12 Experiments in Electric and Magnetic Fields. 



The same will happen to the tube if it move during rotation 

 from position a b to a x h x \ and, when the potential difference 

 between its ends attains a sufficient magnitude, the dielectric 

 of rarefied gas will break down, a convection-current flow 

 from b ± to <-?!, and the tube for the instant will behave like a 

 conductor. During this instant of breaking down the tube 

 will glow. After this the tube will behave like a noncon- 

 ductor, the ends being at the same potential until the tube 

 rotating moves into position a 2 b 2 , and, if the potential differ- 

 ence between a 2 and a 1 is equal to that between a and « 1? and 

 the same for the 6's, the tube will glow for an instant at 

 position a 2 b 2 . 



In the same way it will glow in the positions « 3 b s , a± b±, 

 &c, if continuously rotated in the direction of the arrow. The 

 points a 3 a 4 &c, b 3 b 4 &c. are such that the potential difference 

 between each consecutive two is approximately the same. As 

 the glow takes place at the end of each alteration of the 

 potential difference, it is obvious that the double fan-like set 

 of images obtained will not be symmetrical with respect to 

 the plates, but will be displaced in the direction of rotation 

 by about one image. 



The number of images in a single revolution will be pro- 

 portional to the potential difference between the plates, but 

 will not depend on the speed of rotation, provided that speed 

 exceed a certain limit. The effect of increasing the speed is, 

 however, to apparently increase the number of images ; since 

 those of one revolution do not fall in exactly the same 

 positions as those of the preceding one, and the images of 

 several revolutions are seen simultaneously on account of 

 persistence of vision. 



Discharge through Coil surrounding exhausted Bulb. 



When a Leyden jar is discharged through a coil of wire, 



the varying magnetic induction due to the discharge-current 



will cause an E.M.F. to^ict in the dielectric inside and outside 



the ring in a series of concentric circles. Let r he the radius 



of the coil: consider a circular path in the dielectric of radius 



x less than r. If the magnetic induction passing through the 



coil were uniformly distributed, the number of lines passing 



through the path x would be proportional to its area or to x 2 , 



and the E.M.F. per unit of length along it to x 2 divided by its 



x 2 

 circumference or to — , i. e. to x. Hence the electric stress 



x 



along a circular path x in the dielectric is greater the larger x. 

 Of course the actual distribution of the magnetic induction is 

 not uniform, but increases as we pass from the centre to the 



