Effects due to Magnetization, 485 



tively small ones at "make/' especially with certain salt- 

 solutions, such as copper sulphate. 



The outer envelope which held the iron salts together, and 

 limited the immediate influence of the magnetized point, was 

 distinctly defined within the liquid, and easily observed by 

 the reflexion of the light from its convex surface. 



The persistency with which the iron salts were held about 

 the point was shown by moving the cell with respect to the 

 electrodes, when the contour remained approximately intact, 

 passing bodily through the liquid without being broken up. 



E. Electromagnetic Rotations. 



The small dust particles present in the liquid were drawn 

 radially toward the point until they reached the surface 

 described, when they pierced it and began to revolve rapidly 

 about the point inside this surface, in the opposite direction to 

 the currents of Ampere. Reversing the poles of the magnet 

 produced surfaces of the same appearance but opposite rotations. 



When the current from a Daniell ceil was sent through it 

 seemed to have very little effect upon the rotations, showing 

 them to be controlled by the powerfully magnetized point. 



The electromagnet was arranged with its field vertical, and 

 the point electrode along the lines of force as before. This 

 arrangement gave better control of the surfaces formed, 

 since gravity now acted symmetrically about the point. 



When a single iron rod about 3 millim. in diameter, and 

 placed vertically in the cell, was substituted for the two elec- 

 trodes, two rotations were observed which were uniformly 

 dextro about the north-seeking pole of the rod, and lsevo 

 about the south-seeking pole. About the central neutral 

 portion no rotations were observed. When the rod was covered 

 with a thin coating of vaseline the rotations entirely disappeared 

 as expected. Wartmann* observed similar rotations about 

 soft-iron cylinders adhering to the poles of a magnet, and he 

 ascribed them to electric currents in the liquid which proceed 

 from the periphery of the cell radially to the surface of the rod. 



The explanation of these rotations follows at once from 

 what we know of the time-effects produced by the magnet. 

 A higher potential is always produced at points of greater 

 magnetization, causing electric currents in the liquid from 

 the more strongly magnetized to the weaker parts of the iron. 



Applying this fact to the exposed conical point electrode, 



we see that local electric currents exist from its vertex to the 



other parts of the surface, returning by way of the metal. In 



the case of the vertical rod, these currents pass from the poles 



* Philosophical Magazine, xxx. p. 268 (1847). 





