This paper correlates the appearance of the discharge around a probe with data obtained with a fixedand a movable wall probe. A relatively small probe at the axis of a discharge has much less effect than theinsulating sleeve around the probe lead. Since ion depletion is the controlling factor, the disturbing effectincreases a little as the probe is made more negative. The presence of the insulating sleeve enhances theelectric field on the anode side of it, increasing ion production, and displaces the region of maximum ionproduction toward the part of the tube beyond the end of the probe. Corresponding increases in ion flow tothe wall beyond the end of the probe and to either side of it are observed. In noble gas discharges (tube 34mm i.d., filling pressure 1.0–7.0 mm Hg, are current 1.0–1.4 A) increases in wall current beyond the end of aprobe at the axis of the discharge, with an insulator 0.15 mm in diameter around its lead, ranged from12 to 29%.