Telegraph

Towards the end of the 18th century, however, Galvani and Volta conducted electrical experiments which revolutionized pre conceived ideas of electricity and its effects. In 1786 Galvani accidentally discovered that it was possible to cause a direct or continuous flow of current along an electrical conductor by bring ing two dissimilar metals into contact with a moist substance, such as animal tissues. This led Volta in 1800 to introduce an electric battery which became known as the voltaic pile, the principles of which are still embodied in the modern battery; and that year Salva demonstrated that voltaic currents could be used for transmitting signals.

Electrolytic Telegraphs.

An immediate result of the intro duction of the voltaic pile was the discovery by Nicholson and Carlisle in 1801 that the passage of an electric current caused decomposition of liquids into their constituent elements. Based on this principle, Salva, in 1805, and Soemmering, in 1809, each introduced an apparatus in which a voltaic pile was used for send ing signals whose presence were indicated at the receiving end by the liberation of bubbles of hydrogen. In the latter's instrument 35 wires, each allotted to the German alphabet and the ten numerals, terminated in a receptacle containing water (fig. I). At the sending station a key which brought a voltaic battery into circuits was connected,- as required, to each of the line wires, and the message was read by observing at which end of the terminals the bubbles of gas appeared. The later researches of Davy on the decomposition of chemical compounds by the electric current were applied in America to another type of telegraph by Coxe in 1816, and Dyer in 1828, who each operated a signal telegraph using the electric current to produce electrical decom position at the end of the wire conductors. In the former's apparatus a voltaic current was employed. He used 36 wire con ductors and 36 "return wires." Dyer, who operated his telegraph line over 8 m. on a racecourse in Long Island, used a single wire and ground circuit, and obtained red marks on litmus paper by discolouration, but he used frictional electricity, and the system was abandoned in 1830.

Electro-magnetic Needle Telegraphs.

At the beginning of the 19th century a new set of ideas followed the introduction of the electromagnet. In 1819 Oersted discovered that a magnetic needle could be deflected from its normal position by passing a current through an adjacent wire; and that the deflection was to the right or left, according to the direction of the current. Schweigger a year later found that the deflection of the needle could be increased by surrounding it with a number of separated turns of naked wire. Following upon these experiments Sturgeon

in England, in 1825, produced and named the electromagnet (fig. 2). This consisted of a piece of soft iron in horseshoe form which was surrounded by a number of turns of wire. Upon the passage of a current through this wire the soft iron horseshoe attracted a smaller bar or armature of soft iron towards it. Four years later Henry, in America, improved Sturgeon's magnet by surround ing it with nine coils, each com posed of 6o f t. of insulated wire, thus greatly increasing the power with which the armature was at tracted. In 1831 Henry con structed and successfully operated an electro-magnetic signalling ap paratus which consisted of a magnetized steel bar Io in. in length, which was supported on a pivot in an horizontal position with its north end between the two arms of a horseshoe magnet. When the latter was energized by the current, the end of the bar was attracted by one arm of the horseshoe and repelled by the other, causing its further extremity to strike a suitably adjusted bell, and a code was arranged by varying sound combinations.

The action of the electric current on a magnet was first applied to electric telegraphy by Ampere, in 1820, at the suggestion of Laplace that the deflections of small magnets placed at the re ceiving ends of 26 wires could be used to indicate the letters of the alphabet. Ampere's apparatus was the pioneer of a number of needle instruments which came into extensive use between the years 1829 and 1841. The names of Gauss and Weber, Cooke and Wheatstone, and the brothers Highton, are associated with some of the more important needle instruments. Gauss and Weber also conducted important experiments on line conductors, in which they demonstrated that the conductor need not be insulated over its entire length. In their experiments they used a single-needle telegraph to communicate with each other and proved that by proper combination five signs are sufficient for communication. In the early experiments of Cooke and Wheatstone five needles were employed; the number was afterwards gradually reduced to two, which indicated letters by their separate and combined move ments. In the single-needle instruments a magnetic needle was pivoted in the centre of a wire coil and a pointer, attached to the needle, swung in front of a dial. Deflections to right or left signi fied given letters. These deflections were produced by sending over the wire pulsations of one polarity, or alternations of both, as required by the letter to be transmitted.