MAGNETISM: 



verse of this experiment was attended 

 with still more striking effects. When 

 a plate of copper, for example, is made 

 to revolve with a certain velocity under 

 a magnetic needle, supported on its 

 centre, and contained in a vessel closed 

 on all sides, the needle is found to de- 

 viate from its natural position in the 

 magnetic meridian ; and the deviation 

 is greater in proportion as the rotation 

 of the plate is more rapid. If the ra- 

 pidity of revolution be sufficiently great, 

 the needle will be brought to revolve 

 also, and always in the same direction 

 in which the plate is made to re- 

 volve *. The experiment was varied in 

 the following manner : A circular plate 

 of copper, balanced on a point at its 

 centre, was placed immediately under a 

 strong magnet, to which a rapid rota- 

 tory motion was given; the copper- 

 plate soon began to turn in the same 

 direction, and acquired by degrees a 

 very rapid ^velocity of revolution. It 

 was found, also, that the oscillations of 

 a copper-plate in a vertical plane, when 

 suspended by an axis which passed at a 

 small distance from its centre of gra- 

 vity, were much impeded, and soon de- 

 stroyed, when the plate was inserted be- 

 tween the two poles of a very powerful 

 horse- shoe magnet. M. Arago was of 

 opinion that these phenomena are inex- 

 plicable upon principles of ordinary 

 magnetism, and considered them as the 

 effects of some hitherto unknown power 

 in nature. But subsequent inquiry 

 seems to render it j probable, that they 

 are all reducible to the operation of 

 known laws of magnetism. 



(355.) Mr. Christie having observed 

 a permanent change in the magnetism 

 of an iron plate in consequence of a 

 mere change of position on its axis, it 

 occurred to Mr. Barlow that this 

 change would be increased by rapid 

 rotation. But on trial this was found 

 not to be the case, the effect pro- 

 duced being merely temporary. The 

 first experiments were made with a 

 mortar-shell fixed to the mandril of a 

 powerful turning lathe, worked by a 

 steam-engine. When the ball was made 

 to revolve at the rate of 640 times in a 

 minute, the needle was deflected several 

 degrees from its natural position, and 

 there remained stationary during the 

 motion of the ball ; whenever the rota- 

 tion ceased, the needle immediately re- 

 turned to its original situation. On in- 



* Bulletin Universe!, vol.; iii,, p, 328 ; Annaleg 

 de Chimie, xxviii., 325. 



verting the motion of the shell, an equal 

 and contrary deflection took place. But 

 although numerous trials were made 

 under various circumstances, the law of 

 the phenomena could not be deduced 

 until the influence of the earth's action 

 had been neutralized by means of other 

 magnets perfectly adjusted. All the ano- 

 malies before met with now disappeared, 

 and the following law was rendered ma- 

 nifest. When the needle and ball are 

 both in the same horizontal plane, 

 whatever may be the direction of the 

 axis of rotation of the ball, if its mo- 

 tion at its upper part be made towards 

 the needle, the north pole of the latter 

 will be attracted ; and if the contrary 

 way, repelled. Hence he concluded, 

 that when an iron body is put in rapid 

 rotation on any line not coinciding with 

 its magnetic axis, a temporary derange- 

 ment takes place in its magnetic powers, 

 equivalent in effect to the influence of a 

 new axis of polarization, perpendicular 

 to the planes passing through its axes 

 of rotation and of ordinary polarization. 

 (356.) The next series of experiments 

 on this subject are those of Mr. Chris- 

 tie*, who ascertained that a plate which, 

 in a given position, produced a certain 

 deviation of the compass, no longer pro- 

 duced the same deviation, after it had 

 been carried round one entire revolution 

 in its own plane, although brought to 

 rest, and every part of the apparatus re- 

 stored to its former place. This change 

 in the directive power of the plate, pro- 

 duced by rotation, was greatest when its 

 plane was parallel to the line of dip, and 

 at the same time as little inclined to the 

 horizon as this condition would allow ; 

 or, in other words, when the axis of ro- 

 tation was in the plane of the magnetic 

 equator, and also in a vertical plane, 

 that is, in the magnetic meridian. Hence 

 he deduced a law which may be thus 

 expressed : If we conceive a dip- 

 ping-needle to lie in the centre of an 

 imaginary sphere, having an equator 

 situated in a plane intersecting perpendi- 

 cularly the direction of the dipping- 

 needle, and a circular plate of iron to be 

 placed with its centre in the surface of 

 that sphere, its plane being a tangent to 

 that surface, when the plate revolves, 

 the effect of its revolution upon the dip- 

 ping-needle will be such that each side 

 of the equator of the latter (that is, the 

 portion of the equator which is situated 

 in a line at right angles to the line join- 

 ing the centres of the needle and plates) 



Philosophical Transactions for 1825, p. 347. j 



