1884.] On an Explanation of HalVs Phenomenon. 341 



II. " On an Explanation of Hall's Phenomenon." By Shelford 

 Bid well, M.A., LL.B. Communicated by Professor G. 

 Carey Foster.. F.R.S. Received February 14, 1884. 



Mr. E. H. Hall's first paper relating to what is now generally known 

 as the " Hall effect," appeared in the " Phil. Mag." for March, 1880. 

 It was entitled " On a New Action of the Magnet on Electric 

 Currents," and in it was described the following experiment : — A 

 strip of gold-leaf was cemented to a plate of glass and placed between 

 the poles of an electromagnet, the plane of the glass being perpendi- 

 cular to the magnetic lines of force. A current derived from a 

 Bunsen cell was passed longitudinally through the gold, and before 

 the electromagnet was excited two equipotential points were found, 

 by trial, near opposite edges of the gold-leaf and about midway 

 between the ends ; when these points were connected with a galvano- 

 meter there was, of course, no deflection. A current from a powerful 

 battery being passed through the coils of the magnet, it was found 

 that a galvanometer deflection occurred indicating a difference of 

 potential between the two points, the direction of the current across 

 the gold-leaf being opposite to that in which the gold-leaf itself would 

 have moved across the lines of force had it been free to do so. On 

 reversing the polarity of the magnet, the direction of the transverse 

 electromotive force was reversed ; and when the magnet was 

 demagnetised the two points reverted to their original equipotential 

 condition. The phenomenon is attributed by the author to the direct 

 action of the magnet upon the current in the gold leaf. 



In a second paper, published in the " Phil. Mag." for November, 

 1880, experiments are described in which other metals were used in 

 addition to gold. With silver, nickel, tin, and platinum the nature of 

 the effect was the same as with gold though differing in degree ; but 

 in the case of iron the direction of the transverse current was found 

 to be reversed, a result which Mr. Hall was naturally inclined to 

 connect with the magnetic properties of iron, and which he says had 

 been predicted by Professor Rowland. But he was greatly puzzled 

 to account for the fact that the two strongly magnetic metals, iron 

 and nickel, gave opposite effects. 



A third paper contained in the " Phil. Mag." for September, 1881, 

 confirmed the previous results with regard to nickel, and added that 

 the direction of the transverse electromotive force in the strongly 

 magnetic metal cobalt was the same as in iron. 



Mr. Hall's latest paper on the subject appeared in the " Phil. Mag." 

 for May, 1883, and is entitled, in compliance with a suggestion of Dr. 

 Hopkinson's, "Rotational Coefficients of various Metals." It is here 

 stated that aluminium, copper, and brass behave in the same manner 



