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Conduction and polarization mechanisms in the oil and paper of a power transformer are discussed in this article. Some of the electrical characteristics of an insulating oil sample can be determined accurately at room temperature using the IEC 61620 test procedure. High voltage stress and high temperature may modify these characteristics. Deterioration products, both acid and non-acid types, influence the measured conductivity, but moisture or contaminants do not do so. Reasonable upper limits have been suggested for new and in-service oil. Pressboard moisture content and conductivity of the oil in the main ducts of a transformer may be measured using dielectric response techniques, which also give information on the state of the insulation at interfaces. Measurement of polarization and depolarization currents in a transformer without oil allows conduction and polarization phenomena occurring in the insulating paper to be identified. The depolarization current characterizes the type of aging in the paper, provided the applied voltage stress is very low.