OF WHICH VARIOUS SUBSTANCES ARE SUSCEPTIBLE. 
75 
and .156434, which may be considered as very nearly in the inverse ratio of 
the squares of the respective distances. 
In the following Table are given the deviations of the bar, corresponding to 
other distances and velocities. 
Table I. 
(A) (B) (C) 
Velocity of rotation, 178.5 
Velocity of rotation, 357* 
Velocity of rotation, 714. 
Distance 
in turns of 
Mic r - Screw. 
Angle of 
Deviation. 
Sine of 
Deviation. 
Distance 
in turns of 
Mic r -Screw. 
Angle of 
Deviation. 
Sine of 
Deviation. 
Distance 
in turns of 
Mic r -Screw. 
Angle of 
Deviation. 
Sine of 
Deviation. 
4 
o 
18 
.309017 
4 
o 
38 
.615661 
o 
5 
12 
.207912 
5 
24 
.406737 
5 
56 
.829038 
6 
16 
.275637 
8 
4.5 
.078459 
8 
9 
.156434 
10 
3 
.052336 
10 
6 
.104528 
10 
12 
.207912 
It appears by the above Table, that the influence of the ring is directly as 
the rapidity of the motion, and inversely as the squares of the distances : we 
observe a little discrepancy in some of the numbers, but the general agree- 
ment is very close and remarkable. A complete agreement cannot be expected ; 
for supposing even the most perfect manipulation, there will always arise in 
experiments of this kind many causes which disturb a numerical identity*. 
13. Being desirous to extend these inquiries concerning the transient mag- 
netic state, of which various substances appear to be susceptible, I subsequently 
laid aside the rotating discs, as a means of detecting these minute forces for 
the no less refined, and perhaps still less exceptionable method of a vibrating 
bar; since what is termed the magnetism of rotation, seems in fact to be 
* In estimating the distance between the surfaces of the attracting bodies, the revolving disc was 
purposely made as thin as possible, so as to admit of its being considered without any sensible thick- 
ness. With respect to the magnet, the foregoing experiments with screens clearly show, that an inter- 
vening ferruginous mass completely intercepts the attractive force upon non-ferruginous substances. 
If any portion of the bar beneath the surface contiguous to the revolving body be supposed to operate 
upon the ring, such portion must necessarily act through the intervening iron ; which it cannot do 
(Exp./) (11). This view seems to derive much confirmation from experiment, since on vibrating an 
extremely thin needle of sheet steel over a metallic disc, the comparative results do not vary from 
those obtained by means of a bar of half an inch thick. 
L 2 
