A sensor according to the invention has an amplifier that is capacitively loaded vis-à-vis the ground by a container and by a medium contained therein and that has a capacitive feedback that is dimensioned in such a way that the amplifier only oscillates when the critical filling level has not been reached. According to the invention, the capacitive feedback is influenced by the container in such a way that the capacitive load of the amplifier input caused only by the container is countered. In this manner, it is achieved that the sensor responds only to the capacitance of the medium, largely independently of the magnitude of the container capacitance. A sensor according to the invention is also characterized by such an operating frequency that, on the basis of the conductivity of the medium, it is possible to distinguish between a massive and an apparent filling of the container with the medium simulated by wetting or foam formation, but it largely avoids those problems encountered with sensors having a very high operating frequency of more than 50 MHz.