A sensor (S) for marine measurements of an electric field, the sensor (S) including at least two electrodes (3, 4); signal transmission means (5) for transmitting measured signals from the sensor (S) to a signal processing (6); at least two closed containers (1, 2) which are formed of a non-conductive material and are filled with an electrolyte (E); at least two flexible hoses (7, 8) formed of an electrically non-conductive material; there being attached in a fluid-communicating manner to each of the containers at least one first hose end (7a, 8a), and a second hose end (7b, 8b) being open and attached to means (9a, 9b) for exact positioning of the second hose end (7b, 8b); the hoses (7, 8) being arranged to be filled with a medium (W) of the same type as that, in which the sensor (S) is arranged to be immersed in an operative condition; and two containers (1, 2) forming a pair of containers, the two containers (1, 2), relatively, being placed close to each other under approximately identical thermal, pressure and chemical conditions.