Two types of airbag sensors used in cars are electrical and mechanical. Electrical sensors vary in design. Some use an electromechanical "ball and tube" mechanism, which basically consists of a small tube containing a circuit switch and ball that's held together by a small magnet. If a collision occurs, the ball is dislodged from the magnet and rolls forward in the tube, hitting a switch that completes the electrical circuit. Other electrical designs are similar in principle, using a metal roller or spring loaded weight instead of a ball, or in newer cars, an accelerometer to trip the sensor. Mechanical sensors work independent of the electrical system and respond similarly to the electrical sensors, with a design that actuates a firing pin triggering a small explosion after a crash. Since a mechanical sensor does not require a power source, it cannot be deactivated like an electrical sensor can when the battery is disconnected