CONTEMPORARY ADVANCES IN PHYSICS 



109 



pieces of apparatus, of which one is figured in Fig. 20. The discharge 

 is in the tube /, between the electrodes D and E, of which the latter 

 is a sheet of metal separating / from the evacuated tube //; the beam 

 of fast electrons, proceeding from the filament F and formed by the 

 diaphragms B, passes between E and the lower plate C which is 

 constantly at the same potential with D. The voltage between C 

 and E is the same as that between the electrodes of the discharge; 

 the measure of its amplitude is the length of the arc which the tip 



1.2X107 



■ — 2.3X107 

 3.5XI07 



0.2 0.4 0.6 0.6 1.0 |.2 1.4 1.6 1.8 2.0 



p(mm H9) 



Fig. 21 — Least maintaining-potential vs. pressure, in rarefied air, at the indicated 



frequencies. (Kirchner.) 



of the beam describes, as it dashes back and forth over the surface 

 of S, a fluorescent screen. 



Mostly the curves of least maintaining-potential versus pressure, 

 like those of onset-potential versus pressure, are concave-upward with 

 single flattish minima. Fig. 21 shows four curves which Kirchner 

 obtained with air. They are not very smooth nor are the minima 

 clearly marked ; I choose them for reproduction because they comprise 

 a curve for direct-current discharge (marked 0) as well as three others 

 for certain high frequencies marked beside them. 



Let me denote by Vmm and pmm the coordinates of the minimum of 



