238 MICHAEL FARADAY — HIS LIFE AXD WORKS. 



momentarv, but in an opposite direction, is developed at the moment nlien Iho 

 magnet is withdrawn. Here, tlierefore, was realized that production i)i elec- 

 tricity by magnetism which Faraday had long been seeking, convinced, as he 

 was, that as electricity produces magnetism, magnetism in its turn must produce 

 electricity. 



Is it necessary to follow Fai'aday in the nmltiplied experiments by which ho 

 demonstrates that the electricity developed by induction possesses all the proper- 

 ties of voltaic electricity, and of the ordinary electricity produced by machines — 

 that it heats fine metallic wires, gives shocks, and even produces the spark ? 

 To produce an electric spark by means of the action of a simple magnet, is one 

 of tliose striking facts which give to the discovery leading to such a result a 

 popularity, if I may venture so to express myself, which is reflected upon its 

 author. 



Faraday soon showed that terrestrial magnetism, like that of a magnet, can 

 develop electric currents by induction in a metallic wire rolled into a coil or a 

 circle, and actuated by a movement of oscillation in a plane perpendicular to 

 that of the magnetic meridian. He found that it was not even necessary to 

 employ metallic wires to ascertain the influ(mce of the terrestrial magnetism 

 upon the production of induced pnrrents, but that it sufficed to set a metallic 

 disk (of copper for example) in rotation in a plane [terpendicular to the direc- 

 tion of the inclination-needle to find that it is traversed by electric currents 

 passing from the centre tt) the circumference, or from the circumference to the 

 centre, according to the direction of the rotation. Still more readil}' does the 

 vicinity of a magnet to a similar disk set in rotation in any plane under the 

 influence of this magnet develop in it induced currents, the presence of which, 

 directly ascertained, explains in a perfectly satisfactory manner the phenomena 

 of magnetism by rotation discovered by Arago. 



These currents, although difficult to perceive, must nevertheless possess con- 

 siderable 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 wliich this 

 mode of production of induced currents may acquire. The first is furnished by 

 a curious experiment of Faraday's, in whicli, on causing a cubical mass of cop- 

 per suspended by a thread between the poles of an unmagnetized electromagnet 

 to turn upon itself, he saw this mass stop suddenly the moment he magnetized 

 the electro-magnet, in consequence of the magnetic action exerted by the cur- 

 rents 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 likewise 

 experienced by a thick disk of copper set in rotation between the poles of an 

 electro-magnet the moment the latter is magnetized. 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 produced in a mass 

 of such considerable size, and that we should experience an attractive action so 

 strong on the part of the electro-magnet, 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 induction which accom- 

 panied his fundamental discovery. I shall only refer to the fact that in 1834 

 he discovered a new important fact, namely : the production of an induced cur- 

 rent in the very wire that conducted the inductive current, and which takes 

 place at first at the moment when the latter current begins to circulate, and 

 then at that when it ceases passing. If this wire is rolled in a coil round a 

 cylinder of soft iron, the effect produced acquires great intensity by the fact of 

 the alternate magnetization and demagnetization of the iron which accompanies 



