Page:A Treatise on Electricity and Magnetism - Volume 2.djvu/457

 845-] MODIFIED THEORY OF INDUCED MAGNETISM. 425

844.] These conditions differ from those in Weber s theory of magnetic induction by the terms involving the coefficient B. If BX is small compared with unity, the results will approximate to those of Weber s theory of magnetism. If BX is large compared with unity, the results will approximate to those of Weber s theory of diamagnetism.

Now the greater y, the primitive value of the molecular current,, the smaller will B become, and if L is also large, this will also diminish B. Now if the current flows in a ring channel, the value

T&amp;gt;

of L depends on log, where R is the radius of the mean line of

the channel, and r that of its section. The smaller therefore the section of the channel compared with its area, the greater will be Z, the coefficient of self-induction, and the more nearly will the phe nomena agree with Weber s original theory. There will be this difference, however, that as X, the magnetizing force, increases, the temporary magnetic moment will not only reach a maximum, but will afterwards diminish as X increases.

If it should ever be experimentally proved that the temporary magnetization of any substance first increases, and then diminishes as the magnetizing force is continually increased, the evidence of the existence of these molecular currents would, I think, be raised almost to the rank of a demonstration.

845.] If the molecular currents in diamagnetic substances are confined to definite channels, and if the molecules are capable of being deflected like those of magnetic substances, then, as the mag netizing force increases, the diamagnetic polarity will always increase, but, when the force is great, not quite so fast as the magnetizing force. The small absolute value of the diamagnetic coefficient shews, however, that the deflecting force on each molecule must be small compared with that exerted on a magnetic molecule, so that any result due to this deflexion is not likely to be perceptible.

If, on the other hand, the molecular currents in diamagnetic bodies are free to flow through the whole substance of the molecules, the diamagnetic polarity will be strictly proportional to the mag netizing force, and its amount will lead to a determination of the whole space occupied by the perfectly conducting masses, and, if we know the number of the molecules, to the determination of the size of each,

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