622 



GENERAL THEORIES. 



smallest particles of bodies, each of which turns about its own axis. 

 This is the hypothesis (601) of molecular vortices. 



Maxwell explained in this way the phenomena of rotatory polari- 

 zation by the conception of molecular vortices; but, without entering 

 upon an explanation of this theory, we may arrive at the same result, 

 as Professor Rowland showed, by bringing in a new action dis- 

 covered by Mr. Hall. 



640. HALL'S PHENOMENON. Let ABCD (Fig. 127) be a cross 

 cut in a very thin metal sheet a gold leaf, for instance ; the two 

 ends A and B of the principal branch are connected with the poles 

 of a battery, the ends C and D of the cross piece are connected with 

 a galvanometer. The apparatus may easily be arranged so that 

 none of the current traverses the galvanometer. 



Fig. 127. 



When this conductor is placed in a very strong magnetic field, so 

 that the lines of force are perpendicular to its plane, a permanent 

 deflection of the galvanometer shows that a constant current traverses 

 the galvanometer. If the current goes from A to B in the principal 

 branch, and the lines of force traverse the plane of the figure from 

 front to back, the branch current goes through the galvanometer 

 from D to C, when the conductor consists of gold, silver, platinum, 

 or tinfoil, and in the opposite direction when the metal is iron. The 

 action ceases to be perceived when the thickness of the conductor is 

 increased. 



In the first case, the current is drawn in the direction of the 

 electromagnetic force which would be exerted on a wire parallel to 

 AB, and traversed by a current from A to B ; this may also e said 

 to be the same in the second case, since in the interior of an iron 

 plate, owing to the magnetisation, the direction of the lines of force, 

 and of the electromagnetic force, have changed their sign. 



