Sir

You recently featured a report, in the “100 years ago” column, (Nature 404, 27; 2000) on German iron manufacturing, written by the British consul in Amsterdam in 1900. He mentioned a uniform system of dimensions for articles of universal consumption, and a system of metric screw threads being fixed by a committee of engineers' associations. Unless Britain joined the metric standard, the consul warned, “Germany and the continent generally will have a constantly increasing advantage over British manufacturers”. His words were prophetic — 150 of the world's 190 export markets are now metric.

The engineers' associations agreed on the Système International (SI) of metric threads at a special meeting, the Congress of Zurich, in October1898. This was to end the confusion caused by different European countries using different systems on their railways, though they didn't enter British consciousness until the Second World War, as BSI War Emergency Standard BS 1095:1943. The 1898 meeting also saw the birth of ISO metric standards for engineering, which entered the British standards environment in 1966 through a book published by the then Ministry of Technology.

Meanwhile, metric screw threads, standard diameters, shaft/hole fits and machining tolerances for the engineering industry had been internationalized during the 1920s and 1930s by a body called ISA, which was absorbed into ISO in 1946. The old SI metric thread with its undesirably sharp fillet radius was modernized by application of the unified profile by ISO in 1958 and standardized by all industrialized nations soon afterwards. That is why all mechanical engineering assemblies made in mainland European factories since 1959 use the same threads up to 80 mm diameter.

In order to assure the free circulation of engineering components and subassemblies in the European Common Market, the UK government — led by Harold Wilson — announced in May 1965 that British industry would abandon the inch within ten years and adopt ISO metric standards. The first attempt to produce a British engine to metric standards was already in progress at Leyland Motors with the Leyland 500 engine. However, this was marketed in continental Europe only in the Leyland National bus, whose full metrication was never completed: it incorporated four different screw-thread standards and was impossible to manage in European workshops.

British Leyland vehicles failed to attract continental dealers because the inch was already alien in mainland Europe — it had actually been outlawed in mechanical engineering in Germany during the 1930s. It remained only in water- and gas-pipes, and in certain items imported from the United States.

Inch threads had no future in Europe, but the planned conversion of British engineering factories did not occur. As a result, the UK's non-metric products were rejected by Europe as a nuisance.

It was the abandonment of Wilson's metrication programme, along with deficient marketing, that caused the stagnation of British Leyland and so many other engineering-based companies in the United Kingdom. The economic effects of Britain's failure to adopt the metric system on schedule have never been quantified. But the recent loss of the $125 million NASA Mars Orbiter probe emphasizes the vast sums of money that can be lost through the unfamiliarity of the average UK or US technocrat with metric units.