494 Prof. E. Edlund on the Magnitude of 



themselves very feeble, the question arises whether unipolar 

 induction is sufficiently powerful to contribute in any essential 

 degree to the production of the phenomena mentioned. 



My previous memoir did not contain any quantitative cal- 

 culations on this point. I was then more particularly occupied 

 with qualitative considerations. Nevertheless, since several 

 writers * who have noticed the previous memoir do not appear 

 to have appreciated at its full value the arguments which I 

 adduced against the correctness of the old theory of unipolar 

 induction, I think it desirable to reproduce briefly these argu- 

 ments in a modified form. 



§2. 



Let us suppose a vertical magnet, having its upper 

 extremity a south pole, capable of turning easily about its 

 geometric axis, and that this magnet is concentrically sur- 

 rounded by a cylindrical metallic envelope also capable of 

 motion upon the same axis, but not in any manner electrically 

 connected with the magnet. If a galvanometer be inserted 

 in a metallic circuit, one end of which presses against the 

 cylinder, above one of the poles of the magnet, the south 

 pole for example, and the other against the middle of the 

 cylinder, the galvanometer indicates the production of a cur- 

 rent as soon as the cylinder is put into motion. If, when seen 

 from above, this motion is in the same direction as the hands 

 of a watch, the current passes through the galvanometer from 

 the middle of the cylinder to its upper extremity. 



When the cylinder moves in the opposite direction, the 

 current also passes in the opposite direction, and in both 

 cases its intensity is proportional to the velocity of rotation. 

 According to the old theory, induction in both eases takes 

 place in the cylinder itself, whilst the electrodes act onlv as 

 conductors. If, on the other hand, we turn the magnet with 

 the same velocity and in the same direction as the cylinder — 

 for example, by fixing the one to the other — induction, accord- 

 ing to the same theory, does not take place in the cylinder, as 

 in the first case, but in the fixed conductor, and the cylinder 

 acts only by conducting the current. In one word, according 

 to i he old theory induction can only take place in the portion 

 of the conductor ol^ which the position relative to the magnet 

 is modified by the rotation, without taking into account 

 whether or not this modification is produced at the expense 

 of mechanical work. It is useless for our purpose to consider 

 further the details oi' this theory. 



Let us now suppose that the galvanometer has been removed 



* Tidnkrifl for populmert F\ r tm $ t%Um^m - of Naturvidemahtkem^ - e livrai- 



son, p. 101, note, 



