Abstract: Apparatus for detecting the amount or presence of material in a detection zone comprising a first electrode, input signal means for supplying an electric input signal to the first electrode; a second, smaller electrode parallel to the first electrode; the detection zone being located between the first and second electrodes; a third electrode which is larger than the second electrode and substantially surrounds the second electrode in a plane parallel to the first electrode, and receiving means for detecting signals on the second electrode, the signal on the second electrode being responsive to material in the zone. The second and third electrodes are not electrically connected but are maintained at near the same electric potential whereby the electric field between the first and second electrodes is, in use, substantially linear and fringing fields, if any, occur only between first and third electrodes at their outer edges.