Magnetic Effect of Electric Convection. 145 



charge on the disk at a distance r from the centre, and H 

 the total magnetic induction through the magnet, then the 

 impulsive couple on the disk when the magnet is reversed is 



.0 



cm 



■rdQdt=^\\dH..dQ = 



H Qo 



2trrdt 27rJ^ 



where Q is the total charge on the disk and H the total 

 induction through the magnet when the current is steady in 

 either direction. 



Fig. l. 



N 



When the charge on the disk is reversed, there is a current 

 through the suspending wire and frame ; and since these are 

 in the magnetic field of the magnet, there will be an impulsive 

 couple on the frame when the charge is reversed *. 



Consider an element of the frame of length rfS anywhere 

 in ACDB. Let dR be the number of magnetic lines of force 

 cut by this element when the frame and disk are turned 

 through one complete revolution. 



Then since tl 



ie 



magnet is 



mmetrical about the vertical 



* I am indebted to Dr. Larmor, F.R.S., for the suggestion to take into 

 account the effect of the charging current. 



Phil. Mag. S. 6. Vol. 2. No. 7. July 1901. L 



