Abstract: A sensing probe unit includes an electrically nonconductive polymeric layer charged with a permanent electric charge to form an electret and intimately bonded to a compensating metal electrode. In the fabrication, the polymeric layer is cleaned and the metal electrode affixed to one surface by vapor deposition, lamination or baking. The polymeric layer may also be applied by coating the electrode with dielectric particles and then heating the subassembly to fuse the particles into an integral layer and to bond the fused layer to the electrode. The subassembly is thermally destressed and the polymeric layer is thereafter charged by triboelectric charging, corona discharge or electron beam charging, or by liquid contact charging processes. In the latter, a Freon-type or other similar liquid having good wetting properties and high evaporation rate is used as a contact charging liquid which creates a particularly stable electret. The sensing probe is stored to prevent loss of charge and sensitivity.