432 Prof. H. A. Eowland on 



of investigation ; and if the above analogies be general, these 

 bodies must be characterized by a rapid rate of diffusion as 

 well as by good conductivity. There are several phosphates 

 and carbonates which likewise deserve investigation; and these 

 points I expect to make the subject of future examination. 



Tubingen, 1879. 



LY. Preliminary Notes on Mr. Hall's Recent Discovery. 

 By H. A. Eowland*. 



THE recent discovery by Mr. Hallf of a new action of 

 magnetism on electric currents opens a wide field for 

 the mathematician, seeing that we must now regard most of 

 the equations which we have hitherto used in electromagnetism 

 as only approximate, and as applying only to some ideal sub- 

 stance which may or may not exist in nature, but which cer- 

 tainly does not include the ordinary metals. But as the effect 

 is very small, probably it will always be treated as a correction 

 to the ordinary equations. 



The facts of the case seem to be as follows, as nearly as they 

 have yet been determined : — Whenever a substance transmit- 

 ting an electric current is placed in a magnetic field, besides 

 the ordinary electromotive force in the medium, we now have 

 another acting at right angles to the current and to the mag- 

 netic lines of force. Whether there may not be also an elec- 

 tromotive force in the direction of the current has not yet 

 been determined with accuracy; but it has been proved, within 

 the limits of accuracy of the experiment, that no electromotive 

 force exists in the direction of the lines of magnetic force. 

 This electromotive force in a given medium is proportional to 

 the strength of the current and to the magnetic intensity, and 

 is reversed when either the primary current or the magnetism 

 is reversed. It has also been lately found that the direction 

 is different in iron from what it is in gold or silver. 



To analyze the phenomenon in gold, let us suppose that the 

 line A B represents the original current at the point A, and 

 that B C is the new effect. The magnetic pole is supposed to 

 be either above or below the paper, as the case may be. The 

 line A C will represent the final resultant electromotive force 

 at the point A . The circle with arrow represents the direction 

 in which the current is rotated by the magnetism. 



* From the American Journal of Mathematics. Communicated by the 

 Physical Society. 



t Phil. Mag. [5] vol. ix. p. 225. 



