Home         Up         Back      


measurement and units

    The Systeme International [S I]

    Le systeme international d'Unites officially has been officially recognised and adopted by nearly all countries. It was adopted by the Conference Generale des Poids et Mesures
    (CGPM, Paris, October 1960.)

    Based upon 7 principal units:

    category name abbreviation
    length metre m
    mass kilogram     kg
    time second s
    electric current ampere a
    temperature kelvin k
    amount of substance     mole mol
    luminous intensity candela cd

    The SI units have been defined as:

    metre [m]
    The metre is the basic unit of length. It is the distance travelled by light in a vacuum during a period of time of 1/299 792 458th of a second.

    kilogram [kg]
    The kilogram is the basic unit of mass. It is the mass of an international prototype in the form of a platinum-iridium alloy cylinder kept at Sevres, near Paris in France. This is the only basic unit still defined in terms of an object. It is also the only one with a prefix (kilo) already in place.

    second [s]
    The second is the basic unit of time. It is the duration of 9 192 631 770 periods of the radiation associated with a specified transition, between two hyperfine energy levels of the cesium-133 atom (Cs).

    ampere [A]
    The ampere is defined as: the constant magnitude of the electric current that, when flowing through each of two parallel, straight and infinitely long conductors of negligible circular cross section separated by one metre in vacuum, results in a force between the two conductors of 2 x 10-7 newton [N] per metre of length. It is named after the French physicist Andre Ampere (1775-1836).

    kelvin [K]
    The kelvin is the basic unit of temperature. It is equal to 1/273.16 th of the thermodynamic temperature of the triple point of water. It is named after the Scottish mathematician and physicist William Thomson 1st Lord Kelvin (1824-1907).

    mole [mol]
    The mole is the basic unit of substance. It is the quantity of matter that contains as many elementary entities as there are atoms in 0.012 kg of carbon-12 (12C). The elementary entities must be specified: they may be atoms, molecules, ions, electrons, other particles or specific groups of like particles.

    candela [cd]
    The candela is the basic unit of luminous intensity. It is the luminous intensity (normal to the surface) of an area equal to 1/600 000 of a square having sides of one metre of an integral cavity at the temperature of freezing platinum at a pressure of 101.325 kPa.

    From the 7 principal units the most important derived units are: farad [F]
    The farad is the SI unit of the capacitance of an electrical system, that is, its capacity to store electricity. It is a rather large unit as defined and is more often used as a microfarad. It is named after the English chemist and physicist Michael Faraday (1791-1867).

    hertz [Hz]
    The hertz is the SI unit of the frequency of a periodic phenomenon. One hertz indicates that 1 cycle of the phenomenon occurs every second. It is named after the German physicist Heinrich Rudolph Hertz (1857-94).

    joule [J]
    The joule is the SI unit of work or energy. One joule is the amount of work done when a constantly applied force of 1 newton moves through a distance of 1 metre in the direction of the force.It is named after the English physicist James Prescott Joule (1818-89).

    newton [N]
    The newton is the SI unit of force. One newton is the force required to give a mass of 1 kilogram an acceleration of 1 metre per second per second [m/s2]. It is named after the English mathematician and physicist Sir Isaac Newton (1642-1727).

    ohm [Ω ]
    The ohm is the SI unit of resistance of an electrical conductor. Its symbol is the capital Greek letter 'omega'. It is named after the German physicist Georg Simon Ohm (1789-1854).

    pascal [Pa]
    The pascal is the SI unit of pressure. One pascal is the pressure generated by a force of 1 newton acting on an area of 1 square metre [N/m2]. The magnitude is rather small and it is more frequently being used as kilopascal [kPa] or Megapascal [MPa]. It is named after the French mathematician, physicist and philosopher Blaise Pascal (1623-62).

    volt [V]
    The volt is the SI unit of electric potential. One volt is the difference of potential between two points of an electical conductor when a current of 1 ampere flowing between those points dissipates a power of 1 watt. It is named after the Italian physicist Count Alessandro Giuseppe Anastasio Volta (1745-1827).

    watt [W]
    The watt is used to measure power or the rate of doing work. One watt is a power of 1 joule per second. It is named after the Scottish engineer James Watt (1736-1819).


    The S I allows prefixes as multiplication factors.
    yotta  [Y]  1 000 000 000 000 000 000 000 0001024
    zetta [Z] 1 000 000 000 000 000 000 000 1021
    exa [E] 1 000 000 000 000 000 000 1018
    peta [P] 1 000 000 000 000 000 1015
    tera [T] 1 000 000 000 000 1012
    giga [G] 1 000 000 000 109
    mega [M] 1 000 000 (a million) 106
    kilo [k] 1 000 (a thousand) 1000
    hecto [h] 100 (a hundred) 100
    deca [da] 10 (ten) 10
    deci [d] 0.1 (a tenth)1/10
    centi [c] 0.01 (a hundredth)1/100
    milli [m] 0.001 (a thousandth)1/1000
    micro [µ] 0.000 001 (a millionth)10-6
    nano [n] 0.000 000 001 10-9
    pico [p] 0.000 000 000 001 10-12
    femto [f] 0.000 000 000 000 001 10-15
    atto [a] 0.000 000 000 000 000 001 10-18
    zepto [z] 0.000 000 000 000 000 000 001 10-21
    yocto [y] 0.000 000 000 000 000 000 000 001 10-24

    [µ] the symbol used for micro is the Greek letter known as 'mu'
    Nearly all of the S I prefixes are multiples (kilo to yotta) or sub-multiples (milli to yocto) of 1000. deca also appears as deka [da] or [dk] in the USA and Contintental Europe.

    Top of Page

     Home             About Us       Tell a Friend       e-mail       + Favorites