Abstract: Measurement of a mechanical characteristic of a long metallic product. The long product is passed through at least one turn of an electromagnetic coil energized with a variable current, the impedance of the electromagnetic coil is measured continuously and the mechanical characteristic of the long product or variations therein are determined by comparing the measured impedance with at least one predetermined impedance value. The method can be used in particular to regulate the production of hyperquenched austenitic steel spring wire containing martensite (.alpha.') by drawing the wire through successive dies. The impedance of the coil through which the wire passes when it leaves the die is measured in order to regulate at least one of the following parameters: the temperature of the die and the wire and the wire speed.