﻿of Permanent Magnet. 19 



Magazine for November 1861 by the Astronomer Royal, inves- 

 tigated the question of " the most advantageous form of mag- 

 nets," or that form in which "are united the greatest possible 

 magnetic moment with the smallest possible mass and the small- 

 est possible moment of inertia." In all the forms be experi- 

 mented upon, the breadth was always less than one-third of the 

 length, and generally about one-fifth ; and he does not seem to 

 contemplate the possibility of magnetizing a square plate, and still 

 less an oblong plate, in a direction transverse to its greatest 

 length. Now I find that, by cutting slits nearly up to the 

 middle of a steel plate, a square plate in one piece can with such 

 slits be regularly magnetized ; and by this means even an oblong 

 square plate can be regularly magnetized, aud with a3 many 

 poles as may be required, in a direction transverse to its greatest 

 length. I herewith beg to forward a square plate magnetized in 

 this way. It is of watch-spring steel, 0*0075 inch thick and 

 I inch x J inch ; it has four pairs of slits £ inch wide cut 

 from its edges, and leaving a central web £ inch wide, uni- 

 ting the whole. On moving a small needle round this square 

 plate, it is seen to be regularly magnetized j and on sprinkling 

 iron filings on the magnet covered by a sheet of paper, they ar- 

 range themselves in lines, proving that the magnet really con- 

 sists of a number of small regular similar magnets arranged 

 below each other in the same vertical plane. On suspending an 

 oblong magnet of this kind with its longer axis in the vertical 

 plane, the needles set themselves to the magnetic meridian ; on 

 suspending it flatwise, with its longer axis in the horizontal 

 plane, the longer axis points east and west. As well as can be 

 judged by subjecting them to slight shocks, the magnets are as 

 permanently magnetized as if they were separate from each other. 

 Only time can prove whether they will lose their magnetism. 

 The important question as to what form is the best for retaining 

 magnetism for a length of time is one which, as Dr. Lamont 

 remarks, no one has yet investigated. 



Though I have not yet been enabled to try to magnetize a 

 parallelopipedon of steel after slotting it vertically and trans- 

 versely into a number of bars held together by a central web, I 

 feel very confident that this could be done. The slots could be 

 cut into the parallelopipedon or cube while in a soft state by a 

 thin tool worked to and fro in an ordinary engineers' slotting or 

 shaping machine, and the whole magnetized in a powerful elec- 

 tric spiral, in the way described by Elias. Besides moving the 

 spiral to and fro, as described by him, no doubt in order to over- 

 come the resistance to induction, the cube while in the spiral 

 could also be struck, in order to produce that mechanical vibra- 

 tion which is so favourable to magnetization and demagnetization. 



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