What is the definition of the meter and the inch?
The current definition of the meter (m) is the SI unit of length and is defined as the length of the path traveled by light in vacuum during the time interval of 1/299 792 458 of a second. For the history of the meter, visit these NIST web sites: Historical Context of the SI: Unit of Length (Meter) Length: Evolution from Measurement Standard to a Fundamental Constant This number was chosen to match up as closely as possible to the old definition of the meter, which was the length of a certain number of wavelengths of Krypton86 radiation (light) from an electrical discharge lamp in a vacuum. In practice the new definition of the meter cannot be used directly in a convenient and accurate manner for laboratory-scale measurements, because it is too difficult to do the time of flight measurement. We still count wavelengths, just like we did for Krypton. In the new scheme of things, however, you can use one of many light sources, the uncertainty in the meter depending on how well characteriz
