Adjusting pin for a ferro-magnetic core

An adjusting pin adapted to be axially displaceable in a cylindrical opening recessed in a ferromagnetic core, said adjusting pin comprising a ferromagnetic body secured to a body of synthetic material, the body of synthetic material having a rigid head on one end of the adjusting pin with the largest transverse dimension of the head being smaller than the diameter of the cylindrical opening of the core, a first end portion of the head facing the end of the adjusting pin while the second end portion of the head faces the ferromagnetic body, a plurality of rigid non-deformable longitudinal ridges being formed on said head which taper uniformly over at least a part of their length, the thickest part being situated near the first end portion of the head, the diameter of a circumscribed circle of the head with longitudinal ridges being larger near the first end portion and being smaller near the second end portion than the diameter of the cylindrical opening of the core.