According to the classical treatments given in standard textbooks, the presence of an electric field in an insulating fluid should favor natural convection. The effect of an electric field on thermal convection was reinvestigated with an experimental setup consisting essentially of a thin platinum wire stretched in the axis of a metal ring. Both electrodes were immersed in a cell containing the liquid under investigation. The wire was heated by Joule effect, and a dc voltage up to 5 kV was applied between the wire and the ring. The cell, fitted with optical windows, was placed in the beam of a striograph (Schlieren optics bench) by which the heat front rising above the wire was observed. The heat exchange between the wire and the liquid could also be monitored by estimating the steady temperature of the wire, through the measurement of its resistance. With these techniques, it was found that the electric field may actually impede thermal convection if the liquid contains a sufficient amount of ionizable impurities. The interpretation is based on the space charge which appears wherever a gradient of temperature—hence of conductivity—and an electric field simultaneously exist. A mathematical analysis, made possible by a few simplifying assumptions, shows that an electrostatic attractive force opposes the upward gravitational pull acting on the heated liquid, and satisfactorily predicts the stable position of the heat front above the wire. Other manifestations of this space charge are briefly described.
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