An Experimental study of fluid flow past a symmetrical aerofoil with a rear suction slot and a retractable flap was described by D. M. Heughan in the October 1953 JOURNAL. The present note describes the electric tank method for studying the same problem. The experimental apparatus is shown in Fig. 1. A and B are electric poles along two of the edges of the tank which is filled with water. The poles A and B together with the central pole P are connected to an electric supply via a resistance R in such a way that the potential of P is intermediate between that of A and B. The aerofoil F is represented by a wooden block coated with paraffin wax and the pole P is located in such a way as to represent the sink in the suction slot. A voltmeter V records the potential at a point in the tank.