Car air bag sensor. Coloured scanning electron micrograph (SEM) of the surface of a micro- accelerometer from a car's air bag. A pencil tip is shown for scale. The mauve squares are tiny pressure sensors which detect the rapid deceleration which occurs in a crash. They trigger a circuit which starts the rapid inflation of a plastic bag with air or nitrogen gas. The air bag deploys between the driver and the steering wheel in about one fifth of a second, preventing injury as the driver is thrown forward. Magnification: x35 at 7x4.5cm size.