Abstract

A mercury switch assembly is adapted to be secured to an anterior horizontal surface of a patient and connected to an electrical alarm system. The switch assembly comprises a head chamber and a foot chamber connected by a throat passageway. A ball of mercury moves from the head chamber through the throat passageway and into the foot chamber to complete an electrical circuit when the switch assembly is tilted from horizontal by a predetermined degree of inclination. The head chamber is generally elliptical with lateral extensions. The throat passageway is narrow and shallow. The foot chamber is deep and rounded. The ball of mercury normally lies within the head chamber in the horizontal position, but establishes electrical connection between a novel form of terminals contained in the foot chamber when the tilt threshold of the switch assembly is reached. The foot chamber retains the mercury ball in contact with the terminals despite subsequent movements of the switch assembly until the switch assembly is removed from the patient and inverted to place the foot chamber vertically above the head chamber thereby causing the ball of mercury to return by gravity to the head chamber. Critical ranges of dimensions and angles of interior surfaces are provided.