Theory of Electric Inertia. 243 



7 rays which could not be deflected, and for the normal 

 ionization in T. 



The result is given in column 3 of the above table ; the 

 maximum when no magnetic field is applied is represented by 

 100 for both primary and secondary rays. The numbers 

 show that while the first small field removes more secondary 

 than primary, it requires a stronger field to remove com- 

 pletely the secondary. The secondary pencil seems to include 

 some which are travelling with a greater velocity than any of 

 the primary. 



XXIII. On the Theorn of Electric Inertia. 

 By S. H. Buebury, F.R.S.* 



1. O IR 0. LODGE, in his address on Electrons read before 

 O the Institution of Electrical Engineers, 1.903, calcu- 

 lates the inertia which a small sphere, charged with electricity 

 e. and moving with velocity n, has by virtue of its charge as a 

 function of the charge. Inertia may be defined as resistance 

 to acceleration. If the sphere have mass in in the ordinary 

 sense of that term, its kinetic energy if without charge is 

 J&=±mu 2 , and its inertia if it be moving freely is 2E/w 2 = m. 

 But when charged it has electric energy in addition to E, 

 namely, „ /* 



E'= g- I H-Vt, 



where H is the magnetic force in the volume element dr due 

 to the motion of the sphere, /uu is a known constant, which I 

 shall treat as unity, and the integration is throughout all 

 space outside of the moving sphere. Further, he takes 



JI = ne- — 5-5 in which r is the distance from S, the present 



1- 



position of the sphere, to the point P where the element dr 

 is, and is the angle between SP and the axis, that is the 

 direction of motion of the sphere. Substituting this value 

 for H, he obtains 



",frf".w" l ;i 9 



"M 



and the additional inertia due to the sphere being charged is in 



2E' 



this theory - - ; which is, as thus expressed, a function of e 



and the radius c of the sphere. 



2. The value of H here given is not generally exact, 



* Communicated bv the Author. 

 11 t 



