MAGNETISM. 



cession wni not instantly receive all the 

 magnetism which the magnet is capable 

 of exciting in them ; their state of maxi- 

 mum polarity, therefore, will not be at- 

 tained until the magnet has passed for 

 some small distance beyond them. Nei- 

 ther will they lose their polarity at the 

 same instant that the magnet is still 

 further removed. Thus, from both 

 causes, there will always be, in the rear 

 of the magnet, a space both more ex- 

 tensive and more strongly impregnated 

 with the opposite polarity than in ad- 

 vance of it. Hence, there will arise an 

 oblique action between the pole of the 

 magnet and the opposite pole of the 

 plate, thus lagging behind it, which will 

 urge it to move in the direction of the 

 magnet's motion. The development of 

 the more distant polarity, similar in 

 kind to that of the magnet, being more 

 diffused, that pole will be both weaker 

 and less oblique in its action, and will 

 be much inferior in power to that of the 

 nearer attracting pole. It is evident 

 that the converse will also be true when 

 the magnet is at rest, but free to move 

 horizontally; and when the different 

 parts of the plate are passed in succes- 

 sion under it, the latter will tend to drag 

 the former after it with a velocity con- 

 tinually accelerating, till they move on 

 together with the same velocities. It is 

 also manifest, that the greater the rela- 

 tive velocity, the more will the pole, de- 

 veloped in the plate, lag behind the mag- 

 net, or the magnet (in the reverse case) 

 lag behind the pole ; the more oblique, 

 therefore, will be the action, and the 

 greater the velocity produced. The 

 application of these principles to circu- 

 lar motion, or to the rotation of plates, 

 is sufficiently obvious ; but in this case, 

 if the velocity be excessive, compared 

 with the retentive power of the revolving 

 substance, the latter may have completed 

 a revolution before there has been time 

 for its being affected to a degree suffi- 

 cient to occasion motion in the magnet; 

 the induced polarity will then be weak- 

 ened, and its effects rendered insensible. 

 This diminution of the total effect by a 

 more general diffusion of polarity was 

 imitated by sticking a great number of 

 magnetized needles, vertically, through a 

 light cork circle, having their north poles 

 downwards, so as to form a coronet of 

 magnets. This apparatus, suspended 

 centrally over a revolving copper disc, 

 was not sensibly affected ; for the poles 

 of all the magnets, acting in rapid suc- 

 cession upon all the points below them, 



produced nearly~"a uniform circle of 

 southern polarity, whose equal and con- 

 trary actions on the needles would de- 

 stroy one another. 



(363.) Plausible as this theory may 

 appear, it is yet embarrassed with many 

 serious difficulties. It does not give any 

 satisfactory explanation of the mode in 

 which an attractive force, resulting from 

 induction, between one pole of a magnet 

 and the consequent polarity of the ad- 

 jacent parts of a piece of copper a force 

 which is so feeble as not to produce any 

 sensible effects when both these bodies 

 are at rest is immediately so greatly 

 increased on their separation and conti- 

 nued removal from one another, so as 

 to occasion a very considerable motion. 

 The force producing that motion is, 

 according to the theory, an attractive 

 force; but Arago has shown that the 

 general resultant of all the forces, which 

 operate between the pole of the magnet 

 and the plate, is a repulsive force, with 

 relation to the line' perpendicular to the 

 surface of the plate. The following ex- 

 periment proves this : Suspend a long 

 magnet by a thread, in a vertical posi- 

 tion, to the beam of a balance, and coun- 

 terpoise it by weights on the opposite 

 side ; if the plate be then revolved 

 under the magnet, the equilibrium will 

 cease, and the magnet will rise, or ap- 

 pear to become lighter, indicating its 

 repulsion from the plate*. 



(364.) The latest experiments on this 

 subject are those of Mr. W. S. Harris, 

 of Plymouth, of which he communicated 

 an account to the Royal Society, in June 

 1830f. Finding that the vibrations which 

 attend bodies in rapid rotation are pro- 

 pagated to a remarkable extent along 

 the solid parts of the apparatus by which 

 a magnetic needle is suspended, and that 

 they are also conveyed to great distances 

 by the surrounding air, even when 

 highly rarefied, he took great pains to 

 obviate these two sources of fallacy. 

 He accordingly conducted all his expe- 

 riments in an exhausted receiver, the 

 parts to be acted upon being effectually 

 secured from the influence both of vi- 

 brations from solids and of aerial vor- 

 tices. He conceives that, in general, 

 sufficient care had not hitherto been 

 taken to eliminate these several causes 

 of error, and that we cannot repose that 

 implicit confidence in the conclusions 

 deduced from them which is required in 



* Annales de Chimie, xxxii. 213. 

 " f It has been since published ifl the Thilosophical 

 Transactions for 1831, P, 67. 



