Wikipedia
A mercury switch (also known as a mercury tilt switch) is a switch which opens and closes an electrical circuit when it is tilted at certain angles. When it is tilted, a small amount of the liquid metal mercury makes contact with metal electrodes to close the circuit.
Usage examples of "mercury switch".
The gadget this time might be something fancy like having to sing A above high C, or stepping on a flat button under the carpet, or maybe you just spoke and said: Let there be light, and a mike picked it up and turned the voice vibration into a low-power electrical impulse and a transformer built that up to enough voltage to throw a silent mercury switch.
Vary your route to the office every day, never set patterns, spend the five to six minutes every morning to see if they've wired a mercury switch and C-4 to your windshield wiper, phone in fake restaurant and airline reservations, change your plans constantly, and at the last minute, reschedule, cancel again.
He saw Tommy plug the grid-dip meter into an electric outlet which was fitted with a silent-type mercury switch.
It armed a mercury switch that would fire the charges if anyone tried to drag the vehicle out or if it took a heavy impact--like the other armored car detonating.
They should have found a radio, or a mercury switch, or a simple timer-anything by which the explosion could be triggered.