XVI. On the magnetism of iron arising from its rotation

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Abstract
As the principles on which phenomena depend can only be discovered by a careful investigation of the circumstances attending every new fact which presents itself, its import­ance must not, in the first instance, be estimated by the magnitude of the effects produced, but by their peculiarity. However minute may be the effects, an inquiry into the laws which govern them, if unattended by any other, will have this advantage, that these laws will serve as an additional test of the correctness of the principles advanced for the ex­planation of the more striking phænomena, firmly establish­ing their truth, if the consequences of those principles, or being incompatible with them, pointing out their fallacy. Thus the severest test that the principle of gravitation has been subjected to, is the explanation of the minute irregula­rities in the planetary motions; and the coincidence of the observed irregularities with those deduced from the applica­tion of this principle would have established its truth beyond dispute, had any doubt previously remained. In the expe­riments which I am about to detail, the effects produced are of this minute character; but as they point out a species of action not hitherto observed, they will not, I trust, be consi­dered unimportant. It has been stated that different effects will be produced on iron, as regards its polarity, when struck, twisted, filed, or scoured in different positions, with respect to the magnetic axis or line of the dip; but I am not aware that it has ever been suspected that the simple rotation of iron, in different direc­tions, would have any effect on the manner in which the iron influenced a magnetic needle. This I have discovered to be the case; and that the laws which govern this peculiar action on the needle are so general and uniform, that I have no doubt their causes are as steady in their operation, as those to which the more striking phenomena of magnetism owe their origin. On observing these magnetical phenomena arising purely from rotation, it appeared to me that they might possibly indicate the cause of the earth's magnetism; and this was a further inducement to me thoroughly to investigate the circumstances connected with them. Before giving the particulars of these phenomena, it is necessary that I should mention how I was first led to observe them.

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