Unit of Length the Meter

The unit of length, the meter, was defined in the first CGPM approval in 1889 by an international prototype the length of a bar made of a platinum-iridium alloy defined a length of 1 m. In 1960 this definition was replaced 

Base quantity Symbol for quantity Unit Symbol for unit 

The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 k58 of a second. 

As a result of this definition, the fundamental constant speed of light in vacuum c is fixed at exactly 299 792458 m/s. 

Since the first CGPM in 1889, the unit of mass, the kilogram, has been defined by an international prototype, a metal block made of a platinum-iridium alloy, kept at the BIPM at Sevres. The relevant declaration was modified slightly at the third CGPM in 1901 to confirm that The kilogram is the unit of mass it is equal to the mass of the international prototype of the kilogram. 

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A unit of measurement is an agreed-upon standard with which other values are compared. Scientists use the meter as the standard unit of length. The meter was originally chosen to be 10 times the length of a line from the North Pole to the equator. Volume can be measured in pints, quarts, and gallons, but the scientific units are the cubic meter and the liter. Temperature can be measured in degrees Fahrenheit (°F), degrees Celsius (°C), or kelvins (K). 

For instance, a length may be expressed as a multiple of the unit of length, the meter, m so we write / = 2.0 m if the length is the product of 2.0 (the multiple) times the unit. All units are denoted by Roman letters, such as m for meter and s for second, that distinguish them from the physical quantity to which they refer. 

In 1960 it was internationally agreed that the fundamental unit of length, the meter, would be defined in terms of the orange-red spectral line of krypton-86. In fact, 1 meter is exactly 1,650,763.73 wavelengths (in vacuo) of this line. This agreement replaced the standard platinum-iridium meter located in Paris. 

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