Possibilities of Future Development 



113 



</> = 



8nic' 



(Ur) 



(39) 



and the electrons will not l)e able to reach the sheath electrode 

 "A" if its potential V against the cathode is less than a certain 

 value Ve which is obtained by substituting the radius of "A" 

 into equation (39). Vp is Hull's critical potential. It is known 

 that in practice \^ has to be made somew^hat smaller than \> if 

 anode currents are to be avoided. This effect is probably due 

 to electron collisions, and has never been fully explained. 



41a 

 Magnetic Field 



41b 



Electrostatic Field 

 Corrected Objective 



Fig. 41 



At the ends, where the magnetic field goes to zero, limitation 

 is obtained by negative potentials, which means that the poten- 



