Abstract: A resettable impact switch is disclosed having a non-conductive body with a cavity therein. A pair of terminals extend from opposite sides of the body and have their reverse ends extending into the cavity. A pool of mercury is disposed in the cavity to electrically connect both terminals under normal operation. The pool of mercury is held in a contact-making position within the cavity by a plurality of concentrically disposed tubular members. Longitudinal, relative movement of the tubular members allows openings in their walls to be covered or uncovered to selectively position the mercury, thereby enabling opening or closing a circuit between the terminals. One cylindrical member is carried by a post member which is adjustable within the body. The post member also carries a second contact for making or breaking electrical contact between the terminals.