A sturdy square bar of soft iron (a magnetic shunt), fixed to a brass support by means of a screw, transversally joins the end of the shorter arm to that of the longer one which lies on the former; starting from the position in which it is perpendicular to the two arms of the magnet, it can be rotated up to a maximum of forty-five degrees, remaining constantly in contact with the two arms. Once the tension on the antagonist spring and the positions of the contacts have been regulated, the relay is brought into the position of maximum sensitivity and rapidity of action by orienting the shunt so that the armature's rest position is indifferent with respect to the two contacts.