A current threshold sensor

A novel current threshold sensor of simple construction utilizes the precipitous reversal of magnetization within a cylindrical ferromagnetic shell endowed with a helical easy axis. When driven by a coaxially conducted alternating current of sufficient magnitude, the sense of the helical magnetization is periodically reversed. Emf pulses induced by these reversals in a coaxial solenoid distinguish currents slightly above or below a threshold value. An eminently practical source of the helical anisotropy is torsional stress. Residual stress distributions resulting from torsional overstrains during processing of wire or rod, create a helical easy axis without requiringexternal torque. The reduction of the effective coercive field with increasing torsional stress in many materials, allows for the continuous adjustment of the threshold value. Minimum current thresholds of about .3 A are determined by the need for immunity from ambient fields and limitations on current density. Maximum thresholds may reach 300 A. Thermal stresses within composite materials contribute to the temperature coefficient of threshold current.