16 



Prof. Tyndall on Magnetic Repulsion. [Jan. 26, 



N 



of the force acting upon every particle of the mass of bismuth 

 tends to turn the lever round its axis of suspension, in the direction 

 of the curved arrow. On exciting the magnetism of P, however, 

 a precisely contrary motion is observed — the lever approaches the 

 pole. This result, which, as far as the lecturer could see, was 

 perfectly inexplicable on the assumption that the dia-magnetic 

 force was purely repulsive, is explained in a simple and beautiful 

 manner on the hypothesis of dia-magnetic polarity. According 

 to this, the end b of the bar of bismuth is repelled by P, and 

 the end a is attracted : but the force acting upon a is applied 

 at a greater distance from the axis of suspension than that 

 acting upon b ; and as it has been arranged that the absolute in- 

 tensities of the forces acting upon the two ends differ very 

 slightly from each other, the mechanical 

 advantage possessed by a gives to it the 

 greatest moment of rotation, and the bar is 

 attracted instead of repelled. Let a mag- 

 netic needle n s (Fig. 3,) be attached like 

 the bar a b (Fig. 2) to a lever, and submit- 

 ted to the earth's magnetism. Let the 

 north pole of the earth be towards N ; the 

 action of the pole upon n is attractive, 

 upon s repulsive, the absolute intensities of 

 these forces are the same, inasmuch as the 

 length of the needle is a vanishing quantity 

 in comparison with its distance from the 

 pole N : hence the mechanical advantage 

 possessed by the force acting upon s, on ac- 

 count of its greater distance from the axis 

 of rotation, causes the lever to recede from 

 N, and we obtain a result perfectly analogous to that obtained with 

 the bar of bismuth (Fig. 2).* 



[J. T.] 



Fig. 3. 



* A paper submitted to the Koyal Society last November, and a portion of 

 which formed the subject of the Bakerian Lecture for the present year, con- 

 tains a more comprehensive discussion of this subject. In it are explanations, 

 which it is hoped will be deemed satisfactory, of the difficulties adduced by 

 M. Matteucci, in his instructive Cours Special^ recently published. 



