AbstractAn array of sixteen flush‐mounted probes has been installed in the upper, outboard divertor plate of the TdeV tokamak, giving a radial resolution of 3.3mmat the strike point. It has become apparent, in the case of grazing angles of incidence of the magnetic field with the plate, that the finite size of both the probe sheath and the sheath of the surrounding plate are the dominant factors which determine the effective probe area for charge collection. We can modify these sheaths by biasing the probes with respect to the plate, and the plate with respect to the vacuum vessel, with very illustrative results. A model, based on sheath expansion, has been developed which can be used to calculate the electron density. The model has been tested experimentally by comparing the results obtained from a flush probe and a standard cylindrical probe which were operated simultaneously in the same plasm