Abstract: An electric field sensor for measuring the electric field outside a body having an electrically conductive surface, particularly a hovering aircraft, consists of a sensor element including a plate of electrically conductive material, a slab of dielectric material having a pair of opposite surfaces, a first of the pair of surfaces being fixed to the plate, and an electrometer means positioned within the body adjacent to a second of the pair of surfaces and arranged for measuring the electric field within the slab of dielectric material, the electrometer being calibrated to indicate the electric field outside the body on the basis of the measured electric field within the slab of dielectric material.