Well, an odometer is just such a gauge for cars, bikes, and other vehicles. Whether it is a mechanical or an electronic device, an odometer tells how far a vehicle has traveled.
Today is NOT Odometer Day because today marks the anniversary of the first invention of an odometer. No, that day is lost in the mists of time. We know for sure that Ancient Greeks, Romans, and Chinese used odometers, and apparently the odometer has been invented many times, independent of each other. Who knows? Perhaps a crude odometer was invented by someone, somewhere, soon after the invention of the wheel!
We know that Englishman Thomas Savery invented an odometer that could be used on ships, and we know that Benjamin Franklin fashioned a simple odometer to measure the mileage of routes he traveled as Postmaster General under the Continental Congress. We know that a Mormon pioneer named William Clayton wanted to keep track how far his wagon traveled each day. He determined that 360 revolutions of his wagon wheels made a mile, so all he had to do was count wheel revolutions. Clayton tied a red rag to one of his wheels and started counting. It wasn't too many days before Clayton tired of this method of measuring distances. He discussed the problem with another pioneer, and they came up with an idea of fixing wooden cog wheels to the hub of a wagon wheel in such a way that they could tell the mileage. A third man built the device, which they called a roadometer. (Even though they weren't traveling on roads!)
The Mormon-invented roadometer was first used on May 12, 1847. Hey! That's where we got the May 12 date for Odometer Day!
Read the How Stuff Works article to find out how modern odometers work.
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