DISTINCTION OF MAGNETISM AND MAGNETO-ELECTRIC INDUCTION. 189 
240. If, instead of intersecting the magnetic curves of a straight wire carry- 
ing a current, by approximating or removing a second wire (235.), a revolving 
plate be used, being placed for that purpose near the wire, and, as it were, 
amongst the magnetic curves, then it ought to have continuous electric cur- 
rents induced within it ; and if a line joining the wire with the centre of the 
plate were perpendicular to both, then the induced current ought to be, accord- 
ing to the law (114.), directly across the plate, from one side to the other, and 
at right angles to the direction of the inducing current. 
241. A single metallic wire one twentieth of an inch in diameter had an 
electric current passed through it, and a small copper disc one inch and a half 
in diameter revolved near to and under, but not in actual contact with it 
(fig. 39.). Collectors were then applied at the opposite edges of the disc, and 
wires from them connected with the galvanometer. As the disc revolved in 
one direction, the needle was deflected on one side ; and when the direction of 
revolution was reversed, the needle was inclined on the other side, in accordance 
with the results anticipated. 
242. Thus the reasons which induced me to suppose a particular state in 
the wire (60.) have disappeared ; and though it still seems to me unlikely that 
a wire at rest in the neighbourhood of another carrying a powerful electric cur- 
rent is entirely indifferent to it, yet I am not aware of any distinct facts which 
authorize the conclusion that it is in a particular state. 
243. In considering the nature of the cause assigned in these papers to 
account for the mutual influence of magnets and moving metals (120.), and 
comparing it with that heretofore admitted, namely, the induction of a feeble 
magnetism like that produced in iron, it occurred to me that a most decisive 
experimental test of the two views could be applied (215.). 
244. No other known power has like direction with that exerted between 
an electric current and a magnetic pole ; it is tangential, while all other forces, 
acting at a distance, are direct. Hence, if a magnetic pole on one side of a 
revolving plate follow its course by reason of its obedience to the tangential 
force exerted upon it by the very current of electricity which it has itself caused, 
a similar pole on the opposite side of the plate should immediately set it free 
from this force ; for the currents which tend to be formed by the action of the 
two poles are in opposite directions ; or rather no current tends to be formed, 
