An electrical switching device consisting of an oval glass bulb which contains two electrical contacts (penetrating the bulb, and at one end of the oval) and a drop of mercury. In hobby rocketry, mercury switches are used to detect the absence of acceleration in a moving rocket. When the rocket launches, acceleration forces hold the mercury drop at the bottom of the bulb, away from the electrical contacts. When the rocket's acceleration ceases, the mercury - which is a heavy metal and stores considerable momentum - moves forward in the bulb, bridges the gap between the contacts, and closes the electrical circuit. The electrical circuits controlled by mercury switches are most commonly used to activate recovery system deployment charges or sustainer motor igniters.