Odometer and Speedometer

An odometer registers the distance traveled by a vehicle. An odometer consists of a train of gears (with a gear ratio of 1,000:1) that causes a drum, graduated in 10ths of a mile, to make one turn per mile. 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 1 revolution of the second, and so forth; the numbers appearing in the window represent the vehicle's accumulated mileage. A speedometer is an instrument that indicates the speed of a vehicle. The speed-indicating mechanism of the speedometer is run by a circular permanent magnet that is rotated 1,000 revolutions per mile of vehicle travel by a flexible shaft driven by gears at the rear of the transmission. The magnet turns within a movable metal cup that is attached to the shaft carrying the indicator. As the magnet rotates, it exerts a magnetic drag on the cup that turns i t against a spiral spring. The faster the magnet rotates, the greater the pull on the cup and the pointer. Thomas Jefferson was the first to use the word odometer in writing in 1791; speedometer did not appear in writing until 1904.