1826.] Magnetism of Metals, S^c, arising from their Rotation, 261 



in this, or in the general case, either the retentive power of the 

 shell, disc, or lamina, great, or the velocity of motion excessive, 

 it may be instructive to consider the modifications thus intro- 

 duced into the effect. It is evident that the induced pole Vi^Ul 

 lag farther and farther behind the magnet in proportion as 

 either of these conditions obtains. In the case of rectilinear 

 motion, this will, up to a certain point, increase the oblique 

 action, and the dragging effect will be strengthened ; but if the 

 velocity be excessive, or the retentive force considerable, as in 

 steel, the pole may lag so far behind as to carry it altogether, 

 out of the sphere of the magnet's attraction ; and the magnetized 

 portion, remaining within its limits, may have not had time 

 enough to acquire a high degree of polarity. From both causes 

 the drag (the expression, though uncouth, is convenient) should, 

 be weakened. In the case of circular motion this effect may go 

 so far, that a complete circumference shall have been described 

 before the polarity of any one point shall have been either com- 

 pletely induced, or completely destroyed. In this case the 

 effect observed will be a general weakening of the total polarity 

 of the disc or sphere ; and (supposing the latter of iron, or soft 

 steel) a directive virtue on a small compass-needle placed near 

 it, not probably towards any particular place, but to a resultant 

 imaginary point depending on the situation of the compass, the 

 dip, and the axis of rotation, by laws not very easy to assign. 

 This will explain some expressions quoted by Mr. Barlow from 

 his correspondence with one of the authors of this paper, which 

 may appear otherwise to militate against the general view here 

 taken. 



This diminution of the total effect by a more general distribu- 

 tion of the magnetism, was imitated by sticking a great number 

 of needles vertically through a light cork circle, all being strongly 

 magnetized, and having their north poles downwards, so as to 

 form a circle, or, as it were, a coronet of magnets. This appa- 

 ratus suspended centrally over a revolving copper disc, was not 

 sensibly set in rotation. In this case, when at rest, the south 

 polarity induced in the plate would be disposed in spots accu- 

 mulated under each needle ; but these spots, elongated and 

 blended by the effect of rotation, must produce a nearly uniform 

 circle of south polarity, whose equal and contrary actions on all 

 the needles would keep up the equilibrium, and prevent the 

 coronet from acquiring a tendency either way. 



One consequence of this reasoning, which deserves trial, is 

 this — that if the axis of rotation of an iron shell be situated in 

 the direction of the dip, the spots occupied by its poles will not 

 change their places by rotation, and consequently no deviation 

 of the compass ought to take place from that cause. The expe- 

 riment, however, is very delicate ; and care must be taken to 

 remove any magnetized bodies whose influence might iiiduce 



