44 Dr. H. Pender on the Magnetic 



the single condenser thus formed than the two condensers 

 above described, for a given speed of the Yoss machine and 

 the interrupter. (It may be here noted that the Voss machine 

 was always run at the highest possible speed. A machine 

 of greater capacity would have made possible the procuring 

 of a greater deflexion.) 



To calculate the deflexion which should be expected on the 

 assumption that a moving apparent charge has a magnetic 

 action, a method similar to that employed in the previous 

 calculation was adopted. Let a be the surface-density of 

 the apparent charge on the surface of the disk next to the 

 coil, assumed uniform as a first approximation, N the number 

 of revolutions of the disk per second, v the ratio of the two 

 systems of magnetic units, r the radius of an imaginary 

 ring on the surface of the disk with its centre at the centre 

 of the disk and of width dr, B the deflexion of the galvano- 

 meter-needle produced by a unit current in such a ring on 

 the surface of the disk next to the induced coil, rapidly made 

 and broken the same number of times a second as the disk is 

 polarized and depolarized, 8' the same for a unit current in 

 such a ring on the opposite surface of the disk. Then the 

 deflexion of the galvanometer-needle due to the rapid polar- 

 izing and depolarizing of the rotating disk will be 



v Jo 



a is determined from the formula (Webster, Elec. and Mag. 

 p. 364) 



lirtfifa + d^ + dy 



where jjl is the dielectric constant of the disk (for ebonite 2*5), 

 di the distance between the outer condensing plate C x and 

 the surface of the disk, d 2 the distance between the inner 

 condensing plate C 2 and the surface of the disk, d the 

 thickness of the disk, and V the potential of the outer plate, 

 the inner plate being earthed. 



The integral 1 r(S — S')dr was determined by a u calibra- 

 Jo 

 tion of the apparatus " in a manner similar to that employed 

 in the first experiments. A set of coils of known radii was 

 clamped up against the surface of the disk next to the induced 

 coil. A known current i was sent through the reverser and 

 one of these coils. While the current was being thus reversed 

 in this coil, the frame carrying the disk was drawn back from 



