Magnetic Fields, Constant and Varying. 71 



through the tube until the charges at the ends neutralize 

 each other. 



Fk. 8. 



If the change in the electric field take place rapidly 

 enough, the conveel ion-current will produce sufficient dis- 

 turbance of the molecules to cause luminosity. 



Theory of a Vacuum-tube rotating in a constant Electric 

 Field. 



Consider the tube in a constant field in position a b (fig. 9). 

 There will be a — charge at " a " and a + one at " b ", and 

 the tube will behave as a nonconductor. We shall in fact 

 have conditions similar to those in fig. 10, where " c " and 



Fi£. 9. 



Fig. 10. 



is 



O 



a 



o 



+ 



o 



*1 



"d" are two insulated conductors, which have been previously 

 connected together by a conductor, and which correspond to 

 the ends of the tube. 



Imagine now " c " and " d " to be moved to positions C\ 

 and c/i ; they will now be no longer at the same potential, 

 and, if connected together, a current will flow from d x to c x , 

 again equalizing their potentials. 



