424 Mr. E. H. Hall on the Rotation of the 
EY. ¢Nowl -) 
Length of main strip when weighed . 3°70 cm. 
Width of main strip when weighed. . 1:06cm. 
Area of whole strip when weighed . . 4°28cm.sq. 
Weight he tes EM Sos EE CO lena 
Detistty\) Aen egies ae alti, 
Average thickness from the data . . °00934cm. 
By calipers, thickness at one end ‘0098 ; at other end -0100 ; 
between arms ‘0100; average ‘(00993 cm. Take for true thick- 
ness between arms, ‘00941 cm. Distance between strips B, B, 
about 3°2 cm. 
G. GNovd:) 
Length of main strip when weighed . 4:17 cm. 
Width of main strip when weighed. . 1°09 cm. 
Area of whole strip when weighed . . 95:03cm.sq. 
Weight ore 5,0 2e a a eer ore ane 
Density to GOL00" scree pelt aos Gd, eee 
Average thickness from these data. . ‘00383 cm. 
From a somewhat careful use of the calipers it appeared 
that the thickness between the arms was about 4% per cent 
greater than the average thickness. 
Take, then, for thickness between arms, ‘00348 cm. Distance 
between strips B, B, about 3°2 cm. 
The main object in the experiments upon these metals and 
their alloys was to determine whether the alloys would range 
themselves according to any simple law, so that the magnitude 
of the rotational power in any alloy might be inferred from its 
known proportions of copper and zinc. Some attempt was 
made, moreover, to determine the effect of change of tempe- 
rature in different specimens. 
It will be seen that the intensity of the magnetic field was 
kept nearly the same throughout the experiments of August 
upon copper, zinc, and their alloys. The strength of the 
direct current was, moreover, of about the same magnitude in 
all cases, except the experiments of August 11th on copper. 
On that day the current used was less than one half as strong 
as that used later. The same strip of copper was tested again 
August 29th with a current of the usual strength ; and the 
agreement between the results obtained under so widely 
different conditions is quite close. 
In the following table and throughout this article the 
C. G.S. system is used wherever no statement is made to the 
contrary. The following symbols used with the tables need 
explanation :— 
C means current along line C D (fig. 1). 
