Three short papers were read after the review paper by T. W. Johnston.
P. E. Vandenplas (Ecole Royale Militaire, Brussels, Belgium) gave a survey concerning resonances observed in laboratory plasma capacitors. For such a capacitor with plate distance l and plates covered with rarified sheaths, cold plasma theory gives the main resonance of an infinite plane capacitor as a serial one at a frequency ƒN (1 − a/l)1/2, where a is the thickness of the (homogeneous) plasma sheath between the plates. In warm plasma theory apart from a correction term of the order λDe2/a2(λDe is electron Debye-length) this resonance splits up into a narrow series of successive resonances, the distance of which is controlled by the same parameter. Furthermore, since the ‘warm’ dispersion equation is of fourth order now, other eigenvalues lead to new resonances of another type, which have been found in the laboratory also.