A probe having a spacially periodic potential is created by a bifilar winding of small tungsten wire. This geometry results in an electric field configuration having an extremely short penetration distance into the plasma. When the behavior of this probe is studied in extended negative glow and back‐diffusion type plasmas, the floating mode current‐voltage characteristic curve is seen to be quite different from that of the usual double probe in that no effects of the electrons are observed. The behavior of the periodic probe is controlled by the ion inertia established in the diffusion region which lies just in front of the probe. The important parameter of the characteristic curve is the slope near zero applied voltage difference which can be related in a simple manner to the flux of plasma diffusing to the probe and to the diffusion drift energy of the ions as they reach the probe. The electrons make no contribution to this slope for low applied voltage differences. This probe thus provides a very sensitive method for measuring the diffusive flux of plasma to a containing wall.