A Reply to the Comments of M. F. Hoyaux on the Paper Entitled 'On the Low-pressure Mercury-vapour Discharge Mechanism and the Origins of Langmuir's Paradox'

To determine theoretically whether or not a positive column is stable requires a complete description of the energy balance, and hence precise knowledge of all excitation, ionization and other loss mechanisms. If such a description were available it would indicate either that the plasma is d.c. stable, i.e. perturbations tend to be damped, or that its equilibrium is one in which oscillations are present. In either case, it is to be expected that, the steady-state conditions obey the 'minimum energy' principle, as pointed out by Professor Hoyaux.
The purpose of our article was to show that both the experimental evidence, and the inferences to be drawn from knowledge of the relevant collision processes, indicate that the mercury-vapour positive column can be d.c. stable. The low-frequency oscillations commonly observed, and the more rarely observed high frequency ones, can be identified as originating in the cathode region. They propagate with damping along the column. By increasing the length of the column, the amplitude of these damped oscillations may be decreased indefinitely, far from the cathode.
It is certainly true in other gases, which exhibit standing or moving striations, that the uniform column is unstable against the perturbation corresponding to the striations, the stability being determined by the form of the relevant, collision cross-sections and we have no doubt that Steenbeck's principle would reveal, for such gases, that the striated . column has a lower average voltage drop than the uniform column. This is no reason, however, to attribute to plasma an evil genius for finding modes of instability. Plasmas are only as inventive as the laws of physics allow. Professor Hoyaux's 'guess' as to the ability of a mercury-vapour plasma always to succeed in finding an unstable mode of operation is no substitute for experiment and analysis. [ABSTRACT FROM AUTHOR]