ACTION OF MAGNETS ON COPPER AND GOOD CONDUCTORS. 
37 
to the lines of magnetic force, and not when they are parallel to it ; for the horizontal 
bar, or the vertical bar, or the cube or sphere, rotate with perfect facility when they 
are suspended above the vertical pole (2246.), the rotation and vibration being then 
equally free, and the same as the corresponding movements of bismuth or heavy 
glass. The obstruction is at a maximum when the axis of rotation is perpendi- 
cular to the lines of magnetic force, and when the bar or cube, &c. is near to the 
magnet. 
2329. Without going much into the particular circumstances, I may say that the 
effect is fully explained by the electric currents induced in the copper mass. By 
reference to the Second Series of these Researches (160.)*, it will be seen that when a 
globe, subject to the action of lines of magnetic force, is revolving on an axis per- 
pendicular to these lines, an electric current runs round it in a plane parallel to the 
axis of rotation and to the magnetic lines, producing consequently a magnetic axis 
in the globe, at right angles to the magnetic curves of the inducing magnet. The 
magnetic poles of this axis therefore are in that direction which, in conjunction with 
the chief magnetic pole, tends to draw the globe back against the direction in which 
it is revolving. Thus, if a piece of copper be revolving before a north magnetic pole, 
so that the parts nearest the pole move towards the right-hand, then the right-hand 
side of that copper will have a south magnetic state, and the left-hand side a north 
magnetic state ; and these states will tend to counteract the motion of the copper 
towards the right-hand : or if it revolve in the contrary direction, then the right-hand 
side will have a south magnetic state, and the left-hand side a north magnetic state. 
Whichever way, therefore, the copper tends to revolve on its own axis, the instant it 
moves, a power is evolved in such a direction as tends to stop its motion and bring it 
to rest. Being at rest in reference to this direction of motion, then there is no 
residual or other effect which tends to disturb it, and it remains still. 
2330. If the whole mass be moving parallel to itself, and be small in comparison 
with the face of the magnetic pole opposite to which it is placed, then, though it pass 
through the magnetic lines of force, and consequently have a tendency to the forma- 
tion of magneto-electric currents within it, yet as all parts move with equal velocity 
and in the same direction through similar magnetic lines of force, the tendency to the 
formation of a current is the same in every part, and there is no actual production of 
current, and consequently nothing occurs which can in any way interfere with its 
freedom of motion. Hence the reason that though the rotation of the bar or cube 
(2324. 2328.) upon its own axis is stopped, its vibration as a pendulum is not affected. 
2331. That neither the one nor the other motion is affected when the bar or cube 
is over the vertical pole (2328.), is simply because in both cases (with the given 
dimensions of the pole and the moving metal) the lines of particles through which 
the induced currents tend to move are parallel throughout the whole mass ; and 
* Philosophical Transactions, 1832, p. 168. 
