CONTEMPORARY ADVANCES IN PHYSICS 



593 



in the reading of the galvanometer, when the discharge and the 

 ionization commence in the tube (resonance being restored in the 

 circuit, by shifting the "bridge" across the wires), is the quantity 

 which supposedly varies inversely as conductivity. In the curves of 

 Fig. 6, one sees the minima. Such curves were plotted for four 

 frequencies — 340, 500, 550 and 625 millions — and the maxima occurred 

 at the pressures 0.08, 0.11, 0.12 and 0.15 mm. Hg. The values of mn 

 corresponding were 1.9, 2.8, 3.1 and 3.5 times lO-^^; these by the 

 theory are the values of g for the corresponding pressures. They 

 are roughly proportional to the pressure, as they should be. (Inci- 



15 



10 



100 120 140 160 



PRESSURE (SCALE UNITS) 



220 



Fig. 6 — Evidence of a maximum of conductivity occurring when the frequency is re- 

 lated in a particular way to the frictional coefficient g. (Appleton & Chapman.) 



dentally, the resonance-frequency of the system was shifted, when the 

 ionization commenced, in the proper sense — the sense corresponding 

 to a dropping of the dielectric constant below unity.) 



Before considering further data, let us compare these values of g 

 with those deduced from observation (or theory) of electrons drifting 

 through the same or a similar gas under a constant field. In such a 

 condition, they attain a "terminal drift-speed" u given by the equa- 

 tion, 



gu = eF, (12) 



