A complete set of units which is used to measure all kinds of fundamental and derived quantities is called a system of units.
Some of the commonly used systems of units are as follows :
(i) cgs system:
It was set up in France. It is based on centimetre, gram and second as the fundamental units of length, mass and time respectively.
(ii) fps system:
It is a British system based on foot, pound and second as the fundamental units of length, mass and time respectively.
(iii) mks system:
It is also a French system based on metre, kilogram and second as the fundamental units of length, mass and time respectively.
(iv) SI: The international system of units:
SI is the abbreviation for “Systeme Internationale d’ Unites”, which is French equivalent for international system of units. It is a modernised and extended form of the metric systems like cgs and mks systems. This system was adopted by eleventh General Conference of Weights and Measures in 1960. It covers all branches of science and technology. It is based on the following seven basic units and two supplementary units.
Basic SI quantities and units
| s. No. | Basic physical quantity | Basic unit | Symbol |
| 1. | Length | metre | m |
| 2. | Mass | kilogram | kg |
| 3. | Time | second | s |
| 4. | Temperature | kelvin | K |
| 5. | Electric current | ampere | A |
| 6. | Luminous intensity | candela | cd |
| 7. | Quantity of matter | mole | mol |
Supplementary SI units
| S. No. | Supplementary quantity | Basic unit | Symbol |
| 1. | Plane angle | radian | rad |
| 2. | Solid angle | steradian | sr |
Important Points Regarding System of Units:
> The cgs, mks and SI are metric or decimal systems of units. This is because the multiples and sub-multiples of their basic units are related to the practical units by powers of 10.
> The fps system is not a metric system. This system is not in much use these days.
The seven basic SI units are defined as follows:
(i) Metre (m). It is the SI unit of length. One metre is defined as the length of the path travelled by light in vacuum during a time interval of 1/299, 792, 458 of a second.
(ii) Kilogram (kg). It is the SI unit of mass. One kilogram is the mass of prototype cylinder of platinum-iridium alloy (whose height is equal to its diameter) preserved at the International Bureau of Weights and Measures, at Sevres, near Paris.
(iii) Second (s). It is the SI unit of time. One 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 cesium -133 atom.
(iv) Ampere (A). It is the SI unit of electric current. One ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per metre of length.
(v) Kelvin (K). It is the SI unit of temperature. One kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The triple point of water is the temperature at which ice, water and water vapour co-exist.
(vi) Candela (cd). It is the SI unit of luminous intensity. One candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540× 2012 hertz and that has a radiant intensity of 1 / 683 watt per steradian in that direction.
(vii) Mole (mol). One mole is that amount of a substance which contains as many elementary entities as there are atoms in 0.012 kg of carbon-12 isotope. The entities may be atoms, molecules, ions etc.
The two supplementary SI units are defined as follows :
(a) Radian (rad). It is defined as the plane angle subtended at the centre of a circle by an arc equal in length to the radius of the circle.
(b) Steradian (sr). It is defined as the solid angle subtended at the centre of a sphere by a surface of the sphere equal in area to that of a square, having each side equal to the radius of the sphere.
A HISTORICAL VIEW OF BASIC SI UNITS
metre (m)
> In 1791, the Paris Academy of Sciences defined metre as one ten millionth part of the distance measured along the meridian from the north pole to the equator.
– In 1889, the General Conference of Weights and Measures defined metre as the distance between two lines marked on a platinum-iridium rod preserved at a constant temperature of 273.16 K at 1 bar pressure in the International Bureau of Weights and Measures at Sevres, near Paris.
> In 1960, the standard metre was defined in terms of wavelength of light. One metre is defined as the distance which contains 1650, 763.73 wavelengths of certain orange-red radiation (of wavelength 6057.8021 A) emitted by krypton-86 source kept at the temperature of triple point of nitrogen.
kilogram (kg)
> In 1791 in France, originally kilogram was defined as the mass of one cubic decimetre (or 1 litre) of water at 4°C. Water has maximum density at this temperature.
> In 1889, the General Conference of Weights and Measures defined kilogram as the mass of platinum- iridium cylinder preserved at Sevres.
second (s)
The Paris Academy of Sciences defined second as the time taken by a simple pendulum of one metre length to swing from one extreme position to the other.
Advantages of SI System
Advantages of SI over other systems of units:
(i) SI is a coherent system of units. All derived units can be obtained by simple multiplication or division of fundamental units without introducing any numerical factor.
(ii) SI is a rational system of units. It uses only one unit for a given physical quantity. For example, all forms of energy are measured in joule. On the other hand, in mks system, the mechanical energy is measured in joule, heat energy in calorie and electrical energy in watt hour.
(iii) SI is a metric system. The multiples and submultiples of SI units can be expressed as powers of 10.
(iv) SI is an absolute system of units. It does not use gravitational units. The use of ‘g’ is not required.
(v) SI is an internationally accepted system of units.
Guidelines for Writing SI Units in Symbols:
Rules for writing SI units in symbolic form:
(i) Small letters are used for symbols of units.
(ii) Symbols are not followed by a full stop.
(iii) The initial letter of a symbol is capital only when the unit is named after a scientist.
(iv) The full name of a unit always begins with a small letter even if it has been named after a scientist.
(v) Symbols do not take plural form.