How British metallurgists helped to make the first international kilogram

We reported a few weeks ago on the redefinition of the kilogram. In this article, Martin Vlietstra, one of our regular contributors, outlines Britain’s contribution to the creation of the prototype kilogram upon which the definition had relied since 1889.

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 an 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 our substantial contributions. In particular how 19th Century metallurgists in London helped make the first international kilogram and metre.

Although the quest for a rational, decimal measurement system may have begun elsewhere, it was in France that the crucial steps in the creation and development of the metric system occurred. Despite the chaos of revolutionary France, many eminent scientists there recognised the need for a unified set of measures. These varied from province to province, hampering trade (yes, the benefits of a single market were recognised then too). The National Assembly formed a Commission in 1789 to make proposals for a unified system. Eminent French scientists were involved including the “father of chemistry” Lavoisier. They came up with some familiar units including the metre and the gram. Standard prototype standards were produced, including a platinum cylindrical kilogram made by Marc Etienne Janety. Despite a few hiccups (such as Napoleon’s imposition of mesures usuelles), 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 the potential of metric units and proposed ways of further developing the system.

With the metric system taking an international dimension, the French government sought to make it an international rather than a national standard. 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, including the USA. 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 to signatories of the Metre Convention. For the kilogram, it was agreed that the new prototype would be made from an iridium-platinum alloy. The 10% iridium/90% platinum mix was chosen because of its high density, corrosion resistance and stability.

Initial attempts to manufacture the alloy had failed, and the London firm Johnson, Matthey & Co, which had previously manufactured standard measures for Russia, was brought in. This led to the production in 1879 of the International Prototype Kilogram (IPK), one of three prototypes.

International prototype kilogram

The contract also included the production of copies: 30 standard metres and 40 standard kilograms in iridium-platinum alloy. The kilograms cast in London by George Matthey were delivered in 1884 and were then hammered, polished and adjusted to match the IPK by M Collot in France – a truly international effort typical of the development of the international system of units.

In 1883, it had been determined that the mass of the IPK was indistinguishable from that of the Kilogramme des Archives made eighty-four years prior, and it was formally ratified as the kilogram by the first Conference general des poids et mesures (CGPM) in 1889.  This definition, relying on the prototype produced by Johnson Matthey & Co and Collot, survived until last month’s meeting of the CGPM.

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