Abstract: A mechanism for the instantaneous tripping of an electrical contact in response to a progressive variation in applied fluid pressure. Contact is made by a conductive element movable along its longitudinal axis between an open position and a closed position. The upper portion of the interior wall of the contact element comprises opposed first and second sloping sections of substantially triangular cross-section having a side in common at their connection. A throttle is defined by the connection between the sloping sections. A cage member partially positioned within the conductive element is movable therein in response to the applied fluid pressure. The cage member includes a head portion having a channel therethrough perpendicular to the longitudinal axis of the conductive element. At least one ball is housed in the channel and is movable along the longitudinal axis of the channel and up and down the sloping sections in response to the movement of the cage member within the conductive element.