# Odometer

An odometer comes from the Greek word hodós, meaning "path" or "way" and métron, "measure". It is a device used for indicating distance traveled by an automobile or other vehicle. It may be electronic or mechanical.

Mechanical odometers usually appear as a row of wheels with the edge of the wheels towards the person viewing it. There are digits written on the edge of these wheels. A mask obscures these wheels from view, except for one row of digits which can be seen through a window in the mask.

In the U.S., odometers on older cars could only indicate up to a value of 99,999. At 100,000, the odometer would restart from zero. This is known as odometer rollover. Newer cars usually have odometers that can indicate up to a value of 999,999.

A common form of fraud is to tamper with the reading on an odometer. This is done to make a car appear to have been used less than it actually has been, to get a higher price for the car.

## History

An odometer for measuring distance is described by Vitruvius around 27 and 23 BC. The actual invention may have been by Archimedes during the First Punic War. Hero of Alexandria describes a similar device in chapter 34 of his Dioptra. In ancient China, independent of the Greeks, the scientist Zhang Heng invented the odometer.

Chariot wheels of 4 feet (1.2 m) diameter turn 400 times in one Roman mile (about 1400 m). On each revolution a pin on the axle engaged a 400 tooth cogwheel thus turning it one complete revolution per mile. This engaged another gear with holes along the circumference, where pebbles (calculus) are located, that drop one by one into a box. The distance travelled is given simply by counting the number of pebbles. Whether this instrument was actually built is disputed. Leonardo da Vinci tried to build it according to the description but failed, however, Andre Sleeswyk was able to make a working model using gears similar to the Antikythera mechanism as opposed to the traditional cogwheel.

The odometer used in most modern systems, where separate gears control each digit, was invented by William Clayton with help from Orson Pratt. Clayton, a Mormon Pioneer, developed the odometer (dubbed the "roadometer") to keep track of wheel revolutions on the pioneer carts. The odometer had multiple gears, including one which turned every quarter-mile and one which turned every ten miles.

## References

• Sleeswyk, André Wegener "Vitruvius' Odometer", Scientific American 245.4 (October, 1981), pp. 188-200
• Sleeswyk, Andre W. "Vitruvius' Waywiser", Archives internationales d'histoire des sciences Vol. 29 (1979), pp. 11-22.

• Art and Cultures
• Countries of the World (http://www.academickids.com/encyclopedia/index.php/Countries)
• Space and Astronomy