Prof. Thomson, Velocity of Secondary Cathode Rays, etc. 541 
On the velocity of Secondary Cathode Rays from gases. By 
J. J. Thomson, M.A., F.R.S., Cavendish Professor of Experimental 
Physics. 
[Read 10 February 1908.] 
When a discharge passes from a Wehnelt cathode, on which 
the lime is concentrated in a small patch, the clearly defined bine 
pencil of cathode rays emanating from the lime is seen to be 
surrounded by a bluish haze which may, under favourable circum- 
stances, fill the vacuum tube. This haze is due to secondary 
cathode rays produced by the impact of the primary cathode rays 
against the molecules of the gas in the vacuum tube. That this 
is the case may be shown by placing a piece of metal in the tube ; 
when the metal is uncharged the haze will extend right up to it, 
but if the metal is charged in such a way that the electric force in 
its neighbourhood tends to repel a negative particle from it, then 
it will be found that if the force exceeds a very moderate value, the 
haze will be repelled from the metal and there will be a very well 
defined dark space round the metal, the thickness of the dark 
space increasing with the charge on the metal. If however the 
metal is charged so that the electric force in its neighbourhood 
acts so as to attract a negatively electrified particle there is no 
dark space round the metal, the haze comes into contact with it 
and no diminution in luminosity is perceptible. 
It is evident that if we measure the electric field in the 
neighbourhood of the metal when this is surrounded by the dark 
space we can calculate the difference of potential required to stop 
the negative particles which produce the haze and hence determine 
the velocity with which they are projected from the molecules of 
gas struck by the primary cathode rays. The method employed to 
do this is represented in Fig. 1. A pencil of cathode rays starting 
from the lime covered cathode A, passed through an aperture and 
then close over the top of the box C. At the top of the box there 
