570 



HISTORY OF SCIENCE. 



an ordinary galvanic battery produce its effect on the needle at very 

 great distances. It was the idea of a physicist named SCHWEIGGER that 

 the effect of a current on the magnetic needle could be multiplied in- 

 definitely by carrying the conducting-wire many times round the needle. 

 Fig. 285 represents such an arrangement, and the reader can have no 

 difficulty in seeing that by Ampere's rule (page 562) every part of the 

 coil A will concur in producing one and the same effect on the needle 

 mounted on a pivot within the coil. In fact, with such a voltaic ele- 

 ment as that shown, ordinary water might be the liquid in the vessel 

 B, and the current would suffice to deflect the needle to be nearly per- 

 pendicular to the magnetic meridian. In all the needle telegraphs such 

 " multipliers " were used. BARON SCHILLING in 1833 constructed a 

 telegraph at St. Petersburg with five horizontally-mounted needles, 



FIG. 285. THE GALVANOMETER. 



which could be made to point out anything required on a dial. The 

 idea was soon improved upon, notably by Professor Wheatstone in 

 England, who made the five needles move in vertical planes. This 

 was in 1837, and soon afterwards it was found possible to signal letters 

 by the movement of a single needle. This, the simplest form of the 

 needle telegraph, soon came into very general use. Fig. 286 represents 

 the external appearance of the combined receiving and sending instru- 

 ment. The handle in the base of the instrument, according as it is 

 moved to the right or to the left, connects the line wire with one of 

 the poles of the battery, and the current circulating through similar 

 coils at each end of the line moves the needles right or left, at the will 

 of the operator. There is a conventional system of signs : thus, two 

 movements of the needle to the left indicates " A," etc. 



Ampere's and Arago's discovery of electro-magnetism has been em- 

 ployed in a great number of ingenious telegraphic systems, in which 

 messages are printed, or even written, at the distant station. As a 

 printed message has certain obvious advantages over one received 

 merely by the eye or ear, and leaving no permanent record, several 

 kinds of printing telegraphs have been used; but, from its simplicity and 

 efficiency, the telegraph devised by MORSE has been far more generally 

 employed than any other, and in the public service of England and 

 America it has almost entirely superseded the needle telegraph. The 



