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