CH. v.] OF CATHODE RAYS 51 



particles, the simplicity of combining this with 

 magnetic deflexion, and thereby making a couple 

 of measurements simultaneously, is so great that 

 it has practically replaced the more elaborate plan 

 first described, namely the method by observ- 

 ing the aggregate charge, the aggregate energy, 

 and the magnetic deflexion only, a method which, 

 first as an original determination, and now as a 

 check, has been of high value. 



The simple plan has now been applied to rays of 

 various kinds, and it may be well to give its 

 simplest possible form, before proceeding to a more 

 complicated case. It consists, (1) in observing the 

 radius of curvature r caused by a magnetic field 

 of measured strength H ; (2) in finding the electric 

 field E which, applied at right angles to the magnetic 

 field, just succeeds in neutralising all deflexion ; then 

 a centrifugal force equation 



e u 



applies to experiment No. 1 ; and a balanced force 

 equation ... E 



E 

 or u= 



applies to experiment No. 2. 



This second experiment gives . the velocity, by 

 itself; and the first in combination with it gives the 

 electro-chemical equivalent. 



So the determination of 'velocity for the case of 

 particles 'flying all with one speed is remarkably 

 easy : it comes out, in centimetres per second, as 



