How things are measured

When we measure a weight, length or temperature, we use some kind of unit for easy reference. However, in ancient times, people used the size of their palm or foot to measure a length, but it greatly depended on each person's body size.

Therefore, the whole world needed a standard measurement that could be used everywhere. In order to fulfil this need, a group of scientists formed a system of units known as the 'International System of Units' or SI units.

This is a modern form of the metric system. It is the world's most widely used system of units, both in everyday commerce and science.

Formation of SI units

A group of scientists (among them, Antoine-Laurent Lavoisier, who is known as the 'father of modern chemistry') which had been commissioned by King Louis XVI of France to create a unified and rational system of measures conceived the metric system.

The metric system has a long history and has seen a number of changes. Its use has spread around the world, replacing many traditional measurement systems.

It was recognized that additional steps were needed to promote a worldwide measurement system.

As a result, the ninth General Conference on Weights and Measures (CGPM), held in 1948, asked the International Committee for Weights and Measures (CIPM) to conduct an international study of the measurement needs of the scientific, technical and educational communities.

Based on the findings of this study, the 10th CGPM held in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities.

The six base units recommended were the metre, kilogram, second, ampere, degree Kelvin (later renamed the kelvin), and the candela. In 1960, the 11th CGPM named the system the International System of Units, abbreviated SI from the French name Le Systeme international d'unites. The seventh base unit, the mole, was added in 1971 by the 14th CGPM.

The SI units

The units of SI can be divided into two sub-sets. There are seven base units. Each of these base units is nominally (in numbers) and dimensionally independent. From these seven base units, several other units are derived.

SIS writing style

Symbols do not have an appended (attached) period or full stop (.) unless at the end of a sentence. Symbols are written in upright Roman type (m for metres, l for litres), to differentiate from the italic type used for mathematical variables (m for mass, l for length).

Symbols for units are written in lower case, except for symbols derived from the name of a person. For example, the unit of pressure is named after Blaise Pascal, so its symbol is written 'Pa' whereas the unit itself is written "pascal".

The one exception is the litre, whose original symbol 'l' is similar to the numeral '1' or the uppercase letter 'i' (depending on the typeface used), at least in many English-speaking countries. The SI rule for pluralising units is that symbols of units are not pluralised, for example '25 kg' (not '25 kgs').

A space separates the number and the symbol, e.g. '2.21 kg', '7.3x102 m2', '22 K'. Exceptions are the symbols for plane angular degrees, minutes and seconds, which are placed immediately after the number with no intervening space.

Spaces may be used as a thousands separator (1 000 000) in contrast to commas or periods (1,000,000 or 1.000.000) in order to reduce confusion resulting from the variation between these forms in different countries.

In print, the space used for this purpose is typically narrower than that between words. Symbols for derived units formed from multiple units by multiplication are joined with a space or centre dot (.), e.g. 'N m' or 'N.m'.

Symbols formed by the division of two units are joined with a solidus (/), or given as a negative exponent. For example, the 'metre per second' can be written 'm/s', 'ms-1', 'm.s-1'. A solidus should not be used if the result is ambiguous (uncertain), i.e. 'kg.m-1.s-2' is preferable to 'kg/m.s2'.

When writing dimensionless quantities, the terms 'ppb' (parts per billion) and 'ppt' (parts per trillion) are recognised as language-dependent terms since the value of billion and trillion can vary from language to language. SI therefore recommends avoiding these terms. However, no alternative is suggested by BIPM.

Compiled by Janani Amarasekara