Abstract: Process and apparatus for measuring the difference in the concentration of paramagnetic components between an unknown gas and a standard gas by periodically alternate entry of both gases into the gap chamber (2) of a magnetic circuit (1) and by detecting the periodical variations of the magnetic flux caused by the gases in the magnetic circuit.In this process,(a) the two gases are brought into the gap chamber (2) from opposite directions through gas lines (5, 6),(b) the boundary zone between the two gases perpendicular to the magnetic lines of force is moved back and forth in the gap chamber periodically by an alternating flow generator (7), and(c) constantly a portion of the gas mixture is made to escape through an exhaust line (9) from the region of the gap chamber (2) by the inflowing of at least one of the two gases.