can get a positive shriek. The
precise character of the sound depends entirely upon the rapidity of the
intermittency.
Now it is easy to see that the current passed through an electric-bell
mechanism is intermittent. It is the very nature of the apparatus to
make the current intermittent. It is by so doing that it works.
Therefore, if we pass the same current which works a bell through a
telephone we get a buzzing or humming sound according to the speed of
interruption.
The vibration of the armature itself also causes a humming sound of a
similar note or tone to that heard in the telephone, but it must be
clearly understood that these two sounds are quite different. One is the
result of mechanical motion, the other is the result of electrical
action producing motion in the diaphragm of the telephone. When you
listen in the telephone it is not that you hear the sound of the bell
mechanism, you hear another sound altogether, although, since both have
the same origin, both have the same note or tone.
Take any old bell, then, which you may happen to have or be able to
procure and an old telephone such as can be bought for a shilling or so
at a second-hand shop, and these together with a pocket-lamp battery can
be formed into a military field telegraph.
The way to connect these up is to run a wire from one of the copper
strips on the battery to one of the terminal screws on the bell, a
second wire from the other screw on the bell to one of the flexible
wires of the telephone, which may be a mile away if you like, a third
wire returning from the other flexible wire of the telephone back to the
battery. To send signals all you have to do is to touch the return wire
upon the second strip of the battery for short or long intervals,
thereby making the dot-and-dash signals. Or a simple form of key can
easily be contrived for the purpose.
Every time you complete the circuit the buzzer will buzz, in other
words, it will permit an intermittent current to pass round the circuit
and a buzzing or humming sound will be heard in the telephone, no matter
how far away it may be.
This arrangement, however, involves two wires between the two stations,
and in practice only one is usual. This could be arranged by running the
third wire from the telephone not back to the sending station but to a
peg driven into the earth, connecting the second pole of the battery in
like manner to an earth pin at the sending end. Thus the return wire
would be done away with and the earth utilized instead. To do that,
unfortunately, you would need to increase very greatly the power of your
battery, for although the path through the earth itself offers
practically no resistance at all to the current, the actual places where
the current passes to earth and from earth, especially if they be simply
temporary pegs driven into the ground, offer very considerable
resistance, so that in order to get enough current through the buzzer
to make it work would need a powerful battery. There is another way,
however, by which that difficulty can be overcome quite easily.
Probably all my readers know something of the induction or shocking
coil, wherein intermittent currents in one part of the coil induce
intermittent currents of a somewhat different kind in another part of
the coil. Few people realize, however, that the same effect can be
attained, within limits, in a single coil such as the winding upon the
magnet of an electric bell.
Watch a bell at work and you will notice a bright spark at the place
where the contact is made and broken. That spark is due to a sudden rush
of current which takes place in the coil when the original current is
stopped, in other words, when the contact is broken. It is as if the
coil gives a rather vicious "kick" every time the current is stopped.
There is not much electricity in this "kick" current, but it is very
forceful, and it is that force which makes it actually jump across the
gap after contact has been broken, thereby causing the spark.
Now we can capture most of that energy and make it go a long distance
through wire and through earth carrying our messages for us. To do this
we need to make a new connection on the bell at the place where the
spring is fixed. Then we can make two circuits. One is between the two
terminal screws of the buzzer, in which circuit we must include the
battery and the key. That circuit will be just as it would be if we were
fixing the buzzer to announce our visitors at the front door.
The second circuit is different: lead one wire from the new connection
just made and take it to a pin driven into the ground. If the ground is
just a shade moist a wire meat-skewer will answer admirably. Then lead a
second wire from that one of the two terminal screws which is connected
directly to the winding of the magnet (not to that one which is
connected to the contact screw) and lead it away to your distant
station.
At the other station connect the single wire to the telephone as before
and the other "end" of the telephone to a pin in the earth. You will
find that the "kicks" from the coil will traverse wire and earth-return
quite easily, while there will be no difficulty about working the bell,
for the small battery will do that quite well. In fact, after cutting
the hammer off and so converting a bell into a buzzer, I have got quite
good results with one-third of a pocket-lamp battery. The little flat
batteries so familiar to us all if divested of their outer covering will
be found to consist of three little dry cells any one of which is quite
capable of sending messages in the way described as far as any amateur
is likely to want to send.
To be able to send and receive at either end it is only necessary to
connect both telephones and both coils "in series." That is to say,
connect one end of the coil to the long wire and the other to one wire
of the telephone, the other wire of the telephone being connected to
earth. If this be done at both ends signals can be sent and received
both ways.
