The smaller case surmounting the instrument, Fig. [282], contains a bell or alarum, which serves to call the attention of the clerk at the receiving station. The first electric bell-alarum was invented by Wheatstone and Cooke. It was simply a clock alarum, put in motion by a wound-up spring. The spring was released at the proper moment by a detent, which was removed by the attraction of a soft iron armature to the core of a small electro-magnet, formed by the line wire itself; but when the current, on account of the length of the line, was too weak to produce a sufficiently strong electro-magnet, Wheatstone caused it to close the circuit of a local battery. The electric alarum has been modified in a thousand ways, and as electric alarums or bells are now coming into common use in hotels, and even private houses, we give in Fig. [283] a representation of one of the simplest forms, in which the bell is rung continuously by the electric current so long as the circuit is closed. The action is very simple: a soft iron armature, A, is attached to the steel spring, B, and prolonged into a hammer, C, which strikes the bell, D, every time the armature is attracted to the electro-magnet. The armature and the spring, E, form part of the circuit, which is continued by connectors to F, and through the coils to G. The spring, E, does not follow the armature in its motion towards the electro-magnet, and consequently the circuit is broken before the armature touches the magnet; but the hammer strikes the bell, and the elasticity of the spring, B, brings the armature back into contact with E, the circuit is closed, and the motions are repeated, so that the bell is struck a rapid succession of blows. This make-and-break movement is precisely similar to that with which Ruhmkorff’s coils are usually provided.
Fig. 283.—Electro-Magnetic Bells.
Below the dial of the instrument, in Fig. [282], may be seen two handles. Each of these is connected with an arrangement constituting the transmitting apparatus, by which the metallic contacts are varied according to the position of the handles. When the handle is vertical, all communication with the battery in connection with the instrument is cut off, but the coils are ready to receive any current from the line-wires. When the handle is turned to the right or left, the contacts are such that the battery current flows into the line, and deflects to the right or left the needles of both receiving and transmitting instruments. The single-needle instrument as now made is of a very simple and inexpensive construction, and it is the form principally used in connection with the working of lines of railway. One may see at every station in the United Kingdom the little vertical needle, mounted in the centre of a small perfectly plain green dial-plate; for the letters and signs with which it was formerly the practice to cover the dial have been found to distract the eye more than they aid the memory. A boy will after a few weeks’ practice learn to read the signals and to transmit messages with considerable rapidity.
Fig. 284.—Portable Single-Needle Instrument.
The field telegraph lines, which are used in actual warfare to enable the commander of an army to communicate with every part of his forces, require as the essential condition for their construction rapidity of erection and removal, and the greatest possible simplicity and portability in the sending and receiving instruments. The wires are fastened to trees, or other fixed supports, where such are available, but artificial supports are provided in light poles which admit of being readily planted in the ground and removed. In cases where it is inexpedient or impossible to use these, the conductor may be laid along the ground, but must then be well insulated with some non-conducting material, which is capable of withstanding the action of the weather. A kind of cable is usually employed, in which is the conductor, made of copper, protected and strengthened by hemp fibres and covered with some non-conducting material. No form of needle telegraph instrument could be simpler than that represented in Fig. [284], which has been designed for military purposes. The communicator, or transmitting apparatus, here shows an arrangement very compact, and not easily deranged. The springs, A B, press against the piece of metal marked C, with which good contact is insured by providing the springs with several projecting steel points. D, E are finger-keys of ebonite or ivory; underneath are two points of a metallic conductor on which the springs can be pressed down by a touch of the finger. This conductor is in communication with the binding-screw, F, from which a wire proceeds to the negative or zinc end of the battery, while the piece, C, is in metallic connection with G, to which a wire proceeding to the positive or copper end of the battery is attached. From B a wire, H, communicates through the hinge with one end of the coil, the upper end of which is connected through the upper hinge with a binding-screw not visible in the figure, and to this the end of the line conductor is attached. From A a wire K passes to another binding-screw, by which the earth connection is made. A current arriving by the line traverses the coils and passes through H and B into C, hence by A into the earth through K. When D is depressed the current from the battery passing from G through C, A, and K, into the earth, and thus to the distant station, returns through the coils of the instrument there and along the line wire, through the coils, L L, and by H, B, D and F, to the negative pole of the battery. The reader will have little difficulty in tracing the course of the reverse currents, whether sent or received, which deflect the needles in the opposite direction.
The field telegraph instrument selected by the War Department of the United States Government is also extremely simple, communicating its signals, not by the deflections of a needle, but by the blows on an electro-magnet of its armature. The letters are indicated by various combinations of two signals—one, a single stroke of the armature; and the other, two blows in very rapid succession. The alphabet used is the “General Service Flag Code” of the American army and navy, and the signal numerals of this code are indicated by contacts of the transmitting key—one contact producing a single blow of the armature, implying the numeral 1, and two rapidly succeeding contacts causing two blows, which stand for the numeral 2. The signals are read merely by the sound made by the stroke of the armature. In the table below the code is given, dots being used to represent the contacts of the key in the “sending” instrument, and the blows of the armature in the “receiving” instrument—the single dots standing for one contact or sound, and the double dots for the double blows:
| Letters. | Flag Code. | Telegraph Signals. |
|---|---|---|
| A | 2 2 | ·· ·· |
| B | 2 1 1 2 | ·· · · ·· |
| C | 1 2 1 | · ·· · |
| D | 2 2 2 | ·· ·· ·· |
| E | 1 2 | · ·· |
| F | 2 2 2 1 | ·· ·· ·· · |
| G | 2 2 1 1 | ·· ·· · · |
| H | 1 2 2 | · ·· ·· |
| I | 1 | · |
| J | 1 1 2 2 | · · ·· ·· |
| K | 2 1 2 1 | ·· · ·· · |
| L | 2 2 1 | ·· ·· · |
| M | 1 2 2 1 | · ·· ·· · |
| N | 1 1 | · · |
| O | 2 1 | ·· · |
| P | 1 2 1 2 | · ·· · ·· |
| Q | 1 2 1 1 | · ·· · · |
| R | 2 1 1 | ·· · · |
| S | 2 1 2 | ·· · ·· |
| T | 2 | ·· |
| U | 1 1 2 | · · ·· |
| V | 1 2 2 2 | · ·· ·· ·· |
| W | 1 1 2 1 | · · ·· · |
| X | 2 1 2 2 | ·· · ·· ·· |
| Y | 1 1 1 | · · · |
| Z | 2 2 2 2 | ·· ·· ·· ·· |
There are similar signals for the numerals and for a few often-recurring syllables.