66 LEAKAGE IN ULTRA-VIOLET LIGHT [CH. VL 



circles, since they are not subjected to any but a 

 deflecting force when they have passed through the 

 screen. When the maximum is received by the plate 

 F, the tangent to the mean circle will be horizontal 

 at the position of the hole B ; and this fact, together 

 with the passing of this circular trajectory through 

 the centre of the plate F, is sufficient to determine it, 

 and so to give its radius of curvature, which will 

 be equal to 



The velocity u is separately estimated by assuming 

 that it is acquired under electrical influence between 

 C and B, the equation being 



the latter factor being the difference of potential 

 between C and B. 



Thus the two quantities u and e/m are determined. 



Positive and Negative Carriers. 



In connexion with the above experiments it is 

 important to notice that the above value of e/m for 

 the negative carriers is only obtained at low pressures. 

 If the pressure is high the ordinary electrolytic value 

 of e/m, or something still smaller, can be obtained; and 

 this suggests that at ordinary pressures the electron 

 becomes loaded up by attaching itself to an atom, or 

 even by collecting round itself a group of atoms. 

 Further evidence of this is afforded by the fact that 

 there is but little difference between the velocities of 

 the positive and negative ions, when urged by an 

 electric field, in a gas at atmospheric pressure. It 

 will be seen, further on, that in the case of the 



