22 THE ELECTRIC TELEGRAPH. 



position of the observer must be supposed to be reversed 

 also head at a, feet at b'. While . he faces the needle the 

 north pole is still found on his left hand. When currents 

 pass, therefore, in both wires at the same time in opposite 

 directions, they act in the same sense on the magnetic needle 

 N s, and, other things being equal, their combined force is 

 double that of a single wire. The same would be reached by 

 joining b and b' by a wire, and letting a current of equal 

 strength pass from a to b, b' } and a. 



28. Professor Schweiger, of Halle, the same who suggested 

 improvements of Sommering's telegraph, soon after the 

 discovery of electro-magnetism by Oersted, invented an 



apparatus based on this 

 principle, which he 

 made by coiling a wire 

 several times round a 

 magnetic needle, and 

 found that the deflect- 

 ing force increased with 

 the number of turns. 

 Such an apparatus, 

 called an electro-mag- 

 iietic multiplier, is 



shown in Fig. 9. It has since become one of the most 

 essential instruments for the measurement and indication of 

 galvanic electricity, 



29. The brilliant discovery of electro-magnetism* was 

 speedily followed by attempts to employ it for the telegraph, 

 which made, from this time, gigantic progress towards its 

 present state of perfection. 



The idea of substituting magnetic needles suspended in 

 multipliers of wire in place of the voltameters of Sommering 



* It is said that the same discovery had already been made in 1802, by 

 Grrandominico Romagnosi, of Trent, and made known in a book entitled 

 "Manuel du Galvanisme," published in Paris in 1805. If it be true that 

 Romagnosi discovered the deflection of a magnetic needle by the current, the 

 discovery could have excited no interest whatever, and must have been known 

 within a very limited circle, as the discovery of Oersted in 1820 was imme- 

 diately hailed as a landmark in science. 



