CONTEMPORARY ADVANCES IN PHYSICS 



685 



One scheme of apparatus was realized first by E. G. Dymond. I 

 show in Fig. 8 a slight modification, set up by G. P. Harnwell. On the 

 left, the electrons come from a filament contained in the cylinder C, are 

 accelerated through a slit (1) in the cylinder and a second slit (2) in the 

 front wall of a chamber beyond. (For arrangements of this sort, the 

 word "electron-gun" has become the standard metaphor.) They 

 meet the molecules in the center of the tube, and those which are 

 scattered in directions passing through slits 3 and 4 go through these 



Fig. 8 — Harnwell's apparatus for studying scattered electrons. {Physical Review.) 



slits (unless they make further collisions) and onward into the "analy- 

 zer." The electron-gun can be revolved around the axis cutting the 

 plane of the paper normally at 0, thus making it possible to study scat- 

 tering as function of angle. 



In the analyzer there is a region pervaded by a field, and beyond it a 

 stationary collector, to which electrons go if they are suitably deflected 

 by the field,- — as in many a well-known type of analyzer used for identi- 

 fying ions, for instance Aston's or Dempster's for finding isotopes. 

 There, however, it is the charge-to-mass ratio of the ions which is 

 computed from the strength of the deflecting field which sends them to 

 the collector, their kinetic energy being taken as known. Here it is the 

 energy of the electrons which is computed from the strength of the 

 field, their charge-to-mass ratio being accepted as known. One plots 

 the collector-current against the field-strength, and then, translating 



