# International System of Units (SI Units)

International system of units (SI Units) is the most widely accepted system of measurement and this system is built on 7 primary units which are:

**Length **

**Time **

**Weight **

**Amount of substance**

** Electric current**

** Temperature **

**Luminous intensity **

This international system of units was earlier referred as meter-kilogram-second (MKS) system.

The principle behind international system of units is that it used to provide the same values across the world for measurements such as length, height, weight etc.

SI plays a vital role in international conferences and is also used in scientific and technological research.

## History of SI Units

The international system of units was introduced in 1960 which was adopted by 11th general conference on weights and measure or CGPM.

This system was invented to modify the definition of units and to be used as technology for measuring objects that we use in our daily lives.

The United States further introduced its own system of units or system of metric units which is now called as United States customary units or USCS

### Difference Between USCS and SI Units

The United States metric units are also called as “imperial units.” The key difference between the SI units and the American metric units is the terms and the form of units used.

For instance, In SI unit, the length is measured using the metre whereas In USCS foot is used for measurement.

The international system of units consisted of following three categories mentioned below; Base units Supplementary units Derived units

The seven base units are given below:

###### Length

This unit length is used for measuring the size of an object or the distance that an object travels from one end to the other end. There are different units of length which are metre, kilometers, feet etc. The most common tool which is used to measure length is called a ruler.

For Example; The height of this blackboard is about 3 metres. Smallest unit of measuring length is millimeter and the largest unit is kilometers.

1 kilometer = 1000 metre

###### Time

The standard unit for measurement of time is seconds. Other metric units of time are minutes, hours etc.

1 minute = 60 seconds

1 hour= 60 minutes

1 day = 24 hours

1 week = 7 days

###### Weight

Weight is the unit that is used to measure the mass of an object. The standard unit that is used for the measurement of mass is kilogram, gram ton etc.

The most common tool which is used to measure the weight of an object is the weighing scale. For instance; the weight of this bottle is 250 grams.

1 kilogram =1000 gram

###### Amount of Substance: Mole

The amount of matter of a system that comprises as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. Elementary entities are subatomic units that comprise matter and energy.

The symbol of unit mole is mol.

###### Luminous Intensity

The luminous intensity, in given route, of a source produces monochromatic radiation of frequency 540×1012 hertz and has a radiant intensity in that path of 1/683 watt per steradian.

The unit of luminosity is candela which is denoted by cd.

###### Current

Electric current is well-defined as the rate of flow of negative charges of a conductor. Since the charge is calculated in coulombs and time is in seconds, the unit of electric current is coulomb/Sec (C/s) or amperes. The SI unit of current Ampere is denoted by unit symbol A.

###### Thermodynamic Temperature

The kelvin (abbreviation K), is the SI unit of temperature. One Kelvin is 1/273.16 (3.6609 x 10 ^{-3}) of the thermodynamic temperature of the triple point of a pure water that is H 2O.

###### Supplementary Units

###### Plane Angle

The name of the unit which is used to calculate the plane angle is radian. Symbol of radian is rad.

Radian describes the plane angle that is subtended by an arc of a circle.

###### Solid Angle

The name of the unit which is use to describe the solid angle is steradian Symbol of steradian is sr.

A steradian describes the solid angle at the centre of sphere that is subtended on a section of surface.

###### Derived Units

Few examples of derived units are given below;

**Area:** Unit name of area is square metre.

**Frequency:** Unit name is hertz (Hz)

**Volume:** Unit name is cubic metre

**Speed:** Unit name is metre per second

**Magnetic:** Field strength Unit name is ampere pe metre

#### International System of Units (SI Units) Citations

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