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## The International System of Units In earlier time, scientists of different countries were using different systems of units for measurement: the CGS, the FPS (or British) system, and the MKS system. These were used extensively until recently. The base units for length, mass, and time in these sy...

## The International System of Units In earlier time, scientists of different countries were using different systems of units for measurement: the CGS, the FPS (or British) system, and the MKS system. These were used extensively until recently. The base units for length, mass, and time in these systems were as follows: - In the CGS system, the base units were centimeter, gram, and second, respectively. - In the FPS system, the base units were foot, pound, and second, respectively. - In the MKS system, the base units were meter, kilogram, and second, respectively. The internationally accepted system of units for measurement is the *Système Internationale d'Unités* (French for International System of Units), abbreviated as SI. SI units were developed and recommended by the General Conference on Weights and Measures in 1971. They are used for international usage in scientific, technical, industrial, and commercial work. SI units use decimal systems, making conversions within the system simple and convenient. ### Fundamental Units and Derived Units In SI, there are seven base units. Additionally, there are two more units that are defined for: (a) plane angle as the ratio of the length of arc *ds* to the radius *r*, and (b) solid angle *dΩ* as the ratio of the intercepted area *dA* of a spherical surface to the square of its radius *r*. The following table describes fundamental units and their symbols: | S.No. | Fundamental Quantities | Fundamental Units Symbol | |---|---|---| | 1. | Length | Meter (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) | The unit for plane angle is a radian (*rad*). The unit for the solid angle is a steradian (*sr*). Both these are dimensionless quantities. ### Derived Units Other physical quantities derived from the base quantities can be expressed as a combination of the base units. These are called derived units. A complete set of units, both fundamental and derived units, constitutes a system of units. Examples include volume and density.