Abstract: A measuring device for measuring the potential, relative to the earth, of a metallic structure buried in the earth, and cathodically protected by an externally applied DC voltage. The measuring device includes the combination of a metallic, rod-shaped probe adapted to be driven into the earth, a major portion of which is coated with an electrically insulating material. A small portion of the probe near the bottom end thereof is left uncoated and constitutes a measuring electrode. A reference electrode is situated in a transverse bore near the bottom end of the probe, and is electrically insulated therefrom. The probe is adapted to be electrically connected to the metallic structure, for measurement of and the potential difference between the measuring and reference electrodes.