and it is kept separated at least in part from one of the electrodes 3 by insulating gaps as is visible in fig2 in the other case permanent electrical polarization shown diagrammatically in fig4 the foil 1 possesses piezo electrical properties and it is contiguously enveloped by the two electrodes 2 and 3 each of the electrodes 2 and 3 is advantageously constituted by surface metallization of a foil of plastics material in the case of the charged sensitive foil fig2 one 2 of these electrodes is attached to this foil 1 whilst the other electrode 3 held slightly separated from said foil 1 by any desirable means is attached to an additional plastics foil 4 in the case of the polarized sensitive foil 1 fig4 the two electrodes 2 and 3 are attached respectively to the two surfaces of this foil 1 the assembly of the three foils 1 2 and 3 is itself positioned between two external protective foils 5 and 6 whose thickness is advantageously comprised between 0 1 and 0 5 mm constituted of an electrically insulating flexible material these protective foils 5 and 6 extend beyond the assembly of the three foils 1 2 and 3 and their projecting margins 7 and 8 fig1 are assembled against one another so.