On the Low-pressure Mercury-vapour Discharge Mechanism and the Origins of Langmuir's Paradox†

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Abstract
In l925, Langmuir described probe measurements on the positive columns of low-pressure mercury-vapour discharges yielding results that then seemed paradoxical: the electron energy distribution appeared to be Maxwellian up to several times the mean energy. Since fast electrons are rapidly lost to the walls, and collisions are relatively rare, it is not immediately obvious how the high-energy tail is generated, nor how the electron energies are Maxwellinriized. Low-pressure discharges commonly exhibit natural oscillations. If these are of large amplitude, they will influence the electron energy distribution, and it has been suggested that such oscillations play an essential role in maintaining the discharge. It is important for an understanding of the discharge, and from the point of view of possible device applications, to establish whether random collision processes can maintain the column, or whether additional oscillatory mechanisms are required. The experimental and theoretical work relevant to the paradox is discussed; it is concluded that the low-pressure positive column in mercury-vapour can be stable on a basis of random processes only.

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