1826.] Magnetism of Metals, S^c, arising from their Rotation. 187 



deceived us) a decisive result has been obtained ; and, what is 

 very singular, this body is carbon, in that peculiar state in which 

 its density, lustre, degree of hardness, and high conducting 

 quality, both as regards heat and electricity, seem to give it 

 some title to a place among the metals. This is the state in 

 ' which it is precipitated by a red-heat from coal-gas. The mag- 

 netism developed in this singular substance is, however, too 

 feeble to admit of precise measurement, and is only rendered 

 barely sensible by delicate management. 



The second process alluded to as employed by us to compare 

 the relative magnetic forces of the different bodies examined, 

 consists in suspending magnetized bars over revolving discs of 

 them, and observing not the point of equilibrium but the velocity 

 generated, or the time required for the description of certain 

 spaces ; in other words, by measuring not the statical, but the 

 dynamical effect. These methods, for distinction's sake, may 

 be called the statical and dynamical methods of observation. 



In the original experiment of M. Arago, a magnetic needle 

 was made to deviate or revolve by ths rotation of a plate 

 beneath it. The motion of the needle must of course be rendered 

 irregular by the effects of its polarity, and subject to periodical 

 accelerations or retardations ; and it is obvious, that in the case 

 of a very weak magnetic force in the plate it can never execute 

 an entire revolution, but must oscillate backwards and forwards 

 till reduced to rest by the friction and resistance of the air. It 

 occurred to us, however, that much more regular and uniform 

 results might be obtained by this means, could the polarity of 

 the needle be destroyed without at the same time destroying its 

 magnetism ; in other words, could the earth's action on it be so 

 precisely neutralized as to allow of its resting indifferently in all 

 directions. The obvious mode of doing this, by the approach 

 of a powerful magnet acting in opposition to the earth, proved 

 much too coarse for our purpose, which, however, after a few 

 trials, we found might be accomplished to any required degree 

 of precision by the following simple contrivance. 



If two exactly equal and similar magnets of equal strength be 

 placed parallel to each other, but in a reverse position, and at 

 such a distance as not mutually to affect each other's magne- 

 tism, and if in this situation they be firmly attached to a piece 

 of wood, glass, &c. the system so formed will have no polarity, 

 i. e. no tendency to rest in one rather than another situation, 

 however suspended. This is clear; because whatever be the 

 inclination (9) of one of the magnets to the line of dip, that of 

 the other will necessarily be (180 + fl),and the directive forces, 

 being represented by the sines of these two angles, will always 

 be equal and opposite, so that each magnet urges the system 

 with equal force, but in opposite directions. The truth of this 



