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The experimental determination of electric potential in gaseous insulating materials is performed using a corona probe. This method can be used to measure point values of electric potential from dc to ac at industrial frequency values. The proposed method, after an initial calibration of the probe, allows the determination of the amplitude of the electric potential at the point in space where the tip of the probe is located. The measurement is performed by adjusting the feeding potential of the probe until a complete corona suppression is obtained. To discriminate the corona effect of the probe from other possible corona sources, a device for probe current detection is proposed. The corona probe method has been verified by measuring electric potential, under both dc and 50-Hz ac conditions, in known field configurations and in natural and controlled environments. Finally, the proposed method has been applied to determine the space distribution of the electric potential around high-voltage insulating structures under dc and 50-Hz voltages in a natural environment.