Semi-evacuated electron tube filled with low-pressure helium and containing a tangential and axial electron gun. In the case of tangential bombardment and a perpendicularly aligned magnetic field, the fundamental electron charge-to-mass ratio can be determined from the diameter of the observed beam pattern. In the case of axial bombardment and a co-axial magnetic field, kinetic gas theory (e.g. impact excitation) and particle collisions can be studied through observations of the fine, luminescent beams resulting from excitation of the helium atoms.