Chase. 1 



152 



[October. 



tained bj the aid of a Gaussian modification of Faraday's apparatus 

 for showing the electric currents developed in moving metallic bodies 

 by terrestrial magnetism. It consists of a heavy copper ring (a), 

 fixed to horizontal supports (b) (c), on which it can be rapidly rotated 

 by a wheel (c/) and pinion (e), the supports being relieved by fric- 

 tion-wheels (/). The outer diameter of the ring is 6 inches at each 



edge, and Qj\ inches in the middle ; the inner diameter, 5 inches ; 

 the thickness, 1 J inches. The axle opposite to the gearing (g) is 

 hollow, to allow the insertion of a brass rod (Ji), which supports a 

 compass (0 horizontally in the interior of the ring. The compass is 

 not shielded by a glass, and it is therefore easily affected by aerial 

 currents. If the ring is made to revolve around the compass needle, 

 the N. pole of the needle is deflected in a direction opposite to the 

 motion of the top of the ring.* 



For my special purpose, I replaced this copper ring by a wooden 

 one of the same dimensions, and prepared a number of discs {k) (I), 

 in the form of semicircles and circular segments, fitted so that they 

 could be fixed, in different vertical positions, in the compass box, 

 above the needle. Causing the wheel to rotate, with the axis vari- 

 ously placed, I tried the following experiments : 



* For the use of the apparatus, I am indebted to the kindness of President 

 Richard S. Smith, of Girard College. 



