PROGRESS IN PHYSICS. 329 



uialtii)lied. Henry studied also the best form and aiTant,^enient of the 

 battery under varying conditions of the conductor. ^ An electro- 

 mao-netic telegraph had been declared impossible in 1825 by Barlow, an 

 Englishman, who pointed out the apparently fatal fact that the resistance 

 offered to the current was proportional to the length of the conducting 

 wire, and that the strength of the current would be thus so much 

 reduced, for even short distances, as to become too feeble to be detected. 

 Henry showed that what is known as an "intensity battery" would 

 overcome this difficulty, discovering experimentally and independently 

 the beautifully simple law showing the relation of current to electro- 

 motive force which Ohm had announced in 1827. He also invented 

 the principle of the relay, by which the action of a very feeble current 

 controls the operation of a more powerful local system. It will thus 

 be seen that the essential features of the so-called American system of 

 telegraphy are to be credited to Henrv, who had a working line in his 

 laboratory as early as 1882. 



Morse made use of the scientific discoveries and inventions of Henry, 

 and by his indefatigable labors and persistent faith the commercial 

 value of the enterprise was realh^ established. In the meantime con- 

 siderable progress was made in Europe. Baron Schilling, a Russian 

 councilor of state, devised and exhibited a needle telegraph. The two 

 illustrious German physicists, Gauss and Weber, established a success- 

 fully working line 2 or 3 miles long in 1833, and this system was com- 

 mercially developed by Steinheil in 1837. In England, Sir Charles 

 Wheatstone made man}^ important contributions, although using the 

 needle system, which was afterwards abandoned. Before the middle 

 of the century the commercial success of the electro-magnetic telegraph 

 was assured, and in the matter of the transmission of messages distance 

 was practicallv annihilated. 



Oersted, Arago, Ampere, Sturgeon, and Henry had made it possible 

 to convert electricity into mechanical energy. Motors of various 

 types had been invented, and the possibility of using the new source 

 of power for running machiner3^ cars, boats, etc., was fully recognized. 

 Several attempts had been made to do these things, but the great cost 

 of producing the current by means of a battery stood in the w&y of 

 success. Another epoch-making discovery was necessary — namely, a 

 method of reversing the process and converting mechanical energy 

 into electricity. This was supplied by the genius of Michael Faraday, 

 who had succeeded Davy in the Royal Institution at London. In 1831 

 Faraday discovered induction, the key to the modern development of 

 electricity. He showed that while Oersted had proved that a current 

 of electricity would generate a magnetic field and set a magnet in 

 motion, this process was reversible. A magnet set in motion in a mag- 

 netic field by a steam engine or any other source of power would pro- 

 duce in a conductor properly arranged a current of electricity, and 



