342 



Mr. S. JBidwell. On an 



[Feb. 21, 



as gold ; zinc gives the same result as iron, and with lead no 

 appreciable effect is produced. 



The experimental results may be stated in a different form from 

 that originally adopted by Mr. Hall. The equipotential points on 

 opposite edges of the plate may clearly be joined by equipotential 

 lines, and Hall's results may be expressed by saying that in the case 

 of gold, the equipotential lines are rotated in a direction opposite to 

 that of the current circulating in the coils of the electromagnet, and 

 in the case of iron the lines are rotated in the direction of the 

 magnetising current. In fig. 1 MNOP represents the metal plate 



Tig. 1. 







Ifli 

















through which a battery current is passing from left to right, and AB 

 is the equipotential line (which, if the arrangement is symmetrical, 

 as shown in the figure, will be a straight line) joining two opposite 

 equipotential points. If now the south pole of a magnet is placed 

 beneath the plate and the north pole above it (the axis of the poles 

 being perpendicular to the plate), the equipotential line will be 

 rotated, in the case of gold, counter-clockwise to the position CD, and 

 in the case of iron clockwise to the position EF. Mr. Hall distin- 

 guishes the " rotational coefficient " of those metals which behave like 

 iron by the positive sign ( + ), and those which behave like gold by 

 the negative sign (— ).* 



Of the thirteen different metals which Hall has tested, three — iron, 

 cobalt, and zinc — are classed as positive ; nine — gold, silver, tin, 

 copper, brass, platinum, nickel, aluminium, and magnesium — are 

 negative; and one — lead — shows absolutely no signs of rotation. 



* In the second of Hall's papers the direction of the " transverse electromotive 

 force" is arbitrarily called + for gold, &c, and — for iron. He subsequently 

 adopted opposite signs for their " rotational coefficients." 



