Abstract: A printed circuit type of substrate (1) has a conductive film (2) on one face which is etched to constitute a first electrode (3) shaped as a printed circuit resistance between two connection tabs (4A, 4B) and a second electrode (5) having a connection tab (6), both electrodes are covered in a film of polymer and points (12) of the second electrode (6) are bared of polymer to put it into direct contact with a porous metal film covering the polymer film which serves as a dielectric between the said porous metal film and the first electrode (3).