114 



MAGNETISM. 



The possibility of conferring magnetism on substances which are not ferru- 

 ginous, was shown in 1733 by Brandt, who imparted magnetism to the metal 

 cobalt. Cronstedt, in 1750, showed that nickel is also susceptible of this prop- 

 erty. 



After philosophers had become familiar with the attractions and repulsions, 

 the polarity and directive power of magnets, their attention was directed to 

 the establishment of a numerical measure of the actual amount of attractive or 

 repulsive force which they exerted under given circumstances. For a long 

 period, no estimate of this was formed more accurate than the weight which, 

 by attraction, the magnet was capable of supporting attached to a piece of soft 

 iron adhering to it. In 1780, Coulomb applied to magnetism those beautiful 

 and accurate instruments of investigation which were so successfully employed 

 in electricity and other departments of experimental physics, and determined 

 by their means the intensities and laws of magnetic forces. Two methods of 

 measuring the force exerted were practised by him, similar to those by which 

 electric attractions and repulsions had been measured. These were, the bal- 

 ance of torsion, by which the amount of the force was estimated by the action 

 of a twisted wire, or fibre of silk ; and the observation of the number of oscil- 

 lations which the attracted or repelled body made in a given time, on each side 

 of the line of attraction or repulsion. By these means it was demonstrated 

 that the force of a magnet was, c&teris paribus, in the direct ratio of the abso- 

 lute intensity of the magnetism, and inversely as the square of the distance of 

 the attracted or repelled body from it : a law identical in all respects with that 

 by which electrical attractions and repulsions are governed. He also esti- 

 mated, as he had done with electrified conductors, the distribution of magnet- 

 ism on the surface of magnetized bars ; and found that in bars of equal trans- 

 verse section, of which the length was considerable compared with the mag- 

 nitude of the section, the poles or points of maximum intensity were always 

 at a distance of about an inch and a half from the extremities ; and that, in very 

 short bars, the poles are at one third of their length from the extremities, and 

 that this latter position is the limit to which the poles approach as the bars are 

 diminished in length. 



In making artificial magnets, either by means of natural magnets or by other 

 artificial magnets already made, the process first adopted was to rub the bar to 

 be magnetized, from end to end, with one of the poles of the magnet by which 

 it was to be magnetized. This method succeeded sufficiently well in magnet- 

 izing short needles ; but, when applied to bars of any considerable length, it 

 was attended with the liability of producing consequent points that is, in fact, 

 making the bar into a succession of magnets instead of a single magnet. Thus 

 a certain portion of the entire length, measured from the extremity, would pos- 

 sess two poles and an intermediate neutral point ; then another succeeding 

 portion of the length would possess other two poles with another intermediate 

 neutral point, and so on. 



In 1745, Dr. Go wan Knight, of London, practised an improved method. 

 He placed two strong bar magnets end to end in the same line, the north pole 

 of the one being in contact with the south pole of the other. Over them he 

 laid the bar to be magnetized, its centre coinciding with the united ends of the 

 two magnets, and its length laid along them. In this position the two magnets 

 were drawn asunder, their poles passing under each half of the length of the 

 bar to be magnetized. By this method the bar acquired much stronger mag- 

 netism than by that which had previously been practised, 



Du Hamel further improved this process. The bar to be magnetized being 

 placed between the pieces of soft iron, he took two bar magnets, and placing 

 the north end of one and the south end of the other upon the centre of the bar, 



