What is SI unit of length meter
The basic unit of length in SI units is meter (also spelled as metre in British English). Most of the countries in the world adopted meter as the unit of length. One meter is approximately equal to 39.3701 inches. The meter is the basic unit of length in the International System of Units (SI).
The standard length of one meter was defined by the French Academy of Sciences 1791. Before 1791, there were two definitions to a meter. One definition was based on the length of a pendulum and the other was based one ten-millionth of the distance from the Equator to the North Pole. The French Academy of Sciences in 1791 selected that one meter would be one ten-millionth of the distance from the Equator to the North Pole.
During 1797, many marble samples of meter were installed in Paris to promote the use of meter as the unit of length. Following image shows one of it.
International Metre Commission was held at Paris 1872 with the participation of thirty countries. Later, thirty bars of meter length were made using 90 percent platinum and 10 percent iridium alloy for different countries participated. 90% platinum and 10% iridium alloy is harder than pure platinum. Those bars were made with a special X-shaped cross section called as Tresca section to reduce the effects of torsional strain during handling.
No. 6 of the thirty bars is kept as the international prototype. Remaining bars were calibrated and distributed to different signatory countries.
Image Credit: NIST
Later in 1960, at 11th CGPM (General Conference on Weights and Measures) agreed a new definition of the meter. meter was redefined as the length equivalent to 1650763.73 times the wavelength of orange-red light emitted by krypton 86 isotope in vacuum.
Today there are instruments to calculate exact speed of light in vacuum. In 1983 by the CGPM (General Conference on Weights and Measures) accepted that the value speed of light in vacuum is 299,792,458 meters per second. Means that the light will travel 299,792,458 meters at a time interval of one second in vacuum.
So, the exact length of one meter is now calculated based on the speed of light in vacuum. If the light can travel ’x’ units of length in meters in one second in vacuum, then the accurate length of 1 meter is the distance travelled by light in 1/x seconds in vacuum.
The value of one meter is now redefined as the distance traveled by light in vacuum in 1 over 299,792,458 of a second.