How British metallurgists made the first international kilograms and metres

The introduction of metric units in the UK has been controversial and much of the opposition has been on the basis that it ‘betrays our heritage’  or is a ‘foreign imposition’.  However as a schoolboy then engineer – who has used not just metres and kilograms, but watts, farads, joules and newtons (all international units named after British scientists) – I have always thought we should be proud of using the metric system.

Having learned more about the history of the metric or international system of units I am dismayed that most British people are unaware of the substantial contribution our scientists and metallurgists have made. In particular how 19th Century metallurgists in London helped make the first international kilograms and metres.

Of course, there is no doubt that the metric system originates in France. Despite the chaos of revolutionary France, many eminent scientists there recognised the need for a unified set of measures. Measures varied from province to province and this confused trade. The National Assembly formed a Commission in 1789 to come up with a unified system. Eminent French scientists were involved including the ‘father of chemistry’ Lavoisier and they came up with some familiar units including the metre and the kilogram. Standard prototype measures were developed including a platinum cylindrical kilogram made by Marc Etienne Janety. Despite a few hiccups (such as Napoleon’s distaste for metric units) the new units solved the incompatibility issues in France and rapidly spread to other European countries. Scientists in countries including Britain (especially Joule, Maxwell and Lord Kelvin) saw a huge potential in metric units and proposed ways of further developing the system of measurement.

With the metric system taking an international dimension the French government wisely sought to make metric an international rather than national initiative. This resulted in the signing of the Convention du Metre (Metre Convention) in May 1875; a diplomatic treaty between an initial set of 17 countries. Britain signed the Convention in 1884 and there are now 51 member countries.

A challenge of the newly formed International Bureau of Weights and Measures (Bureau international des poids et mesures, BIPM ) in Paris was to construct international prototypes for the metre and kilogram and to distribute national copies 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 10% iridium/90% platinum alloy was chosen because of its high density, corrosion resistance and stability.

Initial attempts to manufacture the alloy had failed so in 1882 the London firm Johnson, Matthey & Co, which had previously manufactured standard measures for Russia, was brought in. The contract included 30 standard metres and 40 standard kilograms in iridium-platinum alloy. The standard kilograms, delivered in 1884, 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 standard by M Collot in France. The international kilogram was cast in the UK and finished in France; a truly international effort typical of further development of the international system of units.

Today’s definition of the kilogram is still based on the prototype of Matthey & Collot. It is stored in three bell jars at BIPM.

[Courtesy of BIPM]

The first General Conference on Weights and Measures approved the kilogram and metre prototypes in 1889 and the signatories to the metre convention were given copies. Britain had joined the Metre Convention in 1884 and so received copy number 18. This ‘official British kilogram’ is stored at the National Physical Laboratory.

[Courtesy of NPL © Crown Copyright 1996]

Since the 1880s BIPM has commissioned more than eighty platinum/iridium prototype kilograms including those manufactured by Johnson Matthey below.

[Picture orginally published in F.J. Smith, "Standard Kilogram Weights", Platinum Metals Review, 1973, 17 (2), 66-68 reproduced with permission of publisher Johnson Matthey plc]

The International Bureau of Weights and Measures (BIPM) continues its work in Paris with participation from many countries. Both the current Director, Professor Andrew Wallard and his predecessor Dr Terry Quinn are British.

However, the story of the kilogram has probably not yet finished. Although the international prototype has served international metrology for over a century it will probably be superseded; that has already happened with the metre. The standard iridium-platinum metre has been superseded by a more precise definition in terms of the distance travelled at the speed of light during a very precisely defined fraction of a second.

Similarly it is to be expected that the definition of the kilogram will be superseded by a definition that does not depend on a physical prototype. The further development of the International System (SI) of units is the responsibility of Consultative Committee for Units (CCU) at the International Bureau of Weights and Measures (BIPM). Professor Emeritus Ian M. Mills of the University of Reading, UK, is the President. He has set out his views in Chemistry International.

For further reading on this subject I can recommend both the article by F.J. Smith of Johnson Matthey in Platinum Metals Review and the National Physical Laboratory website.

In conclusion, while acknowledging that the kilogram was originated in France, I feel proud of the British contribution to the unit’s history past and present. It was indeed ‘conceived in France, but cast in London’! Never let it be said that the kilogram is unBritish.

[Roddy Urquhart]