The idea for the metric system – a system of units based on the meter and the kilogram – was born during the French Revolution when were built two standard reference artifact of platinum for the meter and kilogram, which was deposited in the National Archive of France in Paris in 1799 – which later would become known as Archiving Kilo and Archiving Meters. The French Academy of Science was commissioned by the National Assembly to build a new system of units to be used throughout the world, and in 1946 the countries of the Meter Convention recognized the MKSA system (meter, kilogram, second, ampere). In 1954, MKSA Union has added Kelvin and plugs. Later on the system took the name International System of Units, SI (Le Système International d’unités).
SI system is established in 1960 by the Conference of the 11th General Weights and Measures CGPM: “The International System of Units, SI, is the coherent system of units adopted and recommended by the CGPM”.
At the 14th conference of the CGPM’s held in 197, SI, as expanded again with the dock as the base unit for amount of substance. From that time, the system consists of the seven SI base units, which together with the current units make a coherent system of units. In addition, some units are recognized outside the SI system, to be used together with SI units.
Base Units
The basic units of the SI system are the system’s building blocks and all units issued by them. When Maxwell introduced the concept of a coherent system, he identified three sizes that will be used as a basis mass, length and time. Giorgi later presented the need for an electricity base unite which was selected unit of electrical current. Three other units of this system (temperature, substance and intensity of light) were added later.
Base units
Size | Symbol | Unit | Symbol | Definition |
Length | l | meter | m | The metre is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 second. |
Mass | m | kilogram | kg | The kilogram is the unit of mass equal to the mass of the international prototype of kilogram and is kept in International System Center in Paris. |
Time | t | second | s | It is vibration interval of chemical element Cs Caesium with atomic number 55, expressed in seconds and is known as “Atomic Clock” (DPM) |
Intensity of electrical current | I | ampere | A | An ampere is that it is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between those conductors a force equal to 2 x 10-7 newtons per metre of length. |
Temperature | T | kelvin | K | The Kelvin, unit of thermodynamic temperature is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water |
Amount of substance | n | mole | mol | The mole is the amount of substance of a system which contains corresponding to the numbers of 0,012 carbon 12C. |
Light intensity | l V | candela | cd | The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 Hz and that has a radiant intensity in that direction of 1/683 W per steradian. |
Prefixes
Prefixes of the International System are added to unit names to produce multiple and sub-multiples of the original unit. All multiples are integer powers of 10. For example, kilo- denotes a multiple of a thousand and milli- denotes a multiple of a thousandth, so there are one thousand millimetres to the metre and one thousand metres to the kilometre. The prefixes are never combined, so for example a millionth of a metre is a micrometre, not a millimillimetre. Multiples of the kilogram are named as if the gram were the base unit, so a millionth of a kilogram is a milligram, not a microkilogram.
10n | Prefixes | Symbol | Name | Equivalent in decimal |
1024 | yotra | Y | quadrillion | 1 000 000 000 000 000 000 000 000 |
1021 | zetta | Z | sextillion | 1 000 000 000 000 000 000 000 |
1018 | exa | E | trillion | 1 000 000 000 000 000 000 |
1015 | peta | P | billiard | 1 000 000 000 000 000 |
1012 | tera | T | billion | 1 000 000 000 000 |
109 | giga | G | milliard | 1 000 000 000 |
106 | mega | M | million | 1 000 000 |
103 | kilo | k | thousand | 1 000 |
102 | hecto | h | hundred | 100 |
101 | deca | da | ten | 10 |
10−1 | deci | d | tenth | 0,1 |
10−2 | centi | c | one percent | 0,01 |
10−3 | milli | m | 1/thousands | 0,001 |
10−6 | micro | µ | 1/million | 0,000 001 |
10−9 | nano | n | 1/ milliard | 0,000 000 001 |
10−12 | pico | p | 1/ billion | 0,000 000 000 001 |
10−15 | femto | f | 1/ billiard | 0,000 000 000 000 001 |
10−18 | atto | a | 1/ trillion | 0,000 000 000 000 000 001 |
10−21 | zepto | z | 1/ sextillion | 0,000 000 000 000 000 000 001 |
10−24 | yocto | y | 1/ quadrillion | 0,000 000 000 000 000 000 000 001 |
Converters for measuring units:
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