Each fundamental quantity in the metric system is defined in terms of a naturally-occurring phenomenon, except for the mass standard.

 

The meter is the length of the path traveled by light in a vacuum during a time interval of 1/299 792 458 of a second.

 

The second is the duration of 9 192 631 770 cycles of the radiation associated with a specific transition of the cesium 133 atom.  The second is obtained by tuning an oscillator to this resonance frequency in an atomic clock.

 

The ampere is that current which, if maintained in each of two infinitely long parallel wires in free space, would produce a force between the two wires due to their magnetic fields of 2 x 10 ^-7 newtons for each meter of length.

 

The candela is the luminous intensity in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10¹² hz and that has a radiant intensity in that direction of 1/683 watt per steradian.

 

The kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.  The temperature 0K is commonly referred to as "absolute zero." On the Celsius scale, water freezes at 0ºC and boils at 100ºC.  One Celsius degree is an interval of 1 K.

 

The standard for the unit of mass, the kilogram, is a cylinder of platinum-iridium alloy kept by the International Bureau of Weights and Measures, near Paris. A duplicate in the custody of the National Institute of Standards and Technology serves as the mass standard for the United States. This is the only fundamental unit still defined by an artifact.

 

The mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilograms of carbon 12.