Abstract: The strip is pulled in its longitudinal direction over two parallel supports, which extend transversely to and are spaced apart in said longitudinal direction. Said strip is subjected between said supports to magnetic forces which are approximately at right angles to the surface of said strip and are exerted by a plurality of solenoids, which are distributed across and clear of the strip and which comprise exciter windings fed with a predetermined alternating current, whereby a local deflection is produced in the strip adjacent to each of said solenoids, which deflections differ in a non-planar strip, in which the tensile stress differs across the strip. The voltage is sensed across each exciter winding during an interval of time shorter than one-half the period of the voltage and including a crossover of the alternating current. Values related to said local deflections are derived from the voltages across the exciter windings of said solenoids.