CH. ix.] SIZE OF ELECTRON 97 



the size of the supposed pure electron to be of the 

 order 10~ 13 centimetre in diameter; or 100 * 000 of 

 the linear dimension known as molecular magnitude, 

 viz. 10~ 8 centimetre. 



The calculation of order of magnitude is quite 

 simple, for all ordinary speeds ; because, for them 





,->i a = . pe = 10 7 x 10~ 20 = 10~ 13 centimetre, 

 m 



though it might with some data be estimated as small 

 as 10 ~ 14 .* Minuteness like that easily explains the 

 penetrating power of cathode rays. Especially if the 

 atoms of matter are themselves composed of such 

 minute particles. For the interspaces will be 

 enormous compared with the filled-up space, and a 

 point can penetrate far into such an assemblage 

 without striking anything. 



Penetrability of Matter by Electrons. 



The mean free path of a particle is a question of 

 probability. In a space containing n^ obstacles to the 

 unit volume, a space Ax will contain n = Axn l of 

 them ; and the chances of a collision, while one of 

 them travels a length x, will be approximately their 

 combined areas, as targets, compared with the total 

 area available for both hit or miss that is to say, 



mra 2 1.1 ., o x 



- ; which we may write px or -, 

 A x 



where x is the " mean free path," or average distance 

 travelled by any one particle without a collision with 

 another, and ft the number of encounters while 



*See Lodge in the Electrician for March 12, 1897, vol. 38, page 644, 

 where the size is deduced from the then just discovered Zeeman effect. 

 L.E. G 



