Abstract: First and second contact bars are spaced apart in fixed positions. A pressure responsive ram has first and second positions of penetration into a casing which contains the bars and other elements. A bolt is disposed between the ram and a first element which is pivotable about the second bar between a first position and a second position. A second elemtn is coupled to the first element. When the ram is in its first position, the bolt places the first element in the first position causing the second element to establish a current carrying path between the bars. When the ram is in its second position, the bolt places the first element in the second position, causing the second element to break the current path. A device disposed between the belt and the ram exerts a biasing pressure on one end of the ram which maintains the ram in its first position. When a larger pressure is exerted on the other end of the ram, the ram is moved into the second position.