#### What is a Unit?

In our daily life, we often need to measure various physical quantities. To measure a
physical quantity, we compare it with some standard quantity. For example, if we
purchase some sugar, we must know how much quantity of sugar we are talking about.
Thus, there is a need of some standard quantity for measuring unknown quantity. This
standard quantity is called unit.

Various standard units have been in use at different times in different parts of the world.
With the passage of time, these units were made more precise and acceptable. People
especially business communities and scientists of different countries faced problems of
converting the units into one another. This problem was solved in a conference of the
scientists from all over the world held in Paris.

In 1960, the eleventh general conference of
International Committee on Weights and
Measures recommended that all countries of
the world should adopt a system of same kind
of standard units. This conference
recommended the use of

*International
System of units*. It is abbreviated as

**SI**.
According to this system, the units of length,
mass, time and volume are given in the
following table.

Table. 1: SI Units
Length | l | meter | m |

Mass | m | kilogram | kg |

Time | t | second | s |

Volume | V | cube meter | $m^3$ |

A practical unit of volume is liter (L). Mostly the liter is used for measuring volume of
liquids such as milk, petrol, cooking oil, etc. It is $1/1000th$ part of a cubic meter ($m^3$ ).

Therefore $1\, m = 1000 \,L$

Also $1 \,L = 1000 \, \text{milli-litre} = 1000\, \text{cubic centimetre (cc)} $

In the past, the systems of units most commonly used were the

**English**
and

**metric**, the English system is based on a single standard, the metric is subdivided into two interrelated standards: The

**MKS **and The

**CGS**
The

__MKS system__ uses

*Meters*,

*Kilograms*, and

*Seconds*, while the

__CGS system__ uses

*Centimeters*,

*Grams*, and

*Seconds*.

Unfortunately, the use of more than one system of units in a world, would introduce unnecessary complications to the basic understanding of any technical data. The need for a standard set of units to be adopted by all nations has become increasingly obvious.

The

*International Bureau of Weights and Measures* located at Sevres, France, has been the host for the General Conference of Weights and Measures, attended by representatives from all nations of the world. Since then, SI has been adopted by the

*Institute of Electrical and Electronic Engineers*, Inc. (IEEE) in 1965 and by the United States of America Standards Institute in 1967 as a standard for all scientific and engineering literature.

#### Prefixes

The main advantage of SI units is that their multiples and sub-multiples can be
conveniently expressed using prefixes. Prefixes are based on multiplying and
dividing the units by powers of 10. The words or letters added before SI units such as
milli (m), centi (c) and kilo (k) are known as prefixes.

- Milli means 1000 part. For example, millimetre (mm) is 1000 part of a
metre, i.e., 1/1000 m. It means, 1 m = 1000 mm.
- Centi means 100 part. For example, centimetre (cm) is 100 part of a metre, i.e.,
1 cm = 1/100 m. It means 1 m = 100 cm.
- Kilo means 1000 times. For example, kilometre (km) is 1000 times of a metre,
i.e., 1 km =1000 m.

Thus, diameter of a thin wire can be written in smaller units of centimeter (cm) or
millimetre (mm) instead of metre. Similarly, the longer distance between two cities may
be expressed better in a bigger unit of distance, i.e., kilometre (km).

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