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 in
terest 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."