The Units in Detail

The origins of the meter go back to at least the 18th century. At that time, there were two competing approaches to the definition of a standard unit of length. Some 

In 1927, the meter was defined as the distance between the axes of the two central lines marked on the bar of platinum - iridium kept at the Bureau International des Folds et Mesures (BIPM) under certain conditions, such as temperature and pressure. In 1960 a definition came up, based upon the wavelength of Kr radiation. In 1983, the CGPM replaced this definition by the following definition [14]  

The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. This definition is based on the constant velocity of light in vacuum at exactly 299792458 m s .  

Around 1800, one kilogram was defined as the mass of a cubic decimeter of water. In 1901, the kilogram was defined as the mass of a certain standard, made of platinum -iridium. This definition is still waiting for a replacement. 

Originally, the second was defined originally as the fraction 1/86,400 of the mean solar day. However, unpredictable irregularities in the rotation of the earth were discovered. In 1960 a definition, based on the tropical year, was presented by the International Astronomical Union. In 1997 the following definition was agreed  

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