S. Kownacki and J. A. Batcliffe outline a method for investigating ‘electron-inertia’ effects in thermionic tubes. They point out that if positive ions of atomic mass are used instead of electrons, the frequencies involved are very much smaller and more manageable. They have investigated some 'electron-resonance' effects with tubes in which positive ion emitters were used, and the frequencies involved were about 5 x 106 and 106 c./sec. when caesium and lithium ions respectively were used.