In every telegraphic system there are three distinct portions of the apparatus, which may be separately considered, as they may be variously combined. We have—
1º. The apparatus for producing the electricity, such as batteries, magneto-electric machines, &c.
2º. The conductors, or wires, which convey the electricity.
3º. The apparatus for sending and for receiving the messages.
Of the first we shall have little to add to what has been said in the last article; and before entering upon the description of the second, it will be better to discuss the third division.
TELEGRAPHIC INSTRUMENTS.
Telegraphs may conveniently be classed according to the mode in which the actions of the sender produce their effect at the point where the message is received. A first class may include those in which the current is made to deflect magnetized needles; a second may comprise those in which the current, by magnetizing soft iron, causes an index to travel along a dial and point to the letter intended; a third may embrace those in which the same action on soft iron is made to print the despatches, either in ordinary type or in conventional signs; while in a fourth class we may put the instruments which give their indications by sounds only. It is obvious that in some of these systems signs only are used, and a special training and acquaintance with the symbols is necessary, while in the rest the ordinary alphabetic letters are shown or recorded. In the former case the apparatus is simpler, and therefore for the general business of public telegraphy it is almost exclusively employed; while for private purposes, where it is often required that the messages should be dispatched and received by persons not acquainted with the symbolic language, the dial telegraph, or that which prints the message in ordinary characters, will continue to be employed, in spite of the greater complexity and greater liability to derangement of the apparatus.
In the needle telegraphs the essential part of the apparatus is a multiplier (page [493]), having its needle mounted vertically on a horizontal axis, to which is also attached an indicator, visible on the face of the instrument, and formed either of a light strip of wood, or of another magnetized needle, having its poles placed in the reverse position to those of the needle within the coil. When the current is sent through the latter, the index is deflected to the right or left, according to the direction in which the current passes. Fig. [282] represents the exterior of one of Wheatstone and Cooke’s double-needle instruments, now almost entirely superseded, where needles are used at all, by the single-needle instrument. The face of the instrument is marked with letters and signs, which were supposed to aid the memory of the telegraphist, and the movements of the needles were chosen rather with that view than any other. We need not here give the code of signals, as the double instrument is now obsolete, and the code for the single-needle instrument, which was devised by Wheatstone and Cooke, has been in most cases superseded by one corresponding with the Morse code, a deflection to the right representing a dot, and a deflection to the left a dash.
Fig. 282.—The Double-Needle Instrument.