Abstract: An adjustable electromagnetic tripping mechanism for a circuit-breaker, includes in each pole, a fixed magnetic circuit excited by the current passing through the said pole. A movable magnetic member is held by the force of a return spring against a stop at a predetermined distance from the fixed magnetic circuit. A movable adjusting element bears the stop and includes a point of support or of attachment of the said return spring, which enables the limiting operating current of the circuit-breaker to be varied. The movement of the said movable adjusting element modifies at the same time and in the same sense of variation, firstly the distance predetermined by the stop between the movable magnetic member and the fixed magnetic circuit, and secondly the force exerted by the return spring upon the movable magnetic member. The invention is characterized in that the movable adjusting element consists of a small plate of generally plane form guided in translation in slides.