CONTEMPORARY ADVANCES IN PHYSICS 671 



by the equation: 



Q - R = aNQdx. (1) 



The greater it is, the greater the fraction of the number of incident elec- 

 trons which are intercepted, the greater the probabihty of being inter- 

 cepted for any one electron; it is thus a measure of a probability of 

 likelihood, the "likelihood of interception." 



The quantity {Qe — Re) is "missing current"; it is the amount by 

 which the current to the collector drops off, when N molecules per unit 

 volume are introduced into the tube. Nothing has yet been said, 

 nothing has even been implied, about the fate of this lost current and 

 about the missing electrons which presumably bore it into the gas. I 

 have, in fact, been using the very neutral word "interception" so as to 

 evade all implications in excess of what the data say, which is, that 

 some of the electrons fail to persist in the beam. Not to suppose that 

 they have been annihilated, there are at least two conceivable things 

 which may have happened to them. They may have made elastic im- 

 pacts against molecules, bouncing off in new directions, and being thus 

 deflected out of the beam without suffering much change in speed. Or, 

 they may have struck and stuck to molecules moving in other directions 

 than that of the beam. Other possibilities are thinkable; but these 

 are enough to hold in mind for the present.^ 



(The word "absorption" is used by some, especially by Germans, in 

 the sense for which I here use "interception." It seems to me to con- 

 vey unwanted implications, but there may be differences of opinion 

 on this point. Much the same problem of language occurs in optics. 

 Usually the term "absorption of light" means in practice "departure of 

 photons from a beam of light" irrespective of whether they are actually 

 swallowed up by atoms, or deflected without any loss of energy; but it 

 is rather common nowadays, especially in treating of X-rays, to use 

 "absorption" for the former mode of disappearance only, and "scatter- 

 ing" for the latter. Since it is necessary now to distinguish two kinds 

 of scattering of photons, the complications are beginning to rival those 

 of electronics.) 



Adopting either the elastic-impact idea or the adhesion idea, we may 



visualize this quantity o- in a familiar way. We may conceive of the 



molecules, for this purpose and for this purpose only, as spheres so 



constituted that when an electron touches one of them it sticks — or 



else rebounds, whichever theory we are using. The value of a is then 



^ Lenard reviewed a number of possibilities, and considered ways of distinguishing 

 them in his brochure Quantitatives ilber Kathodenstrahlen. He made a peculiar dis- 

 tinction between reflection of electrons from molecules, and small deviations of elec- 

 trons by molecules; it seems to have been suggested by his work on very fast cor- 

 puscles. 



