An interaction is described where a spatially and temporally periodic distribution of surface charge is induced on a moving fluid interface by means of adjacent electrodes. The fluid convection shifts the surface‐charge distribution downstream, with an attendant shift in the spatial phase of image charges on the electrodes. This phase shift is sensed electronically through the electrodes to provide a quantitative measure of the relationship between surface convection and charge relaxation. It is theoretically and experimentally shown that the fluid velocity can be measured in a way that is not sensitive to the conductivity, and that the conductivity can be measured independent of the fluid velocity. No electrical or mechanical contact with the fluid is required, and hence the interaction is attractive for basic studies of natural conduction in the region of a moving interface while satisfying the requirements for flow and conductivity measurements that may involve either fluids or solids.