ELECTRIC TRANSMISSION IN VACUA. 151 



the screen and negative electrode, in such a manner that 

 it can be turned so as to reverse the position of its poles. 



By this means the electric current may be attracted 

 or repelled, so as to pass over or under the screen. 

 When it passes over the screen, the upper vanes are 

 attracted towards ihe positive electrode, producing 

 rotation of the wheel in accord- 

 ance with such movement : but 

 when the position of the magnet 

 is reversed,the current is repelled 

 and passes under the screen, and 

 the lower vanes are attracted, 

 reversing the rotation. 



The glass in these tubes is 

 usually of very low insulating 

 power, much lower than that 

 of air at the ordinary density. 

 Hence the electric resistance in 

 high vacua, being much greater 

 than in the glass, electric move- 

 ment takes place through the 

 vacua and glass respectively, in 

 the inverse ratio of the resist- 

 ance of each. 



Fig. 47 represents a tube in 

 which the negative electrode 



. , /. IIP i . i /. Fig. 47 Glass Illuminated 



consists of a half cylinder of 



aluminium, supported, near the center of the tube, on a 

 small glass tube, b; through which a copper wire ex- 

 tends, connecting the aluminium with the platinum 

 terminal below. 



The ends of both electrodes come near the walls of 

 the tube; and when the electric charge passes, its prin- 



