The metric system is the modern approach to weights and measures. Although it is based on scientific principles, it is very flexible and, once the simple basic rules have been grasped, it is easy to use for everyday purposes. The ease of use has been a key factor in its adoption worldwide.

## A system of units

Metric units form a coherent "system", which means that the units are directly related to each other. A wide range of uses are possible ranging from home cooking, DIY, weather reporting, electrical applications and motoring to radiotherapy.

Traditional measurement units mostly originated from particular activities (e.g. the acre was based on the convenient size of a field that a horse could plough in a day). They therefore do not have a consistent relationship with each other. For example, volume in imperial may be based on cubic feet and cubic inches for solids and gases, but gallons and pints for liquids. Power is measured in horse power for car engines but in British thermal units per hour for heating systems. Imperial uses a wide range of awkward number systems (e.g. 8 pints in a gallon, 14 pounds in a stone, 4840 square yards in an acre).

In the international metric system there is a single unit for each physical phenomenon. Thus grams are used for the weight (strictly speaking, the "mass") of anything - whether it is gold, herbs or garden fertilizer. Similarly litres are equally applicable to volumes of cooking oil, the capacity of a refrigerator or car boot, the amount of air in a room, or the wine production of a vineyard.

Units in the international system follow physical principles. For example, power is the rate at which energy is used. Thus, the SI unit of power, the "watt" (W), is defined as a "joule" (J), the SI unit of energy, divided by a "second" (s), the SI unit of time. In other words, a watt is a joule per second.

Or in symbols, 1 W = 1 J/s.

This idea of units fitting together systematically or *coherently *was pioneered by British scientists in the 1860s.

See also UKMA's "Measurement Units Style Guide" for further advice on writing metric units.

## Metric values for water make everyday use easy

Water is essential for life and its properties affect everybody. The freezing point of water is important for weather, for preserving food and for motoring. Similarly the boiling point of water is important for cooking and for many mechanical systems (notably the steam engine). The temperature scales in the metric system (whether kelvins (K) or degrees Celsius (°C)) have 100 units between the freezing point and boiling point of water.

A litre of water weighs exactly one kilogram |

There is also a convenient match between the kilogram and a litre of water. As John Fraser MP said in a House of Commons debate in 1989, “A cube with a base of 10 centimetres contains exactly 1000 cubic centimetres or 1000 millilitres. Filled with water, it weights exactly 1 kilogram … It is a beautiful and delightful system with a particular simplicity”.

## Decimal numbers make calculation simple

The metric system was intended from the outset to be based on decimal numbers. People think and calculate most easily in decimal and money is also decimal. Simple calculations using metric are easily done in the head. More complex cases are readily handled with a pen and paper or with a calculator.

## Everyday units

Units such as the metre, kilogram, litre, volt, watt and degree Celsius are commonly used in Britain. A few examples are given below:

metres for signage | a kilogram of sugar |

a litre of fruit juice | a 1.5 volt battery |

30 watt speaker | a weather forecast in degrees Celsius |

## International development and usage of metric

Various proposals for a new, standard, simplified and universal system of weights and measures were made in England and elsewhere in the 16th and 17th centuries, notably by John Wilkins, Bishop of Chester, in 1668. But it was in France a century later that King Louis XVI asked the French Academy to propose a new practical system of measures. This system was adopted in France during the Revolution in the 1790s, partially reversed under Napoleon, and then reinstated in 1840. Twenty years earlier, the Low Countries, now Belgium and The Netherlands, were the first countries to adopt the metric system permanently, followed by most other European countries and their colonies during the remainder of the 19th century. Since then it has been adopted in most of the world. The development of the metric system was made truly international in 1875 by the signing of the Convention du Mètre (Metre Convention), a diplomatic treaty between seventeen countries. Britain signed the Convention in 1884 and there are now 57 members, including all the major industrialised nations.

In 1960, the 11th General Conference on Weights and Measures adopted the name International System of Units (*Système International d'Unités *or SI) for the recommended practical system of units of measurement. SI consists of different classes of units:

Seven independent base units. These are the metre (m), the kilogram (kg), the second (s), the ampere (A), the kelvin (K), the mole (mol) and the candela (cd). All these units except the kilogram are based on constants of nature (e.g. a metre is the distance travelled by light in a vacuum in a tiny fraction of a second). It is expected that the kilogram will also soon be similarly defined.

A set of derived units which are defined in terms of base units. Examples include the watt (W), the volt (V), the square metre (m

^{2}) and the degree Celsius (°C).

Units are represented by symbols, normally letters of the Roman alphabet. These symbols are internationally recognised, even in countries whose principal language does not use the Roman alphabet such as Greece, Russia, India, Japan, China and the Arabic-speaking countries of the Middle East.

There is also a standard set of prefixes which allows metric units to describe a wide range of quantities from very small to very large. Common examples of prefixes are:

micro- (µ-) meaning millionth, e.g. microvolt (µV)

milli- (m-) meaning thousandth, e.g. millimetre (mm)

kilo- (k-) meaning thousand, e.g. kilogram (kg)

mega- (M-) meaning million, e.g. megawatt (MW)

The writing of prefixes and units is also standardised internationally. Furthermore the use of standard prefixes and units means that pronunciation should follow straightforward rules (the stress should always be on the first syllable).

The Metre Convention has also established the International Bureau of Weights and Measures (*Bureau international des poids et mesures*, BIPM) as an international centre for metrology at Sèvres near Paris. BIPM is responsible for providing a scientific basis for a single, coherent set of units and for coordinating comparisons of national measurement standards.

From the outset, the metric system spread internationally, aided by international participation in its development. The Commonwealth and the United States kept traditional measurements much longer than other countries. After a century of recognising the benefits of metric, Britain finally announced that it would adopt metric units in 1965. Commonwealth countries followed this lead and Australia, New Zealand and South Africa successfully converted from imperial to metric in the 1970s. Britain and the United States are among the countries yet to complete the changeover.