Recently published experimental results of Ellington and Ralph, showing the effect of the diluent gas on the electrical properties of alkali-metal-seeded rare-gas plasmas, are discussed in terms of an extension of the two-temperature conductivity theory first proposed by Kerrebrock. The theory explains the increase in the plasma electric field required to maintain a given current that is produced when helium replaces argon as diluent, with potassium as seed, over the seed pressure range 0.1-40 torr (the factor of increase falls from about 13 to about 2 between these values). The gas pressure is atmospheric. Neon is shown to occupy an intermediate position between helium and argon.

The theory developed is used to give an estimate, based on experiment, of the effect on K-He, K-Ne and K-Ar plasmas of varying the diluent pressure from 0.01 to 20 atmospheres. At low diluent pressures the electric field values required to produce a given current in the three mixtures converge and level out, while at high pressures they diverge and increase rapidly with rising pressure. At high pressures helium requires field values about 20 times as high as for argon, and neon about 4 times as high. It is concluded that argon and neon seeded with potassium (or caesium) appear to be suitable for use as the working fluid in a magnetohydrodynamic generator operating at pressures up to 10 or 20 atmospheres, but that helium appears to be unsuitable as a diluent at pressures above 2 or 3 atmospheres.