36 Voltaic Induction. 



around the wires. I shall illustrate this by supposing the magnet to 

 be stationary, and the coil to revolve, at its center, around the mag- 



netic axis. 



Let N, fig. 5, represent a vertical section Fi S- 5 - 



x x 



c 



of the axis, at the north polar surface, and 

 a, 6, c, a flat coil, revolving, vertically, at its 

 centre, around the pole N, and having its di- 

 rection indicated by the terminal arrow at c. 

 Any particle, as a, included within two lines 

 of polar action, will, in consequence of the 

 supposed motion, receive induction from the 

 force No/, and from the point of intercep- 

 tion, a north pole will move around the par- 

 ticle towards #, and a south one towards N. 

 But as the force N#' lies in the same plane as the coil, so will the 

 induced magnetic circle w, s; and the voltaic currents, which result 

 from this action, must of necessity move across the wire, at a. 

 Hence it cannot be detected at other parts, as b and c of the coil. If 

 we employ a copper plate, the result would be different, but only in 

 consequence of an extension of this transverse current ; for the plate 

 forming a continuous conductor from circumference to centre, the 

 positive aud negative elements w T ould distribute themselves in this di- 

 rection, and even enter the wires of a galvanometer when they are 

 placed upon the radius a, N. By the motion, as here represented, 

 (fig. 5,) a positive current would issue from this side of a. particle, at 

 a, and descending over the near surface of the plate, proceed to- 

 wards its centre. Reversing the motion, will obviously invert the di- 

 rection of the current. 



For the full effect of induction under such circumstances, it is not 

 necessary that there should be a plate at all; for, if a magnet, sup- 

 ported at its extremities by pieces of metal, be made to revolve upon 

 its axis, these currents will be generated in the supports, and maybe 

 drawn off by wires placed in contact with either the magnet or the 

 metallic fixtures. Nobili is, therefore mistaken, when he says that 

 currents cannot be generated by a concentric rotation. The highly 

 interesting researches of Faraday, published in the second part of 

 his memoir,* place this question beyond a doubt ; for he detected 



* Annates de Chimie, June, 1832. 





