Longitudinal waves were excited radially across a plasma column subjected to an axial magnetic field. A strip line placed externally to the column was the source of excitation. The wavelength and propagation direction were measured by a movable antenna immersed in the plasma. The measurements are compared with theory and with observations made with an externally situated receiving antenna. The following conclusions are drawn. (a) Contrary to the commonly accepted notion, the longitudinal waves are propagating rather than standing waves. The propagation is radial towards the axis of the plasma column. Since the waves are backward waves, the energy propagates radially outwards. (b) The internal and external measurements are related to good accuracy through the geometrical optics, “phase quantization” condition,∫ k dr ≈ 2m&pgr;, where k is the propagation constant, r is the radial coordinate, and m is an integer. The left‐hand side of the equation is deduced from measurements ofkmade with the movable antenna immersed in the plasma, and the right‐hand side of the equation is obtained from measurements of the signal received by the external antenna.