Abstract: An inertia switch for sensing shock or vibrations imparted to an object to which said switch is secured. The switch comprises a pair of spaced-apart, electrically-conductive contact plates. Each plate has at least one cavity therein. The plates are held in fixed, spaced, parallel relationship to one another with the cavities facing and aligned with each other to constitute at least one opposed pair of cavities. A metal ball is loosely captive between the at least one opposed pair of cavities so that the ball, when in a condition of rest, establishes four conductive contact points, two with each of the plates.