Small electrodes functioning under convection-diffusion control can be used as hydrodynamic sensors as they are sensitive to flow variations. Analysis of the statistics of the flow fluctuations requires the knowledge of the transfer function between the mass flux and the velocity gradient. Calculation of these functions in the streamwise and in the spanwise directions with respect to the average flow direction is presented here. An experimental measurement of the transfer-function in the streamwise direction with a cone-and plate system confirms the theoretical predictions. As an application, the measurement of the power spectrum density of the wall velocity gradient fluctuations of the turbulent flow in a rectangular duct in the streamwise and in the spanwise directions are reported.