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The paper describes a new probe for the measurement of power flow from a source of supply to a load which is characterised by having an extremely low power factor. Power is measured by sensing the electric and magnetic fields in a defined region between the source and the load. A co-axial configuration is chosen to simplify the integration of the Poynting vector that represents the rate of flow of electromagnetic energy. This leads to a simple design that enables the measurement of voltage and current by sensing the corresponding electric and magnetic fields in an integrated structure incorporating both sensors. The new probe is particularly suitable for the measurement of load losses in large, high-voltage power transformers and shunt reactors in which the power factor is extremely low. A small experimental unit is described, together with the associated electronic circuit.