A mercury switch which is actuated only on a specific movement of an individual by design of an internal cavity of the switch to control movement of a mercury ball into and out of engagement with two electrical contacts. The internal cavity has a truncated cone for receipt of the mercury ball, a surface of revolution sloping outward from the opening of the truncated cone and an interruption ramp in this surface of revolution to guide the mercury ball into the truncated cone for actuation of a switch when a critical angle of the switch has been exceeded.