According to the intended use, an electrical conductor (1) having a magnetically sensitive surface (10) subjected to the magnetic force of a permanent magnet (11) is produced, the area of this surface varying according to its relative position with respect to the magnet, said relative position between the magnet and the magnetically sensitive surface is varied, which causes at least one physical characteristic of this conductor, to vary, and said variation in physical characteristic(s) of the electrical conductor is recorded, this variation being in correlation with the position of the magnet (11).