Abstract: An inertia mass is supported in a housing for movement along a given path. Two pair of spring blades are cantilever-mounted in the housing. Movement of the mass in one direction closes the first pair of blades and movement in a second direction closes the second pair of blades. Adjustment members are mounted in the housing and bear against one of the blades in each pair to adjust the switching sensitivity. The blades and a warning member, such as an electric light, are mounted in the base of a channel shaped member which snaps into an outer housing shell. A transparent cap is mounted on the housing shell making visible the warning light. Housing flanges are convexly formed and have a plurality of parallel ribs which may be adapted to a large variety of supporting flanges. In one embodiment, the mass is slidable on an elongate rod. Also, the mass may be suspended on two of the spring blades.