OF ARTS AND SCIENCES. 109 



• 



magnetized inside of it, so that all parts should be touched at the same 

 time. Even then the position, as will appear upon reflection, will not 

 be so favorable as where the battery current flows around the bar of 

 soft iron ; inasmuch as two concentric systems of small circles cannot 

 be brought into such close proximity as one system of small circles 

 concentric with a single large circle. We may select an experiment 

 in which these comparatively weak currents from the battery no 

 longer enjoy their favorable position, and then their weakness is plain- 

 ly manifested. Let a hollow cylinder of iron be taken and placed out- 

 side of the helix, instead of inside. In this case, the battery current 

 which flows in the helix, though intrinsically possessing the same mag- 

 netic power as before, produces little or no effect on the external iron 

 cylinder. 



" Another cause of the superiority of electro-magnets is connected 

 with a peculiarity in the position of the poles of permanent steel mag- 

 nets. These poles, if the bar has any considerable thickness, are at a 

 little distance inside of the extremities of the magnetic axis. This dis- 

 placement has been well explained by the interference of contiguous 

 currents in different portions of the thickness of the bar ; their mutual 

 action prevents the planes of motion around the individual particles 

 of iron near the extremities from being strictly parallel to one another, 

 or perpendicular to the magnetic axis. In the electro-magnet, the cur- 

 rents of the iron are maintained in a strictly parallel direction by the 

 controlling and ever-present activity of the battery current, the direc; 

 tion of which is preserved uniform by the rigidity of the wire. Every 

 one knows how rapidly the power of a magnet diminishes with the dis- 

 tance from its pole, and may understand, therefore, how much offeree 

 is lost if the pole is inside of the extremity of the bar, and inaccessible. 

 The pole of the electro-magnet is at the extremity of the bar ; we may 

 bring the keeper into actual contact with it ; and for this reason, also, 

 it must appear superior to ordinary magnets. In consequence of this 

 difference in the position of the pole, or the seat of maximum force, 

 the power of the electro-magnet diminishes as the square of the dis- 

 tance from the extremity, while that of the common magnet only dimin- 

 ishes at the same rate as the distance from the extremity increases ; the 

 law being, in both cases, that the force diminishes as the square of the 

 distance from the pole increases." 



