An electrical switchin' device consistin' o' an oval glass bulb which contains two electrical contacts (penetratin' t' bulb, and at one end o' t' oval) and a drop o' mercury. In hobby rocketry, mercury switches are used t' detect t' absence o' acceleration in a movin' rocket. Ahoy! When t' rocket launches, acceleration forces hold t' mercury drop at t' bottom o' t' bulb, away from t' electrical contacts. Aye aye! When t' rocket's acceleration ceases, me bucko, t' mercury - which is a heavy metal and stores considerable momentum - moves forward in t' bulb, ya bilge rat, bridges t' gap betwixt t' contacts, and closes t' electrical circuit. Begad! T' electrical circuits controlled by mercury switches are most commonly used t' activate recovery system deployment charges or sustainer motor igniters.