SI is the International System of Units and consists of meter (m), kilogram (kg), second (s), Ampere (A), Kelvin (K), mole (mol), and candela (cd) which refer to the unit of length, mass, time, current, temperature, amount of substance, and luminous intensity. SI is used by most countries including South Korea and all scientist, so it demands precise definition. SI is adopted by CGPM (General Conference of Weights and Measures) hold by BIPM (International Bureau of Weights and Measures) in 1960. SI has been redefined many times since 1960 for a more reliable and universal criterion.

▲ Almost countries including South Korea use SI.

Meter's first definition is the length of 1/40 000 000 of Earth's meridian. However, meter's definition was changed by another standard, which means wavelength of the particular emission line of Krypton. It was because there is a possibility that Earth's size is not unchangeable. Yet, this definition was changed one more time for the convenience of the length that the light progresses while 1/299 792 458 seconds in a vacuum. The kilogram was the mass of 4℃ water filled in a container of 1000㎤ at 1 atmosphere. But, it made so many errors. So, kilogram's definition was changed to weight of international kilogram prototype that consisted of platinum and iridium. However, this mass of prototype can change, so it is defined by Planck constant which is an unchanging number. The second's first definition was 1/86 400 of the mean solar day, but scientists became to know that rotation of Earth is not constant. Thus, the second was changed to the time that the light that cesium-133 atom releases vibrate 9 192 631 770 times. Ampere's first definition was the magnitude of current that current of equal size flowing on ideal two straight parallel conductors placed 1 meter apart in a vacuum generates a force of 2×10^-7 Newtons per meter in length between these conductors. However, this definition was blamed because of excessive ideality, so it was defined by the elementary quantum of electricity which is an unchanging number. Kelvin's first definition was the size divided by 273.16 between triple point temperature and absolute zero degrees of water. However, it was redefined by Boltzmann constant. Mole is the number of 12 g of carbon-12 atom, and it was redefined by Avogadro's number. Candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

▲ International System of Units

At the 26th CGPM, on November 16th, 2018, the definition of the four units which are kilogram, Ampere, Kelvin, and mole was amended and became unchangeable units, using on fundamental constant. With this, the purpose of BIPM that making units unchanging and accurate was almost realized.

▲ The 26th CGPM, on November 16th, 2018

Jan. 19nd, by Lee Ji-eun