Many people in Britain wrongly believe that metric units are a foreign system being imposed from outside. This mistaken belief is particularly ironic since many imperial units, such as the mile, the foot and the acre, were actually imposed by foreign invaders. The 16 ounce "avoirdupois" pound was introduced from France.

While there is no doubt that the metric system was first adopted in revolutionary France, the underlying ideas also came from England. An Australian metrication specialist, the late Pat Naughtin, drew attention in 2007 to the fact that the key principles underlying the metric system were proposed by Dr John Wilkins in his book "AN ESSAY Towards a REAL CHARACTER, And a PHILOSOPHICAL LANGUAGE" published in 1668. Eminent French scientists defined the metric system in 1790 and ‘Système métrique décimal’ became the legal system of measurement in France in 1795. (Contrary to a commonly held misconception, Napoleon actually disapproved of the metric system and tried (unsuccessfully) to reverse the change). Since then, the further development of the metric system has been an international effort to which many British scientists have contributed, especially since the signing of the Metre Convention in 1875.

There have been quite significant British contributions to this international effort over the last century and a half. The suggestion that the metric system is 'foreign' is a slur on some of Britain's most eminent scientists past and present. The following sections summarise some British contributions to the development of the modern metric system.

The original 1790s metric system had been based on the metre (length), volume (litre), day (time) and franc (currency). It also had a basic set of decimal prefixes such as deci-, centi-, deca- and hecto-. There was originally no concept of derived units. In the early nineteenth century discoveries in electricity and magnetism by Oersted, Ampere and Faraday meant that there was a requirement for new units. Gauss and Weber proposed that magnetism and electricity respectively could be measured in terms of 'mechanical units'.

## British Association for Advancement of Science

A comprehensive study of electrical measurements was undertaken by a committee of the British Association for Advancement of Science (BAAS) in 1861. This committee included William Thomson (later Lord Kelvin), James Clerk Maxwell and James Prescott Joule. Building on earlier work by Gauss they introduced the concept of a coherent system of units which mean that the relationship between the units reflects the underlying physical equations.

For example, Ohm's law expresses the relationship between voltage V, current I and resistance R as V = IR. In a coherent set of units the unit for voltage is equal to the unit of current times the unit of resistance.

Furthermore with a coherent system of units, there are a set of base units and derived units which can be expressed in terms of the base units. This committee proposed the metre, gram and second as base units.

In 1873 a second committee proposed the centimetre, gram and second as base units; the CGS system. The same committee worked on electrical units proposing the "B.A. unit" of resistance that was later known as the ohm. Various BAAS recommendations were adopted by the first International Electrical Congress in 1881 - the first modern set of electrical units.

## Metallurgists Johnson Matthey & Co.

After the Metre Convention was signed in 1875, the newly formed International Bureau of Weights and Measures (Bureau international des poids et mesures, BIPM) wanted to establish international prototype metres and kilograms. Copies were to be made as national standards for members of the Metre Convention. For the kilogram it was agreed that the new prototype should be made from an iridium-platinum alloy; the original kilogram prototype was platinum.

Initial attempts to manufacture the alloy had failed so in 1882 the London firm Johnson, Matthey & Co, was commissioned to make the platinum/iridium standards. The contract included 30 standard metres and 40 standard kilograms in iridium-platinum alloy. The standard kilograms were 39 mm in diameter and 39 mm in height. The kilograms cast in London by George Matthey were then hammered, polished and adjusted to match the previous plantinum standard by M Collot in France.

The international prototypes for the kilogram and the metre are shown above. The kilogram prototype (left) remains the definition of the kilogram, while the metre definition has been replaced by a more modern one based on the speed of light.

## British scientists whose names are used in SI units

In the 19 th century Sir Charles Clark and Latimer Clark proposed naming units after eminent scientists. This practice has been used by the General Conference for Weights and Measures. The following British scientists have been honoured in the modern metric (SI) system.

Michael Faraday, English chemist and physicist (1791-1867). The farad (F) is the unit of capacitance

.

Louis Harold Gray, English physicist (1905-1965). The gray (Gy) is the unit of absorbed dose of ionising radiation.

James Prescott Joule, English brewer and physicist (1818-1889). The joule (J) is the unit of energy.

Sir Isaac Newton, English mathematician and physicist (1642-1727). The newton (N) is the unit of force.

James Watt, Scottish scientist and engineer (1736-1819). The watt (W) is the unit of power.

Lord Kelvin (William Thomson), Scottish scientist (1824-1907). The kelvin (K) is the unit of absolute temperature and is an SI base unit.

## British Contributions to BIPM

The Metre Convention (originally signed in 1875) oversees the further development of the international metric system (SI). Key activities include the General Conference on Weights and Measures (CGPM) which meets every four years to ratify changes to SI, the International Committee of Weights and Measures (CIPM) and the International Bureau of Weights and Measures (BIPM).

BIPM, based at Sèvres near Paris, is jointly financed by the signatories of the Metre Convention (including the UK). BIPM is responsible for providing a scientific basis for a single, coherent set of units and for co-ordinating comparisons of national measurement standards.

Two of the recent BIPM directors have been eminent British scientists. The distinguished British scientist Dr Terry Quinn CBE FRS served as Director of BIPM from 1988 to 2003. His successor, Professor Andrew Wallard CBE, was formerly Deputy Director and Chief Metrologist of the UK National Physical Laboratory (NPL) and served as Director of BIPM until 2010.

The current Director of BIPM is another British scientist, Dr M J T Milton, who also joined BIPM from the NPL.

Dr Quinn, Prof Wallard and Dr Milton maintain the tradition of British scientists actively participating in the further development of the international metric system.

Dr Quinn (right) hands over to Professor Wallard in December 2003

Another important area of work is Consultative Committee for Units (CCU) at BIPM. Professor Emeritus Ian M. Mills of the University of Reading, UK, is the President of CCU.

One of the important issues being considered by the CCU is the question of the definition of the kilogram. The kilogram is the only unit still defined by an object – the international prototype – whereas other units are defined in terms of physical constants or properties of certain atoms. Prof Mills sets out the key issues and possible solutions to a new kilogram standard in Chemistry International.