Inside an accelerating car, the string will swing to the rear as the car speeds off, and it will swing forward when the brakes are applied. The protractor measures the angle through which the string moves as the car accelerates or slows down, and that angle is used to calculate the force of gravity, or G-force, on the driver and passengers. Since weighted strings have obvious shortcomings in real-life devices like airbag triggers sophisticated accelerometers measure the deflection of materials like piezoelectric crystals, which produce electricity when they are bent. In an accelerometer, the crystals are positioned so that acceleration or deceleration puts pressure on them, causing them to bend. Greater acceleration or deceleration increases the G- force and causes the crystal to produce more electricity.
Microprocessors amplify the electricity and measure it to calculate the G-force.
One installed in a washing machine, for example, could cost more than the washing machine itself. The company builds up a chip by etching layer after layer of silicon. The result is an extremely small variation of the string and weight. At the heart of this tiny accelerometer chip is a microscopic weight of about 0.07 micrograms suspended on springs that are even smaller. The weight has several tiny projections that carry a small electrical charge.