SOME COX I l-.Ml'OR.lRY .IPl'.LXCr.S IX I'llVSICS—Xr 



487 



then drawn apart by an appropriate electric field) ; the groups of more 

 than two bear witness of a primar\- ionization followed 1)\- secondary 

 processes of the same t\pe. In V\g. 12 there is an actual long branch 

 to the primary trail; the original fast elec'tron has detached another 

 and endowed it with so great an energ\- that in ionizing-elificienc\- it 



F\g. 11 



rivals its liberator. These figures show that a mere count of all the ions 

 formed by a particle flying through a gas is no estimate of the detach- 

 ments of electrons from atoms which the particle of itself and at first 

 hand effected. 



Various theoretical expressions have been derived for the rate of 

 slowing-down and the rate of ionization of an alpha-particle or fast 



Fig. 12 



electron proceeding through a gas. Most of them lead to what are 

 known as "order-of-magnitude agreements," but none to a close quan- 

 titative agreement— which is, perhaps, after all better than could be 

 expected. They are founded upon an equation originally proposed by 

 J.J. Thomson. Suppose a stratum of an element of atomic number Z, 

 containing N atoms; using the nuclear atom-model, we conceive this 

 as a region containing .V nuclei and NZ electrons. If the electrons (of 



