PHYSICS OF NINETEENTH CENTURY 183 



to the quantity of electricity which passed, and that 

 the mass of substance separated was proportional 

 to its chemically equivalent weight. 



The great industrial applications of electricity, made 

 possible by the invention of the telegraph on one hand 

 and the dynamo on the other, are founded on the 

 scientific discoveries of two physicists. In 1820 Hans 

 Christian Oersted of Copenhagen discovered that, if 

 a wire be placed above or below a magnetic compass 

 needle and parallel with it, the needle is deflected from 

 its north-and-south position when an electric current 

 is passed along the wire. By the construction of 

 galvanometers, this magnetic force was at once 

 applied to the measurement of currents, and underlies 

 all our modern quantitative science of electro-mag- 

 netism. In 1831 Michael Faraday demonstrated at 

 the Royal Institution that, when the current in one 

 circuit of wire is started or stopped, a momentary 

 secondary current is induced in another coil of wire 

 placed near the first one. A similar current is pro- 

 duced by the motion of magnets, or by any other 

 change in the magnetic force in the coil. These 

 currents, barely perceptible to Faraday's primitive 

 instruments, were set up by means the same in prin- 

 ciple as those which now drive our electric trains and 

 supply thousands of horse-power to many industrial 

 manufactures. 



Faraday's experimental researches were guided by 



his instinctive grasp of the importance of the dielectric 



Electric or insulating medium in electric pheno- 



Waves. mena. When a current deflects a magnetic 



needle across space, or induces another current in an 



