546 THE AP PLICA TIONS OF PHYSICAL FORCES. [BOOK v. 



Alexander constructed at Edinburgh in 1837 an apparatus on the 

 same system. It had thirty needles corresponding to as many wires 

 stretched between the two stations and made a corresponding number 

 of signs. Gauss and Weber employed also this kind of apparatus for 

 communication between the physical laboratory and the observatory 

 of Gottingen." (Daguin.) 



The time had now arrived (1837 and 1838) when the electric 

 telegraph was about to pass from the period of attempts arid ex- 

 periments to that of true practical realization, and the names of 

 Wheatstone, Cooke, Steinheil, Morse, Masson, and Breguet recall 

 the important labours, discoveries, and improvements which charac- 

 terise the different systems successively adopted. We will here 

 then leave our historical notices to enter on the description of 

 the systems, but we must draw attention, by an example, to the 

 manner in which the applications of science are bound up with 

 purely scientific progress. Without the discovery of the new forms 

 of battery, without the substitution of constant currents for the 

 currents of the first kind of batteries whose intensity so rapidly 

 decreased, it is probable that the marvellous art of electric telegraphy 

 would be yet in its infancy. It would be still a curious application 

 of physics, and not an invention in use and of universal value. 



II. THE ELECTRIC TELEGRAPH. GENERAL THEORY. 



A piece of soft iron in the form of a horse-shoe, round which is 

 twisted a helix or spiral made of an insulated metal wire, constitutes 

 an electro-magnet, that is to say, a temporary magnet, whose magnetic 

 power continues during the passage of the electric current through the 

 wire and ceases as soon as the current is interrupted. 



This temporary magnetisation is instantaneous, and it ceases with 

 the same rapidity as it commences. It follows from this that if by 

 any means whatever we can make a current of electricity pass through 

 the coil of an electro-magnet, and then cut it off, in a rapid series of 

 operations composed of this double elementary operation the attrac- 

 tion of the pole of the magnet for its armature will be reproduced and 

 suspended the same number of times. This property is made use of 

 to obtain a series of alternating movements of the armature ; it is 



