Digital Odometer: Some cars have a magnetic sensor that detects turns of the transmission shaft. The signal is transmitted to a computer, which calculates and displays the car’s distance traveled. The components of the odometer and their role: Coil spring: The spring holds the pointer at zero when the car and the magnet are at rest. Pointer: The pointer is attached to the drag cup. The faster the magnet spins, the greater the angle the drag cup turns. The higher speed is shown by the pointer. Cable: A cable linked to the transmission rotates at a rate directly proportional to the road speed. Worm gears: The worm gears reduce the cable’s rotational speed and move the odometer dials. Drag cup: The drag cup turns from its resting position through an angle that increases with the magnet’s spin rate. Magnet: The magnet is attached to the shaft. As the shaft spins the magnet, a magnetic field exerts force on the drag cup. Measurement: For each full turn of the worm gear the odometer moves up one digit, indicating that the car has traveled one tenth of a mile.