146 Dr. H. A. Wilson on the 



axis the element will be in a field of strength — -^ normal 

 to itself. The force on the element is therefore 



27TT 



where C is the current in the element. Bnt C==~ -m 



Q being the charge on the disk, and half the current going- 

 down each side of the frame. 



The impulsive couple on the frame when the charge is 

 reversed is therefore 



CCdK r 



JJ 27TT i 



dt 7T 



(For all the lines of force going through the middle section 

 of the magnet are cut by the frame when it makes half a 

 revolution.) 



This effect therefore is equal and opposite to the impulsive 

 couple when the magnet is reversed. 



Consequently, when the charge and magnet are alternately 

 reversed the total average couple on the disk should be zero, 

 as Cremieu found. 



Since the force on a current in a magnetic field is well 

 known to exist, it follows that this experiment of Cremieu's 

 may be regarded as indirectly proving the existence of the 

 equal and opposite electrostatic effect of the reversal of the 

 magnet, for if the latter effect did not exist then Cremieu 

 ought to have observed the former. 



I shall now consider Cremieu's experiments made with 

 the object of detecting the magnetic field due to electric 

 convection. 



In these experiments a charged ebonite disk, coated with 

 gold-leaf divided into sectors, was rotated inside a brass 

 drum. The ends of the drum which were parallel to the 

 disk were lined with mica, also coated with gold-leaf divided 

 into sectors. With this arrangement no external magnetic 

 field could be detected, but on removing the ends of the 

 drum, but leaving the mica in position, a small effect was 

 detected, which was attributed by Cremieu to impulsive 

 currents induced in the sectors on the mica by the passage of 

 the sectors on the disk. 



It is, however, easy to show that the brass ends of the 

 drum ought to cut off the desired effect, for currents will be 

 produced in the sectors and drum-ends by the rotation of the 



