Odometer
The instrument panel displays a variety of information for the driver's use. Some dashboards contain a plethora of instruments that report aspects of an automobile's performance, and others hold "idiot" lights that light up only to indicate hazards. Perhaps the two most common instruments are the speedometer and the odometer, and the two are really a single device that gets its information from the turning of the wheels. The speedometer tells how fast the vehicle is traveling in miles or kilometers per hour, and the odometer translates the rotations of the wheels into the distance traveled. Odometers show the total mileage logged by the vehicle, and they may also include a trip mileage recorder that can be reset at the beginning of a journey.
A vast number of inventions and improvements color the short history of the automobile, yet its odometer is one of the oldest instruments still in use. In Rome during the first century B.C., Vitruvius described the odometer in a taxi-chariot that was used to measure distance and, thus, the passenger's fare. In 1500, Leonardo da Vinci created an odometer that consisted of a disc geared to one wheel. This disc dislodged a stone with each complete turn, and the stone fell into a box. The number of stones at the end of the trip reflected the distance traveled.
Early automobiles recorded the total distance up to five figures. A series of dials were mounted on parallel axes. These were geared together so each turned at one tenth of the speed of the preceding dial. A window on the face of the instrument showed the numbers, except that they were hard to read if they were moving in or out of the window. A second display that could be zeroed for trip measurements was difficult to connect to the dial-type odometer.
The drum odometer solved the problems of the dial system. The drum recorder has the numbers 0 through 9 printed on the outside surface of the drum. The inner portion has two sets of gear teeth. Six of the drums are mounted on a center spindle so that the teeth of one drum mesh with those of its neighbor to form a "train" of gears. As the right hand drum makes one revolution, its teeth move the drum to the left by one-tenth of a revolution. The row of numbers across the driver's side of the six drums is always visible, and, by moving the spindle backwards, the drums can be reset to zero for the trip mileage indicator.
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