Magneto-electric Phenomenon. 217 



plane at right angles to those lines passing through its centre), 

 then, on making the current, it will receive an impulse causing 

 it to move parallel to itself along the lines of force towards 

 the weaker part of the field, and at the breaking it will receive 

 an impulse more evident in the opposite direction. If this 

 radiating field is produced between a pointed and a flat pole, 

 the disk will, on making the current, appear to be repelled 

 from, and, on breaking, to be attracted by, the pointed pole. 

 So powerful is this effect that a piece of impure copper, which 

 is strongly magnetic, is repelled from the pointed pole on 

 making, and attracted on breaking, the current, thus appearing 

 at first sight strongly diamagnetic. 



Though these impulses must have been observed by most 

 experimentalists, their amount has not been, so far as I am 

 aware, determined in absolute units, nor have they been turned 

 to account for making any measurements. As they seem to 

 afford one of the most convenient methods of determining 

 conductivity and field-intensity, perhaps a short paper on the 

 subject, even though it be incomplete, may be of interest to 

 this Society. 



The explanation of the motions described will be obvious, 

 but it may be well to give it at length for the sake of arriving 

 at quantitative results. In the first place, let the lines of force 

 be parallel, so that the field is of uniform strength. Let a 

 ring of (small) section s, of specific resistance p, and of radius r, 

 be placed in the field, with its plane making an angle a w r ith 

 the lines of force. Let the strength of the field be H units. 

 Then during a small increment of field-intensity c/H, in the 

 time dt, a current will be induced in the ring of the strength 



rs sin a. dH 

 2p~ lit" 

 This current in the field H will produce a twisting tendency 

 to increase a with a diminishing, or to diminish a with an 

 increasing field, represented by the couple 

 7T?- 3 sH sin 2a dK 

 4p ~~ It* 



From this it is clear that the couple varies inversely as the 

 time dt occupied in making any small change of field-intensity 

 c?H, but it lasts for the time dt ; therefore the momentum 

 acquired by the suspended disk, if free to move, will be inde- 

 pendent of the rate at which any small change in the magnetic 

 field may be made, but will depend only on its amount, pro- 

 vided that the time is not sufficient for the angle « to have 

 perceptibly altered during the change. Since this is true of 

 any element, it is true of all; so the momentum acquired by 



