Abstract: An apparatus for heat treating coiled springs comprises two electrodes (19,20), an electric current supply unit (10) for feeding a series of electric current pulses through a spring (21) placed between said electrodes (19,20), each of said pulses having the general shape of a portion of a sine half wave, an evaluation unit (11) for evaluating the amplitude of the sine half wave corresponding to each current pulse, and a control unit (12) for controlling the total energy supplied to the spring (21) from said current supply unit (10) in dependence upon said amplitude evaluation. In order to keep the total treatment time equal for all springs within a single series, the control unit (12) is arranged to bring the current supply unit (10) to feed a predetermined constant number of current pulses having low initial values through the spring (21) and to control the length of each current pulse in dependence upon the amplitude evaluation effected by the evaluation unit (11).