An arrangement consisting of a cylindrical anode, an axial thermionic filament and an axial magnetic field provides a high electron density if the magnetic field is sufficient to prevent most of the electrons reaching the anode. End plates, perpendicular to the axis and maintained at a negative potential relative to the filament, serve the double purpose of preventing the escape of electrons and collecting ions if gas is present. Such an arrangement forms a simple vacuum gauge the sensitivity of which may be at least 100 times that of the conventional thermionic gauge; for normal use robust pivoted meters may be used in place of galvanometers or d.c. amplifiers. The ion current is linear with pressure below 10^-4 mm of mercury. Characteristic curves are presented and the action of the instrument is interpreted in terms of these.