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.