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SI Units or System of Measurement

written by: Haresh Khemani • edited by: Lamar Stonecypher • updated: 2/26/2010

This article describes International system of units, popularly known as SI system of units. It also describes the advantages of the SI units, the fundamental and derived SI units and the rules for writing the SI units.

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    What is SI System of Units

    SI system of units is the short name for ‘International System of Units.’ It was originally named in French as “Systeme Internationale d’Unite’s. SI system of units is the international system of units adopted for the measurement of fundamental and derived physical quantities. This system was first proposed in the year 1960 by General Conference of Weights and Measures and was finally accepted by the International Standards Organization (ISO) in the year 1962.

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    Before SI Systems of Units

    Before the advent of the SI systems of units, different countries used different systems of units for measurements like CGS, MKS, and FPS. Some countries would measure length in centimeters, some in meters and others in feet. There was difference in units adopted for most of the physical quantities. This created lots of difficulties and confusion during exchange of information between the scientists, engineers and technologists of the various parts of the world. The different systems of units in various countries led to lots of communication gap and many times misinterpretation of the data.

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    Advantages of the SI Systems of Units

    With the acceptance of the SI systems of unit for the measurement purposes by the ISO, it has become compulsory for the all the countries of the world to adopt this common system. Thus length is now measured in meters, mass is measured in kilograms and time is measured in seconds in all the parts of the world. This has led to very easy flow of information from one country to the other, without creating any confusion, miscommunication and misinterpretation of data.

    The scientists, engineers, technologists now use common language of system of units and symbols, thus the results of any research in one part of the world can be easily compared with results in any other part of the world. This also ensures rapid flow and growth of the knowledge. It is just like if all the people of the world speak the same language. If this were to happen there would not be any language barrier and flow of information would be unhindered.

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    Fundamental Units in SI System

    There are seven fundamental or basic physical quantities in SI systems that do not depend on any other physical quantities. The units of these quantities are called as fundamental or basic units. These seven physical quantities along with their units and symbol are:

    1) Length, unit: meter, symbol: m

    2) Mass, kilogram, kg

    3) Time, second, s

    4) Temperature, kelvin, K

    5) Electric current, ampere, A

    6) Luminous intensity, candela, cd

    7) Amount of substance, mole, mol

    Two supplementary SI units

    1) Plane angle, radian, rad

    2) Solid angle, steradian sr

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    The international Systems of Units or SI units were adopted in the year 1962 by International Standards Organization (ISO). There are two types of physical quantities: fundamental or basic quantities and derived quantities. The units of derived quantities are called as derived units. In our day-to-day life we have come across many physical quantities that are to be measured or written. Many people write them incorrectly, here are certain rules to write the SI units:
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    Derived Units in SI System of Units

    The physical quantities derived from fundamental quantities are called as derived quantities and their units are called as derived units. There are many derived physical quantities; some of them are given below:

    1) Force, newton, N

    2) Work, Energy, joule, J

    3) Power, watt, W

    4) Electric potential, volt, V

    5) Electric resistance, ohm, Ω

    6) Electric charge, coulomb, C

    7) Frequency, hertz, Hz

    8) Velocity or speed, m/s

    9) Acceleration, m/s2

    10) Magnetic induction, tesla, T

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    Rules for Writing SI Units

    In our day-to-day life we come across number of physical quantities that are to be measured or written. Many people write them incorrectly, here are certain rules to write the SI units:

    1) When writing the name of the unit, the first letter of the unit should be in small case and it should be not written in capital case. Thus the unit of length should be written as ‘meter’ and not ‘Meter.’ For example it is correct to write 5 meter and not 5 Meter. Similarly, the unit of mass is written as kilogram and not Kilogram. Some of the units are written after the name of the scientist or other person, even these units should start with small letters. Thus the unit of power should be written as watt and not Watt. Similarly other units are newton, ampere, hertz etc.

    2) When writing the symbols for the SI units, if the unit is named after the person, the symbol should start with the capital letter. Thus the symbol for newton is written as N, for ampere it is A, for hertz, it is Hz, for kelvin it is K and so on. If the unit is not the name after the person, its symbol should start with the small letter. Thus symbol for kilogram is kg, for meter it is m, and so on.

    3) The symbols for the SI units are written in singular form and not in plural form though they may indicate quantity more than one. Thus five meter of length is written as 5 m and not 5 ms. Similarly it is correct to write 10 kg, but is incorrect to write 10 kgs. If one has to write them in plural forms then full name of the units should be written and not the symbols. Thus it is correct to write 5 meters, 10 kilograms, etc.

    4) No full stops or punctuation should be written after the symbols of the SI units. Thus when writing five meters of length it is correct to write 5 m but incorrect to write 5m. or 5m,.