426 Prof. A. de la Rive's Memoir of 



leading to such a result a popularity, if I may venture so to ex- 

 press myself, which is reflected upon its author. 



Faraday soon showed that terrestrial magnetism, like that of 

 a magnet, can develope electric currents by induction in a me- 

 tallic wire rolled into a coil or a circle, and actuated by a move- 

 ment of oscillation in a plane perpendicular to that of the mag- 

 netic meridian. He found that it was not even necessary to 

 employ metallic wires to ascertain the influence of the terrestrial 

 magnetism upon the production of induced currents,, but that it 

 sufficed to set a metallic disk (of copper for example) in rotation 

 in a plane perpendicular to the direction of the inclination-needle 

 to find that it is traversed by electric currents passing from 

 the centre to the circumference, or from the circumference to 

 the centre, according to the direction of the rotation. Still 

 more readily does the vicinity of a magnet to a similar disk set 

 in rotation in any plane under the influence of this magnet 

 develope in it induced currents, the presence of which, directly 

 ascertained, explains in a perfectly satisfactory manner the phe- 

 nomena of magnetism by rotation discovered by Arago. 



These currents, although difficult to perceive, must neverthe- 

 less possess considerable power, since they can drag a rather 

 heavy magnet by the action which they exert upon it. It is 

 probable that this power is due less to their individual intensity 

 than to their number, which appears to be very considerable. 

 We may cite two examples which prove in a striking manner 

 the energy which this mode of production of induced currents 

 may acquire. The first is furnished by a curious experiment of 

 Faraday's, in which, on causing a cubical mass of copper sus- 

 pended by a thread between the poles of an unmagnetized elec- 

 tromagnet to turn upon itself, he saw this mass stop suddenly 

 the moment he magnetized the electromagnet, in consequence 

 of the magnetic action exerted by the currents which induction 

 had set up in the copper. We find the second example in the 

 fact observed by Foucault, of the sudden stoppage which is like- 

 wise experienced by a thick disk of copper set in rotation between 

 the poles of an electromagnet the moment the latter is magnet- 

 ized. This stoppage is such that it can only be surmounted by 

 a considerable effort, and the disk itself becomes very strongly 

 heated if the rotation be continued in spite of the resistance it 

 meets with. In order that such a heating effect should be pro- 

 duced in a mass of such considerable size, and that we should ex- 

 perience an attractive action so strong on the part of the elec- 

 tromagnet, the induced currents thus produced must be of very 

 great power — a power which they owe essentially to the exces- 

 sive rapidity of the movement generating them. 



I shall not follow Faraday through all his works upon indue- 



