copper and other substances during rotation. 
50s 
5.04- 
4.60 
4.62 
4.29 
4.20 
4-45 
4.10 
4.65 
4.07 
3-59 
Mean 4.361 
■d 
lU 
rJ Cl- 
(u a, 
I o 
^ 8 ^ 
l§ 
4 ‘ 37 ' 
3-93 
3.8« 
3-51 
3.60 
3*64 
3‘3i 
3.80 
3-23 
2.78 
Mean 3.605 
•n 
(U 
U 
0 
CO 
a; 
B 
c/> M-i 
U 
(U 
Cm 
Cl< 
o 
>- 
3 
u 
4= 
<u 
u 
U 
V) 
e 
o 
If we suppose that the poles of the needle are urged by 
forces in the direction of the motion of the copper, which 
being constant in the copper, would affect the needle recipro- 
cally as the square of the distance ; then these forces in the 
copper being derived from the needle itself, we must suppose 
that their intensity will vary also reciprocally as the square of 
the distance : so that the force on the needle arising from 
this mutual action, would vary reciprocally as the fourth 
power of the distance. Taking the mean between the mean 
values of n above, when the distance is measured from the 
centre of the copper and from its surface, would give the value 
of 71 for an intermediate point 3.983, which is as near to 4, 
supposing that such ought to be the value, as we could expect 
the observations to give. 
The next experiments which I made were with the view of 
determining the law of force as regards the distance, when 
magnets act upon a copper disk. For this purpose I made 
use of the suspending wire as a balance of torsion. The re- 
sults which I have obtained in this manner give a much less 
rapid diminution of the force, as the distance increases, than 
appears to take place when a thick copper plate acts upon a 
small magnet, as in the former experiments, which agrees 
with what you have mentioned as following from your 
