Abstract: In the heat treatment of aluminium alloy strip, in which hardening of the material is effected by heat treatment to dissolve pre-existing precipitates in the metal matrix followed by quenching and ageing, the electrical conductance of the strip after quenching is measured using a conductivity probe scanned across the width of the strip, the heating being controlled in accordance with the measured conductance so as to obtain uniform electrical conductance across the width of the strip. This measured electrical conductance has a close correlation with the required mechanical properties. A transmitter producing an electromagnetic field is moved across the strip and a receiver, picking up signals due to the eddy currents in the strip, provides an output automatically controlling the movement of flux modifiers of a transverse flux induction heater to control the strip heating.