342 Mr. E. H. Hall on the "Rotational Coefficients " 



Repeating, still in a hasty and rough manner but more care- 

 fully than before, the experiments with all these metals except 

 magnesium, and using indeed the same pieces of metal as 

 before, I found: — 



Name of metal. 



Zinc . . 

 Aluminium 

 Copper 

 Brass . 

 Lead . . 



Rotational 

 coefficient. 



+ 10-5 

 -37 



- 6-5 



- 1-4 



No effect discovered. 



It will be observed that the value obtained for brass, which 

 is small, is but little changed ; but those for zinc, aluminium, 

 and copper have each been reduced about 25 or 30 per cent. 

 We may perhaps by analogy, without actual determination, 

 write 



Magnesium . . . —35 



All these values may still be subject to errors of 10 or 20 

 per cent., but will nevertheless serve present purposes tolerably 

 well if substituted for those given in the list previously pub- 

 lished. Such a list, though rough, may be compared with 

 other lists in which the same metals are arranged relatively 

 to various physical properties; and any analogies thus sug- 

 gested may be tested further by more accurate and detailed 

 investigations. In fact, to go no further than the above table 

 itself, the fact that the small rotational coefficient in brass lies 

 between the positive coefficient in zinc and the negative in 

 copper suggests the advisability of a careful study of the 

 transverse effect in alloys. 



In the Philosophical Magazine for September 1881 1 stated 

 that the transverse current obtained with a nickel strip is much 

 increased, other conditions remaining unchanged, by rise of 

 temperature. It was a question of much interest whether the 

 transverse current in the non-magnetic metals would prove to 

 be affected in a similar manner. It might be found indeed 

 that the transverse current would increase at the same rate as 

 the electrical resistance, in which case it would appear that the 

 transverse effect depends upon the rate of fall of electric po- 

 tential along the strip of metal rather than upon the strength 

 of the direct current. 



Accordingly, from a sheet of No. 2 gold foil*, the thinnest 

 foil used by dentists, a piece was cut in the form of a Greek 

 cross. The extremity of each arm of this cross was soldered 

 to a disk of brass. The four brass disks were screwed to a 



* " Standard," R. S. Williams and Co. 



