Abstract: Instrument for the non-destructive detection of contamination on the surface of a metallic object. A reference electrode contains sufficient radioactive material to ionize the air in a gap between the electrode and the metallic object. A buffer amplifier has its input terminals connected to sense the surface potential difference between the electrode and the metallic object. The air gap is typically from one to ten millimeters wide. The input circuit of the buffer amplifier has a resistance very much larger than that of the air gap when the air is ionized. The buffer amplifier drives an alarm device or a display device. A single voltage pick-up is conveniently embodied in a portable hand-held probe. A plurality of voltage pick-ups is incorporated into an array for simultaneously making multiple measurements of surface condition over a large area.