154 Analyses of Books. [Feb. 



Faraday has, however, obtained the same results from terrestrial 

 magnetism as from magnets. He made a helix communicate with a 

 multiplier and a cylinder of soft iron which possessed no magnetic 

 power. This cylinder was placed in the helix, which had been 

 directed previously according to the magnetic inclination. The iron 

 becoming magnetic, induction was exhibited as if a magnet had been 

 actually employed. And from his experiments it appears impossible 

 that a metallic sphere can rotate without producing electrical currents 

 in its interior, in a planfc perpendicular to the plane of revolution, 

 provided that the axis of rotation does not coincide with the direction 

 of the magnetic inclination. He suggests that in the action of steam 

 engines their metallic mechanism may produce accidental electro- 

 magnetic combinations, which may occasion effects hitherto unob- 

 served. He thinks likewise that admitting the earth to produce 

 currents in her own mass during its rotation, by the electro-magnetic 

 induction, these currents at the surface will be directed into the parts 

 which approach the plane of the equator in a contrary way from 

 those which would take place towards the poles. Hence, if we could 

 examine the subject minutely, we might find negative electricity at 

 the equator, and positive electricity at the two poles. 



He has advanced, but with diffidence, an opinion that the aurora 

 borealis may be derived from a discharge of electricity driven towards 

 the poles of the earth, from whence it might be forced, by natural 

 and particular means, to return to the equatorial regions above the 

 surface of the earth. He has observed that the current excited in a 

 copper wire is more powerful than that which is produced by the 

 same magnet in an iron wire ; and the metals whose properties he 

 has examined in reference to this may be arranged in the following 

 order : copper, zinc, iron, tin, and lead, which corresponds nearly with 

 their electric conducting power, and with what Babbage, Herschel, 

 and Harris have found in their experiments on magnetic rotation. 

 Faraday arranges the metals in three classes, in reference to their 

 connexion with the magnet ; 1, those which are affected when at 

 rest, as iron, nickel, cobalt; 2, conductors of electricity influenced 

 during rotation; 3, those which are perfectly indifferent to the 

 magnet, whether they are at rest or in motion. 



It remains now to enquire into the nature of the currents of in- 

 duction. A striking difference exists between the currents produced 

 by magnetic influence in the helices and the hydro-electric currents, 

 and a remarkable difference between these currents and those derived 

 from an origin connected with heat. This difference has been gene- 

 rally ascribed to the greater tension of the hydro- electric piles than in 

 the thermo-electric piles. 



According to the result of an ingenious experiment of M. Peltier, 

 currents of induction are formed by the union of several equal cur- 

 rents, for he obtained by the action of live helices, superimposed on 

 each other, a deviation four times greater than was obtained by a 

 multiplier with one turn ; it was double with that of ten turns, and 

 produced no effect upon one formed with a wire of 2000. 



In uniting all the helices, so as to produce a single circuit, no effect 

 was produced on the multiplier with a single turn, nor on one with 



