An instrument (1) for measuring the loss factor of an electrical apparatus (2) having an axially extending elongate geometry, in particular a medium voltage cable, comprises: at least a first and a second sensor (3A, 3B) connectable to the apparatus (2) in a first and a second measuring section (4A, 4B) axially spaced along the apparatus (2) by a predetermined quantity, respectively, and designed to measure respective values of the magnetic field generated outside the apparatus (2) by a load current flowing axially through the apparatus; a processing unit (5) designed to receive the magnetic field values measured by the sensors (3A, 3B) and programmed to derive from them a load loss value as a function of the difference between the measured magnetic field values.