Many young readers, scouts, members of cadet corps and the like, will
find great pleasure and interest in constructing and working this
apparatus, besides which it shows precisely what the official "buzzer"
is like.
Although beautifully made, of course, the army instrument is essentially
just that and little more. It has an additional feature, however,
namely, a microphone, so that when desired it can be used as a speaking
telephone for transmitting verbal messages. It also has the bottom of
the case made of a brass plate so that earth pins are often unnecessary,
the case dumped down upon the ground being a good enough "earth."
Buzzers are not used for very long lines: forty miles is about the
limit, and usually the distances are very much less. That is because
long lines rather object to rapidly changing currents flowing through
them. Why, you say, what currents could change more rapidly than
telephone currents carrying speech, yet they go for hundreds of miles?
True, but in that case there are two wires, flow and return, twisted
together all the way, under which conditions they interact upon each
other in such a manner as to abolish the difficulty to which I am
referring. Buzzers and indeed all the telegraph circuits consist of one
wire and the earth, which is quite different.
Another objection to the buzzer is that it is apt to interfere with
others. For instance, if two buzzer sets are at work anywhere near each
other and the wires run parallel for a distance they will be able to
hear each other's signals as well as their own. If two such sets are
earthed near together the same thing happens, the signals of one are
picked up by the other, a very annoying state of affairs for the
operators.
Right at the front, however, amid the rough and tumble of the actual
fighting, the buzzer is supreme. The wire used is sometimes plain copper
enamelled: more often, however, it is a mixture of steel and copper
strands twisted together and covered with a strong insulating covering.
This is carried on reels in properly fitted carts which can advance at a
gallop, paying out the wire as they go. The inner end of the wire is
connected to the axle of the reel in such a way that a telegraphist in
the cart is in communication all the time with the starting-point, the
wheels of the cart providing him with an earth connection.
When laying these wires another interesting little device is often
used--an earth plate on the operator's heel. Thus, while carrying the
wire along, laying it as he goes, he can still be in communication with
the starting-point every time he puts his heel to the ground.
For the longer lines away back from the fighting the methods employed
are just the same as those of peace. "Sounder" instruments are used,
Wheatstone automatic machines, duplex and quadruplex systems, whereby
two and four messages are sent simultaneously over the same wire, indeed
all the contrivances and refinements of the home telegraph office are to
be found in the field telegraph offices. But it would hardly be fitting
to describe them here. Some information on the subject will be found in
"The Romance of Submarine Engineering," where their application to cable
telegraphy is dealt with.
A genuine speciality of warfare, however, is the methods by which
makeshift arrangements can be set up, such as sending telegraph messages
over a telephone wire without interfering with the latter.
Imagine that A and B are the two wires of a telephone circuit running
(for the sake of simplicity) from north to south. At the south end I
connect a telegraph set to both wires while you, we will imagine, do the
same at the north end. You and I can then signal to each other without
the telephone man hearing us at all. To him the two wires are flow and
return, to us they are both "flow," the earth being our return. Thus our
signals never reach his instruments at all. But when we each connect to
both his wires, do we not "short-circuit" or connect them to each other,
thereby destroying his circuit? No, we are too cunning for that. We
first connect the two wires A and B together with a coil of closely
wound wire, having, in scientific language, much "inductance," and
telephone currents shun a coil of that sort. Then we make our connection
to the centre of that coil so that our currents go to A through half the
coil and to B through the other half. This enables us to use the
apparatus without interfering with the other fellow at all. For this, by
the way, we must use ordinary telegraph instruments. We cannot employ a
buzzer, for these coils which we use to obstruct the passage of the
other man's telephone currents would also obstruct the changing currents
from a buzzer. The slow, steady currents of the ordinary telegraph pass
quite easily, however.
Again, suppose you and I want to communicate by buzzer and there is
already a wire laid passing both of us but in use already for ordinary
telegraphy. We only need to add a "condenser" to our apparatus and we
can manage all right. As a matter of fact, the service instruments
generally have condensers partly for this very purpose. Each of us then
connects his instrument to the wire and to earth, after which we can
signal to each other while the telegraphist is unaware of the fact. The
reason that is possible is the reverse of what we saw just now. There we
had a coil which obstructed buzzer or telephone currents but passed
ordinary telegraph currents. Here we use condensers which will pass our
buzzer currents but not the ordinary telegraph currents.
Thus the soldier telegraphist is up to many dodges whereby he can save
time or save material, both of which may be precious. As in bridge
building and other branches, he needs to be quick to adapt himself to
circumstances, to utilize to the full any opportunities which may
present themselves. But his principles are quite simple and do not
differ in any way from those of peace. It is only in applying them that
the differences arise.