160 PROGRESS OF SCIENCE IN THE CENTURY. 



must date from Ohm's work. " Since his day it has 

 been subjected to the severest experimental tests that 

 the scientific mind could imagine, and has stood them 

 all. It is really the basis of our whole system of 

 electrical measurements, and is to electric currents 

 what the law of gravitation is to planetary mo- 

 tions." * 



The instrumental measurement of resistance which 

 Ohm initiated was subsequently brought nearer per- 

 fection, especially by those concerned in the develop- 

 ment of telegraphy. Thus Charles Wheatstone 

 (1802-1875) invented what is known as " Wheat- 

 stone's bridge." Here, as in so many other cases, 

 practical requirements led to improvements which 

 stimulated theoretical science and gave it greater 

 possibilities of precision. 



Faraday. The next great name is that of Michael 

 Faraday (1791-1867), who by common consent is 

 ranked as the greatest experimental genius of the 

 nineteenth century as regards electricity and magnet- 

 ism. Among his numerous achievements three must 

 bo specially mentioned. 



While Oersted had shown the deflection of the mag- 

 netic needle by an electric current, Faraday suc- 

 ceeded in demonstrating the converse, that a magnet 

 reacts upon an electric current. This was the dis- 

 covery of magneto-electricity (1831), and it led him 

 on to another of no less importance, that of induced 

 currents (1831), that a wire through which an 

 electric current is passing may induce in another 

 adjacent wire a state similar to its own. With Fara- 

 day's discoveries there must also be associated tho 

 entirely independent but synchronous work of the 



Prof. C. 0. Knott Article, Electricity, Chamber*' L'ncy 

 clopudia. 



