Effect of a Changing Magnetic Field. 465 



being held coaxial with tho rod by means of a single disk, 

 also of mica or paper. The rod was suspended vertically by 

 stretched phosphor-bronze ribbon attached above and below 

 to torsion heads. The rod passed through holes drilled 

 through the pole-pieces (PP) of a large electromagnet, which 

 were so shaped and placed as to project into the suspended 

 cylinder at both ends, leaving only a 6 mm. air-gap for the 

 disk. In this way a very large magnetic field could be made 

 to pass through the cylinder parallel to its axis, while the 

 cylinder itselt: was free to rotate about its own axis. The 

 cylinder was covered with gold-leaf to render it conducting, 

 and was connected to earth together with the rod, suspension, 

 and magnet core* It was charged inductively by means of 

 a somewhat longer cylinder of hard rubber, 7*6 cm. internal 

 diameter, set coaxial with it. The hard rubber cylinder was 

 coated on its inner surface with tinfoil, and charged by being 

 connected with one terminal of a high potential battery, the 

 other terminal being earthed. If now the movable cylinder 

 is charged 4- and the magnet is suddenly excited so as to 

 make the upper pole a N pole, the cylinder would be expected 

 1o suffer a rotary impulse in the direction east, north, west, 

 south. The magnitude of this impulse can be calculated as 

 follows : — 



Let N = the magnetic flux through the cylinder. Then 



~j~ =the E.M.F. induced in one turn of a conductor sur- 

 dt 



rounding the cylinder, and the intensity of the electric field 

 tangent to the cylinder is 



2>rrr' dt 



Hence the instantaneous value of the moment exerted on 

 the cylinder carrying a charge of Q electromagnetic units is 



L=rQF = TJ 



Q rfX 



77 



dt ' 



or the total rotary impulse due to a change in the magnetic 

 field of N lines is 



T NQ 



ITT 



where I is the moment of inertia of the cylinder and w 

 initial angular velocity imparted to it. 



Phil. Mag. S. 6. Vol. 19. No. 112. April 1910. 2 H 



