Definitions of base units

Definitions of base units


Metre


The distance travelled by light in vacuum during a time interval of  1/299,792,458 second is called 1 metre.


Kilogram


The Kilogram is the mass of cylinder made of planitum-iridium alloy kept at International Bureau of Weights at Sevres near Paris in France.


Second

The time duration in 9,192,631,770 time periods of the selected transition of radiation of Cesium-133 atom is defined as 1 s.

Ampere
It is the steady current (a unit of measure of the rate of electron flow ) that when flowing in straight parallel wires of infinite length and negligible cross section, separated by a distance of one meter in free space, produces a force between the wires of 2 × 10-7 newtons per meter of length.

The ampere is named after Andre Marie Ampere, French physicist (1775-1836).



Kelvin

A temperature scale in which The fraction 1/273.16) of the thermodynamic temperature of triple point of water is called the Kelvin.



Mole


The amount of a substance that contain as many elementary entities (molecules, atoms and  ions if the substance is monatomic) as there are number of atoms in 0.012 kg of carbon-12 is called a mole.The number is 6.0225 × 1023, or Avogadro's number. Also called gram molecule.




Candela

It is the Standard International (SI) unit to measure the brightness of a source of light (  luminous intensity. one square centimeter of a blackbody  at the freezing point of platinum emits one-sixtieth of a candela of radiation.