of Magnetic Polarity m Metallic Bodies. 275 



be effected. In several cases, I have found that one revo- 

 lution of the magnet will excite a polar energy in the disc, 

 which, at six inches distance from the pivot of the needle, 

 would be a counterbalance to the magnetism of the earth at 

 this place. 



Experiment 6. — In this experiment the disc is made to re- 

 volve in its own plane on a vertical axis, by means of a multi- 

 plying wheel and band. Over the plate is a light stage, on 

 which is placed a powerful bar-magnet, with its centre over 

 the centre of motion, and its poles reaching nearly to opposite 

 edges of the plate. 



In this experiment the distribution of polarity is very sin- 

 gular and curious, and the retentive faculty of the iron is beau- 

 tifully displayed. 



Let s 71, (fig. 7.) represent the magnet placed on the stage 

 over the revolving plate, proceeding in the direction of the 

 arrows. In this case it will be found, by holding a dipping- 

 needle over the plate whilst in motion, that north polarity is 

 distributed over every part of the plate on N. side of the mag- 

 net, and south polarity over the other half on S. side of the 

 magnet. When the velocity is considerable, the polarity ex- 

 cited by one pole of the magnet has not tirne to change its 

 character, before the same point of the plate arrives imme- 

 diately at the opposite pole ; so that the intensity of polarity is 

 pretty equal in every part of the circle, excepting the two 

 points under the poles of the exciting magnet, at which points 

 the transitions of polarity are continually going on, with a 

 celerity proportioned to the speed of the plate. 



When the plate revolves in the contrary direction, the two 

 halves of it on different sides of the magnet change their 

 polar character, as represented by fig. 8. 



These versatilities have a very pleasing effect when a dip- 

 ping-needle is placed on each side of the magnet. The alter- 

 nations of dip in the two needles are simultaneous, and as 

 prompt as the mutations of motion. 



Experiment 7. — Place the north pole of a magnet directly 

 under the edge of the iron plate, so as not to touch it : in this 

 case every part of the plate appears to be possessed of north 

 polarity, and the south pole of the dipping-needle inclines 

 towards every part of its surface. This at first sight seems 

 rather curious, because a south pole ought to be exhibited 

 near to the edge where the exciting north pole is placed ; and 

 indeed this is tlie fact, for if a slight motion be given to the 

 plate, south polarity is easily detected. Whilst the plate is 

 at rest, the south pole of it being stationary, and directly over 

 the north pole of the magnet, and also of inferior force, it is 

 2 N 2 over- 



