and magnetic rotations. 525 
the corresponding point on the screen ; then, part of the in- 
fluence of the electrified body N passing through the inter- 
stices of the screen, produces on the metal plate the same 
kind of electricity as it did on the screen. Let the curve of 
electric intensity in the metal plate be A. Whilst the metal 
plate C remains at rest, both its electricity and that of the 
screen tend to draw the needle N directly towards it ; but if 
the plate C move in the direction of the arrow with an uni- 
form velocity, and if induced electricity be not instantly anni- 
hilated, then there will arise a new electric equilibrium, and 
the curve representing its intensity and position will have ad- 
vanced towards that part to which the motion is directed, 
and may be represented by the dotted line a. The influence 
of this electricity, which is of the same kind with that on the 
screen, and which consequently it repels, will be, that the 
electric curve on G must be driven back in the opposite direc- 
tion : let it be represented by b, b. Now the attraction of b , b 
on the needle N tends to give it a motion in the direction con- 
trary to that which is impressed on it by the attraction of a , 
which is on the opposite side of the perpendicular, dropped 
from N to the metal plate. That proportion of the resolved 
action of b, b, which communicates horizontal movement, is 
larger than it is at the greater distance of the plate C, and the 
whole force acts at a shorter distance : it may therefore 
exert a more powerful influence in turning the needle in that 
direction, than a stronger force at a greater distance, C acting 
more unfavourably. Thus the needle may, by the advance 
of the plate C, be caused to retrograde. It does not, how- 
ever, follow, that this must always happen, for the suscepti- 
bility for induced magnetism in the plate may be much 
mdcccxxvi. 3 Y 
