The unit to measure length is the fundamental unit in mechanics. An institute named ‘General Conference on Weight and Measures’ was set up and the authority to decide the units by international agreement.

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It holds several meeting to change any standard unit, which are communicated through the publications of the conference. The unit chosen for measuring any physical quantity should be satisfying some requirements such as it should of suitable size, it should be easily accessible, it should not change with time and any other physical conditions, etc.

Units to Measure LengthThe standard unit to measure length in the metric system is metre. The other units of lengths and their equivalent values in meters are as follows:1 millimeter = 1/1000 metre1 centimeter = 1/100 metre1 decimetre = 1/10 metre1 decametre = 10 metre1 hectometre = 100 metre1 kilometre = 1000 metre1 megametre = 1000000 metre1 gigametre = 1000000000 metre1 terametre = 1000000000000 metreThe other units to measure length are foot, yards, inches, etc.1 inch = 2.54 cm1 foot = 12 inches1 yard = 3 foots

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Definition of the Standard Unit of Measure LengthAs we discuss that metre is the standard unit of measure length, so here we define one metre. The standard metre is exactly equal to 1650763.73 wavelengths in vacuum, of radiation from krypton isotope of mass 86. Krypton – 86 emits light of different wavelengths.

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The light emitted by Kr – 86 due to transition between 2p10 and 5 d 5 levels is orange red in color and has wavelength 6.0578021 x 10-7 m. The number of these wavelengths in 1 m is 1650763.73. The standard metre is so defined because Kr – 86 isotopes can be obtained very easily and cheaply as compared to pure form. Therefore, the standard is easily reproducible. It is sharply defined and it is not affected by time or variation of any other physical conditions such as pressure, density, humidity etc. The new definition of one metre is the length of the path covered by light in vacuum in 1 / 299792458 of a second.