1 73 Prof. Tyndall on the Nature of the Force by ivhich 



took up the equatorial position. The iron bar, if placed directly 

 between the poles, would, on the excitation of the magnetism, 

 infallibly spring to one of them ; hence it was removed to a 

 distance of 3 feet 7 inches from the centre of the space between 

 the poles, and in a direction at right angles to the line which 

 united them. The magnet being excited, the bar was drawn 

 a little aside from its position of equilibrium and then libe- 

 rated, a series of oscillations of very small amplitude followed, 

 and the number of oscillations accomplished in a minute was 

 carefully ascertained. Tables III. and IV. contain the results 

 of experiments made in the manner described with bars of iron 

 and bismuth of the same dimensions. 



Table IV. 



Bar of bismuth. No. 1. 

 length 0-8 of an inch, 

 width 0-13 of an inch, 

 depth 0' 15 of an inch. 

 Strength of current. Repulsion. 



Table III. 



Bar of soft iron, No. 1 . 

 length 0-8 of an inch, 

 width 0-13 of an inch, 

 depth 0-15 of an inch. 



Strength of current. Attraction. 



168 1682 



214 3042 



248 3532 



274 3752 



323 3132 



362 3472 



385 3742 



411 3852 



These experiments prove, that, up to a strength of about 380, 

 the attractive force operating upon the iron, and the repulsive 

 force acting upon the bismuth, are each accurately proportional 

 to the square of the strength of the magnetizing current. "For 

 higher powers, both attraction and repulsion increase in a smaller 

 ratio ; but it is here sufficient to show that the diamagnetic re- 

 pulsion follows pi'ecisely the same law as the magnetic attraction. 

 So accurately indeed is this parallelism observed, that while the 

 forces at the top of the tables produce attractions and repulsions 

 exactly equal to the square of the strength of the current, the 

 same strength of 411, at the bottom of both tables, produces in 

 iron an attraction of 3852, ^^^^ jj^ bismuth a repulsion of 3862. 

 The numbers which indicate the strength of current in the first 

 column are the tangents of the deflections observed in each case : 

 neglecting the indices, the figures in the second column express 

 the number of oscillations accomplished in a minute, multiplied 

 by a constant factor to facilitate comparison : the forces opera- 



