296 Trof. E. Edlund on Unipolar Induction. 



the magnetic force towards that periphery ; and those which 

 lie between that circle and the centre are driven towards the 

 latter. As the formula shows, the value of r of the neuter 

 circle increases with I and p. If the plate be touched outside 

 of the neuter circle with the ends of the electrodes of a galva- 

 nometer, the galvanometer indicates a current which goes from 

 the point of contact situated nearest the periphery, through 

 the galvanometer, to the other point of contact. If, on the 

 contrary, both points of contact lie between the neuter circle 

 and the centre of the plate, the current passes from the point 

 of contact situated nearest the centre, through the galvano- 

 meter, to the other point of contact. If the neuter circle lies 

 between the two points of contact, the positions of these latter 

 can be selected so as to give rise to no current. If the plate 

 moves in the reverse direction, the electric fluid accumulates 

 around the neuter circle, and the current consequently changes 

 its direction. These deductions from theory have been verified 

 by the observations of M. Felici*. 



Plucker has experimentally investigated the following 

 case : — Through the centre of a metal disk, ah (fig. 3), passes 

 an axis, cd, of the same metal, about which the disk can be set 

 in rotation. Nearer to the circumference of the disk an aper- 

 ture is pierced, into which a magnet, sn, is fitted so that its 

 centre is in the plane of the disk, and its axis parallel to the 

 axis of rotation. If the rotation takes place in the direction 

 indicated by the arrow, the poles of the magnet having the 

 position shown in the figure, on uniting by a conducting wire 

 one of the extremities of the axis of rotation to the circumfe- 

 rence of the disk we obtain a galvanic current passing from 

 the former to the latter through the wire. The current con- 

 sequently follows the same direction as if the disk with its axis 

 were in rotation about the magnetf . 



According to what precedes, the induction at any point of 

 the conductor depends on its motion relatively to the magnet 

 regarded as a fixed point. As is known, this relative motion 

 undergoes no modification if to the magnet as well as to the 

 point in question equal velocities in the same direction be im- 

 parted. Thus, in the first place, it is easy to determine the 

 induction in the axis of rotation itself by proceeding as fol- 

 lows : — To the rotation-axis and the magnet a velocity is im- 

 parted equal but opposite to the actual velocity of the magnet, 

 which therefore comes to rest, while the axis rotates about it 

 in the direction indicated by the arrow. The induction in the 

 axirj consequently becomes the same as if the magnet were at 



* Ann. de Clrimie et cle Physique, (3) xliv. p. 343. 

 t 1'ogg. Ann. vol. lxxxvii. p. 352. 



