by a Charged Condenser moving through Space. 



151 



point resolved perpendicular to the velocity v. Rontgen 

 endeavoured to detect this field by its effect on a magnetic* 

 compass-needle, but no such effect was observed. Sir Joseph 

 Larmor has explained this by showing that the induced 

 charge produced on the surface of the compass-needle is 

 such as by its motion to completely annul the magnetic field 

 produced by the charged bodies, at all points in the interior 

 of the compass-needle. I think this can also be seen to 

 follow from the consideration that, since all the bodies are 

 moving in the same direction, and since all the tubes of 

 force which strike the surface of the compass-needle end 

 there, none of them exist within the compass-needle, to pro- 

 duce by their motion a magnetic field. Now although the 

 magnetic field is incapable of being detected by a compass- 

 needle, it seems at first sight that it might be capable of 

 detection, by means of a rotating coil; for let AB (fi 



F\s. 1. 



g. 1 ) be two 



C- 



A + 



z ~~ — ^ 





1 B 



t 







I 

 l 



+ 







i 

 1 



"T 



zz^wM@- - 



D^flli """ 



-1 



1 



+ 







i 

 1 



+ 







1 

 1 



+ 







I 



condenser-plates moving with the earth in a direction perpen- 

 dicular to the plane of the paper, and imagine the coil shown 

 in section to rotate about an axis CD. It is easy to see that 

 the tubes of force will be distributed in the manner quali- 

 tatively illustrated in the figure, and it is not obvious that 

 the magnetic flux produced by the motion of those tubes 

 which strike the side L of the coil will be exactly cancelled 

 by the flux caused b}' the motion of those tubes which bend 

 round, and strike the coil on its inner side at a. Since the 

 flux is certainly zero when the plane of the coil is perpendi- 

 cular to the line of motion, we might expect on rotating the 

 coil to obtain an alternating current which by suitable com- 

 mutation could be detected by means of a galvanometer. 



Unfortunately it appears on examination, that the flux 

 through the coil is at all times zero, the two opposing 

 portions annulling each other, as will be shown in Section 1, 

 provided that the whole of the space in the vicinity of the 

 coil is filled with material oi: constant specific inductive 



