Since ancient times, the measurement of quantities was considered a useful tool. Humans have used numerous primitive units in the history of humanity, with varying and typically low accuracy and consistency. With the advent of technology and scientific temperament, homo sapiens felt the need for a standardised system of units.

The internationally followed system of standardised units, called the SI units, was established in the 11th General Conference on Weights and Measures  (CGPM). SI is actually the abbreviated form of the French term Système international d’unités. The CGPM is the institution that oversees the administration of the SI units. It modifies or adds units as per the latest advances in science and technology.

The numerical part of the unit is called its magnitude. There are two kinds of units- fundamental/base units and derived units.

History

The institution of CGPM was founded as a result of a diplomatic treaty called the Metre Convention by the representatives of seventeen countries. The said treaty was signed in 1875 in Paris. Over the years, the number of participant countries in the Metre Convention has risen to 51, which includes nearly all the industrialised states in the world. 

The treaty of the Metre Convention also facilitated the creation of institutions like the Bureau of Weights and Measures (BIPM) and the International Committee for Weights and Measures (CIPM). The members of CGPM currently meet every four years in Paris. The last meeting of the CGPM was held in the year 2018. 

The Need for SI Units

There are several reasons as to why the need for the creation of a standardised system of units was deeply felt. The following are some of the reasons:

• The system of SI units helps to create consistency around the world. For instance, one-kilogram mass will always remain one kilogram around the world. It would not change with the change in geography. 
• It enables people to have trust and faith in the retailers while shopping. The standardised system ensures that the customer would not get cheated and affirms the faith of people in the system.
• The SI units have increased accuracy, which is the need of the hour in today’s generation of ever-advancing science and technology. It has also helped in the field of medicine.
• The system of SI units has increased stability. The historically used units were often susceptible to change and variations.

Types of Units

There are essentially two types of physical quantities. The quantities that are fundamental in nature and are not dependent or derived from other quantities are called fundamental/base quantities. There are a total of seven fundamental quantities in physics which include time, length etc. The units which are used to measure these quantities are called fundamental/base units. Derived quantities, on the other hand, refer to those physical quantities that are derived from the fundamental quantities. The examples of derived units include speed(derived from distance and time), momentum(derived from velocity and mass) etc. The units which are used to measure derived quantities are known as derived units.

Examples of fundamental units include metres, kilograms, ampere etc. Some of the examples of derived units include metre per second(for speed), kilogram metre per second (for momentum). Each of these units has a fixed definition according to the standardised system of units called the SI system.

Types of System of Units

Apart from the SI system of units, there are also several others. A system of units is essentially a collection of certain units, some of which are taken as the fundamental or base units and the rest of the units are derived from the base units. Each of the systems has separate units of measurement. Therefore, it becomes quintessential to define each system of units.

There are multiple systems of units prevalent in physics. Some of them are listed in the following list:

• C.G.S system of units- The unit of length is centimetre (cm). The unit of mass is grams(g) and the unit of time is seconds(s).
• M.K.S. system of units- The unit of length is metre(m). The unit of mass is kilograms(kg), The unit of time is second(s).
• F.P.S system of units- The unit of length is foot (ft). The unit of mass is pounds (Lb). The unit of time is second.

In order to convert from one system to another, the given quantity is multiplied by the conversion factor. The conversion factor refers to the ratio which describes how many of a given unit equals how many of another unit. For instance, one kilometre equals 1000 metres.

Guidelines for Writing Units in SI Units

The following points should be kept in mind while writing units according to the SI system:

• The units named after scientists are not written in capital letters but their symbols are denoted by capital letters.
• Full stood shall not be used after the symbol.
• All units and their symbols are written in small letters.
• The denominators in compound units shall be denoted by negative powers.
• Plural forms of units are not to be used.
• The symbols of a compound unit derived from two or more physical quantities are separated by either a dot or a space.
• Some space should be ideally left between the magnitude and the unit of a physical quantity.

Conclusion

A system of units is essential for the purpose of accuracy and consistency in measurements. There is a mention of numerous systems which have been used for measurement throughout history. The SI units are the standardised system of units initiated by the Metre Convention in 1875. The quantities that can be measured in some form of units are called physical quantities. They may be either fundamental or derived quantities. The units of fundamental quantities are called fundamental/base units, while the units for derived quantities are known as derived units.