Measurements of prebreakdown currents and breakdown voltages at low values of ND to the left of the Paschen minimum are given for argon. Analysis of the results shows that the model of breakdown previously applied to mercury vapour may also be used for argon. Values of M and γ are given at E/N values down to 2·8 × 10−18 V m2, where M is the total electron multiplication coefficient and γ is the usual secondary coefficient. Theoretical values of M are shown to be in good agreement with measured values.

Using plane parallel stainless steel electrodes, the Paschen curve for mercury vapour has been investigated up to 10 kV. The curve is found to contain a complex re-entrant section between 1 and 8 kV thus confirming the results of Guseva and Klyarfel'd in 1954. These results are not in agreement with recent work by Hackam in 1969.

The Townsend theory of dielectric breakdown of gases does not apply at low gas density, where the mean free path of electrons maybe of the order of the dimensions of the discharge chamber. An alternative model of breakdown for this region between true gas breakdown and vacuum was presented recently, and shown to apply in the case of mercury vapour. The mathematical model has been extended to lower pressures than previously, and now includes an analysis of possible secondary mechanisms. The theoretical section is complemented by experimental work, again at lower pressures and for a different gas, namely argon. It is shown that the alternative model of breakdown still applies at the lower gas pressures.

IN a recent paper,' we pointed out that the determination of oxygen in beryllium by the micro vacuum fusion method was unsatisfactory and that further investigation was required.This has nou7 resulted in the development of a method that appears to be equally as satisfactory as that described for titanium, zirconium and so on.

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