424 ELECTRIC THEORY OF MAGNETISM. [843.
angles to the line of magnetic force, that is, so that the plane of the channel becomes parallel to the lines of force.
An effect of a similar kind may be observed by placing a penny or a copper ring between the poles of an electromagnet. At the instant that the magnet is excited the ring turns its plane towards the axial direction, but this force vanishes as soon as the currents are deadened by the resistance of the copper *.
843.] We have hitherto considered only the case in which the molecular currents are entirely excited by the external magnetic force. Let us next examine the bearing of Weber s theory of the magneto-electric induction of molecular currents on Ampere s theory of ordinary magnetism. According to Ampere and Weber, the molecular currents in magnetic substances are not excited by the external magnetic force, but are already there, and the molecule itself is acted on and deflected by the electromagnetic action of the magnetic force on the conducting circuit in which the current flows. When Ampere devised this hypothesis, the induction of electric cur rents was not known, and he made no hypothesis to account for the existence, or to determine the strength, of the molecular currents.
We are now, how r ever, bound to apply to these currents the same laws that Weber applied to his currents in diamagnetic molecules. We have only to suppose that the primitive value of the current y, when no magnetic force acts, is not zero but y . The strength of the current when a magnetic force, X, acts on a molecular current of area A, whose axis is inclined Q to the line of magnetic force, is
.A. A. ( -I A\
��and the moment of the couple tending to turn the molecule so as
to increase is X 2 A 2
sin2<9. (15)
��2L Hence, putting A
Ay Q = m, /- = *, (16)
L y
in the investigation in Art. 443, the equation of equilibrium becomes Xsin0-.X 2 sm0cos<9 = J9sin(a-0). (17)
The resolved part of the magnetic moment of the current in the direction of X is
XA 2
yAcosO = y ^cos0 -- ^-cos 2 0, (18)
= mcos8(l-3Xco80). (19)
- See Faraday, Exp. Res., 2310, &c.
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