Abstract: The level of the interfacial boundary between two liquids having different dielectric constants is located by means of a detector in which two parallel sectional members constitute external capacitor plates supported by a metallic base plate and in which small metallic plates encased within an electrically insulated central upright member constitute central capacitor plates. The assembly thus forms a plurality of capacitors, the plates of which are immersed in at least one of the liquids. Each capacitor forms part of an oscillating circuit connected to electronic means for effecting a comparison between the oscillation frequency and a reference frequency.