Part 19 (1/2)
CHAPTER XXII
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 hards.h.i.+p, 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 pa.s.ses 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 pa.s.ses 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 b.u.t.ton 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 astonis.h.i.+ngly 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 s.h.i.+llings. 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 bra.s.s 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 pa.s.sed 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 pa.s.s 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 s.h.i.+lling 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 pa.s.s 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 pa.s.ses 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.