On the temporary magnetic effect induced in iron bodies by rotation. In a letter to J. F. W. Herschel, Esq. Sec. R. S. by Peter Barlow, F. R. S

Abstract

Listen

Translate article

The author’s attention having been recalled to the consideration of the effects of rotation in altering the magnetic influence of iron, in the course of speculations on the cause of the rotation of the earth’s magnetic poles ; and knowing, at the same time, that Mr. Christie had found a permanent change in the magnetic state of an iron plate, by mere change of position on its axis, it seemed to him highly probable that this change, due only to a simple inversion, would be increased by rapid rotation. On trial, however, it was found that the effect produced was merely temporary. The experiments at first were made with a 13-inch mortar shell, fixed to the mandrel of a powerful turning lathe, worked by a steam-engine in the Royal Arsenal at Woolwich. This being made to revolve at the rate of 640 turns per minute, the needle was deflected out several degrees, and there remained stationary during the motion of the ball, but returned immediately to its original position on ceasing the rotation. On inverting the motion of the shell, an equal and contrary deflection took place. As the law of the phenomena was not evident with this disposition of the apparatus, and the shell was found too heavy for perfect safety, a Shrapnel shell of eight inches diameter was mounted in a proper apparatus (described in the paper), and a number of experiments made ; the law of which, however, still seemed anomalous, till the idea occurred of neutralizing the earth’s action on the needle, when the anomalies disappeared, and the general law of the effect was placed in evidence. The needle being made a tangent to the ball, if the motion of the ball was made towards the needle (whatever was the direction of the axis of rotation), the north end of the latter was attracted, and if the contrary way, repelled. In the two extremities of the axis there was found no effect, while in two opposite points, at right angles to the axis, the effect was a maximum, and the direction of the needle was to the centre of the ball.