As we know that funamental units cannot be defined in terms of other unit..... isnt it that the unit of metre is defined in terms of time required by the speed of light

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farcher · Answer

"As we know that funamental units cannot be defined in terms of other unit." This statement is not correct and even before the definition of the metre in terms of the speed of light and the second one had the ampere, the mole and the candela depending on other fundamental units. What scientist want is a system of units which can be accurately reproduced without there being a problem if the standard is damaged or destroyed. This link http://www.bipm.org/en/scientific/mass/avogadro/ will take you to an overview of what is being done at the moment to replace the lump of platinum/iridium which is the prototype kilogramme.