Electro-mechanical electricity consumption meter which enable external attachment of an optical monitoring assembly

An electro-mechanical electricity consumption meter (1) of a type having a sealed housing (2) to prevent tamper therewith is described. The meter (1) is adapted to include a rotatable component (11) which is visible through a portion (23) of the sealed housing (2) and whose rate of rotation is related to the rate of consumption of electricity. The component (11) has at least one marking (15) on a surface thereof such that the rate of rotation of the component can be determined using a monitoring assembly (24) located outside the sealed housing (2) without the need to access the interior of the meter (1). The, or each, marking (15) comprises a visually distinctive boundary between two surface areas (15, 16) of the rotatable component in which the boundary is transverse to the direction of movement of the component. In an alternative arrangement, an existing component (11) is adapted to have markings (15) on its surface. In a particularly preferred arrangement the rotating component comprises a part (11) of the magnet suspension arrangement of the rotor assembly (6).