An automobile’s speed is measured by a speedometer fitted next to the steering wheel of a car. It indicates the vehicle’s speed in kilometers per hour or miles per hour. The speed is read on the dial which is numbered from 0 to 160, by means of a pointer. Most speedometers also incorporate an odometer – a device that records the distance travelled by the vehicle.
A speedometer is driven by a flexible cable that is connected to a set of gears in the vehicle’s transmission. When the vehicle moves, the gears turn a core or flexible metal shaft inside the cable. The core turns a magnet inside a metal drum called a speed cup. This is located inside the speedometer housing. The revolving magnet exerts a turning force on the speed cup. In turn the speed cup is held back from revolving freely with the magnet, by the opposing action of a hairspring. The movement of the speed cup is transferred to the pointer on the dial. The hairspring brings the pointer back to ‘zero’ when the vehicle stops moving. Most of the speedometers register 36 km/hr when the core inside the cable revolves at 1000 revolutions per minute.
The odometer registers total kilometers travelled by the vehicle. Some automobiles also have Trip odometers that can be reset to ‘zero’ at the beginning of a particular trip. An odometer consists of a chain of gears (with a gear ratio of 1000 : 1) that causes a drum, graduated in 10th of a mile or kilometer, to make one turn per mile or kilometer. A series, commonly of six such drums, is arranged in such a way that one of the numerals on each drum is visible in a rectangular window. The drums are coupled so that 10 revolutions of the first cause one revolution of the second and so forth, the numbers appearing in the window represent the accumulated mileage.
