272 Mr. W. Sturgeon on the Distribution and Retention 



every part of the plate on n side of that line being north polar, 

 and all that part on 5 side of the Ymeeq being south polar; 

 s the situation of the principal pole. 



Experiment 2. — Let the north pole of the magnet be car- 

 ried once round, and just within the edge of the disc, at the 

 same distance above it as before, and in the direction of the 

 arrows (fig. 2.), commencing and terminating the revolution at 

 the point a, and suddenly quitting it as before. 



The plate will again exhibit polarity; but the north pole of 

 the needle, when reconciled, will be deflected towards the 

 "west, as n' s' (fig. 2.), showing that the attracting pole is situated 

 in the limb a d. 



Experiment 3. — Let the same pole of the magnet be now 

 carried round the plate in the opposite direction, from the 

 point a to the same point again, quitting a as in the last 

 experiment. The north pole of the needle will now repose 

 towards the ea&t of the magnetic meridian, indicating the at- 

 tracting pole to be in the limb ah. 



It appears by the phjenomena exhibited in the last two 

 experiments, that there is a peculiar distribution of magnetic 

 polarity by passing the pole of the magnet round the edge of 

 the disc, differing considerably from that occasioned by passing 

 the pole over a diameter. There is, however, as decided a re- 

 gularity in the distribution of polarity by this process as by 

 the former; for, although the pole last excited be not deter- 

 mined in the point a where the magnet quits the plate, its 

 position is invariably found in that limb which was last passed 

 over by the magnet. The other species of polarity is diffused 

 over a great part of the area of the plate, without any appa- 

 rent determinate pole. 



The north pole of the magnet when first brought over the 

 edge of the disc at the point a, excites south polarity in the 

 iron immediately beneath it; in which point the disc would 

 remain polar, if the magnet were to be withdrawn without 

 passing over any other part of it. When, however, the pole 

 of the magnet is permitted to pass round the plate, the parts 

 over which it passes will become polar in succession, and the 

 point a will receive the terminal exciting impression. 



Now, as every point near to the edge of the disc may be 

 supposed to be equally disposed to retain polarity, it is evident 

 that as the exciting pole proceeds in its revolution, the succes- 

 sive poles which it generates will have an equal tendency to 

 a permanent habitation: but in the same gradual succession as 

 the exciting pole abandons them, and calls forth others in its 

 progress, the primogenial poles will become enfeebled, — pro- 

 gressively vanish, and eventually give place to a display of 

 polarity of the other kind ; which polarity, however, for want 



of 



