Needle instruments are largely used in railway signal cabins, but for general telegraphic work an instrument called the Morse sounder is employed. This consists of an electro-magnet which, when a current is passed through it, attracts a small piece of iron fixed to one end of a pivoted lever. The other end of this lever moves between two stops. At the transmitting station the operator closes a battery circuit by pressing a key, when the electro-magnet of the sounder at the receiving station attracts the iron, and the lever flies from one stop to the other with a sharp click, returning again as soon as the circuit is broken. A dot is signalled when the lever falls back immediately after the click, and a dash when it makes a short stay before returning. [Fig. 28] shows the code of signals for the Morse telegraph.
Fig. 28.—The Morse Code.
In passing through a very long wire an electric current becomes greatly reduced in strength owing to the resistance of the wire. If two telegraph stations are a great distance apart the energy of the current thus may be unequal to the task of making the electro-magnet move the lever of the sounder so as to produce a click, but this difficulty is overcome by the use of an ingenious arrangement called a “relay.” It consists of a very small electro-magnet which attracts a light bar, the movement of the bar being made to close the circuit of another battery at the receiving station. The feeble current works the relay, and the current in the local circuit operates the sounder.
The word “telegraph,” which is derived from the Greek tele, far off, and grapho, I write, strictly signifies writing at a distance. The needle instrument and the sounder do not write in any way, but by modifying the construction of the sounder it can be made to record the messages it receives. A small wheel is fitted to the free end of the lever of the sounder, and an ink-well is placed so that the wheel dips into it when the lever is in the normal position. When the circuit is closed the lever moves just as in the ordinary sounder, but instead of clicking against a stop it presses the inked wheel against a paper ribbon which is kept slowly moving forward by clockwork. In this way the wheel continues to mark a line along the paper as long as the circuit remains closed, and according to the time the transmitting key is kept down a short mark or dot, or a long mark or dash, is produced. The clockwork which moves the paper ribbon is started automatically by the current, and it continues working until the message is finished.
Fig. 29.—A Morse Message.
(a) Perforated Tape. (b) Printed Tape.
TRANSLATION.
Series of alternate dots and dashes indicating commencement of message.