Apparatus for ionizing gases at very low pressures comprising inner and outer electrodes wherein the inner electrode is substantially circular in cross section and the outer electrode surrounding the inner electrode may be other than a figure of revolution or may be eccentrically positioned with respect to the inner electrode which inner electrode is less in length than two times the length of the surrounding outer electrode and is spaced therefrom a distance which is greater than the diameter of the inner electrode for a distance of not less than two-thirds the length of the surrounding outer electrode.