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school, in which over a limited range of conditions V was found to be 

 almost independent of i (the amplitude of the oscillating current) 

 and a linear function of the length /. Also Hayman speaks of observing 

 a minimum in the curve of V versus i, occurring "at a value of current 

 slightly greater than the least which gives a uniform glow in the 

 tube." Often, however, the experimenters simply vary the strength 

 of the oscillating current (usually by varying the filament-current of 

 the vacuum-tube oscillator, which is coupled to the circuit containing 

 the discharge-tube) and measure the voltage across the electrodes just 

 before the glow disappears. This is called the "least maintaining- 

 potential" or the "extinction-potential" or by some equivalent name. 

 By analogy with direct-current discharges, it should depend on the 

 constants of the circuit. 



-/ 



t 



Fig. 20 — Kirchner's apparatus for measuring amplitude of voltage in high-frequency 

 glow-discharge. (Kirchner, Ann. d. Phys.) 



The researches of Kirchner and of Rohde cover between them the 

 widest variety of gases and the broadest range of conditions: in 

 respect of frequency, the former worked over the range from 1.2 to 

 3.5•10^ the latter from 3.1 -10^ to 1.39 -lO^. Kirchner's method of 

 measuring voltage deserves especial mention. Its principle is that of 

 the cathode-ray oscillograph: a beam of fast electrons is deflected to 

 and fro by the P.D. applied between two plates, one on either side of 

 the beam. These could be the electrodes, but that the fast electrons 

 might then perturb and be perturbed by the discharge, and there 

 would be other disadvantages. Kirchner therefore designed three 



