Page:A Treatise on Electricity and Magnetism - Volume 2.djvu/453

 838.] CIRCUITS OF NO RESISTANCE. 421

and since by the hypothesis there is no resistance, R = 0, and we get by integration

Ly + My =. constant, = Zy, say. (3)

Let us suppose that the area of the projection of the molecular circuit on a plane perpendicular to the axis of the molecule is A, this axis being defined as the normal to the plane on which the projection is greatest. If the action of other currents produces a magnetic force, X, in a direction whose inclination to the axis of the molecule is 6, the quantity My becomes XA cos0, and we have as the equation of the current

Ly + XAco$0 Ly, (4)

where y is the value _of y when X = 0.

It appears, therefore, that the strength of the molecular current depends entirely on its primitive value y, and on the intensity of the magnetic force due to other currents.

837.] If we suppose that there is no primitive current, but that the current is entirely due to induction, then

y = -- -COS0. (o)
 * XA

��The negative sign shews that the direction of the induced cur rent is opposite to that of the inducing current, and its magnetic action is such that in the interior of the circuit it acts in the op posite direction to the magnetic force. In other words, the mole cular current acts like a small magnet whose poles are turned towards the poles of the same name of the inducing magnet.

Now this is an action the reverse of that of the molecules of iron under magnetic action. The molecular currents in iron, therefore, are not excited by induction. But in diamagnetic substances an action of this kind is observed, and in fact this is the explanation of diamagnetic polarity which was first given by Weber.

Weber s Theory of Diamagnetism.

838.] According to Weber s theory, there exist in the molecules of diamagnetic substances certain channels round which an electric current can circulate without resistance. It is manifest that if we suppose these channels to traverse the molecule in every direction, this amounts to making the molecule a perfect conductor.

Beginning with the assumption of a linear circuit within the mo lecule, we have the strength of the current given by equation (5).

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