SI Units

I was looking through one of my chemistry textbooks and i found a page that defined the definitions of all the SI units. And so I started wondering things like, "how did they first decide how big a metre was?", or "how did they determine how long a second should last for?". So i read on, and found the definitions to be so interesting that i wanted to put them on my blog. (Only the most interesting ones tho. Cos they are interesting, i think anyway;P)

Metre: the metre is the length of path travelled by light in vacuum during a time interval of 1/(299 792 458) of a second.

Kilogram: the kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

Second: the second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom.

Ampere: the ampere is that constant curren which, if maintained in two straight parallel conductors of equal length, or negligible cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10^-7 newton per metre of length.

Kelvin: the kelvin, the unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Candela: the candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^12 hertz and that has a radiant intensity in that direction of (1/683) watt per steradian.

Mole: the mole is the amount of substance of a system which contains as many elementary entities as there are carbon atoms in 0.012 kilogram of carbon-12. The elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

(Source: Awlward and Findlay. SI Chemical Data. John Wiley and Sons Australia Ltd, 2002.)