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The platinum-iridium cylinder (right) was the primary standard kilogram for all metric measurements in the U. S. Known as Kilogram No. 20, it was a copy of the International Prototype Kilogram, which was preserved at the International Bureau of Weights and Measures at Sevres, France. Kilogram No. 4 (left, under double bell jar), a duplicate of No. 20, was used as a secondary standard. Noth cylinders were 39 mm in diameter and 39 mm. high; they were made of 90 percent platinum and 10 percent iridium. Re-comparison of Kilogram No. 20 with the international standard in 1937 showed that the United States standard had changed by only one part in 50 million during approximately 50 years.

Everyday science

A fond farewell to the humble kilo

Cancer Cell disease types breast cancer research skin symptoms 3D rendering © Getty Images

The Immunotherapy Revolution

There's something inscrutable and mysterious about liquids © Getty Images

Everyday science

There’s something inscrutable and mysterious about liquids

The Story of S*** by Midas Dekkers is out now (£12.99, Text Publishing)

Five reasons to be positive about poo

Dead bee lying next to a burger © Getty Images

The Big Mac and the bee

How do your household gadgets actually work? © Getty Images

Everyday science

How do your household gadgets actually work?

A brief introduction to imaginary numbers © Getty Images

Everyday science

A brief introduction to imaginary numbers

Where is your experience? © Getty Images

Where is your experience?

Can you weigh the world? © Getty Images

Everyday science

Can you weigh the world? Revolutionary experiments in physics

Five recipe books to tantalise your scientific senses © Getty Images

Everyday science

Five recipe books to tantalise your scientific senses

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Future Technology

Meet the robots taking on tasks we’d rather not

An Alfred Edward Chalon watercolour of Augusta Ada King-Noel, Countess of Lovelace (nee Ada Byron) circa 1940 © Donaldson Collection/Michael Ochs Archives/Getty Images

Future Technology

How Ada Lovelace’s notes on the Analytical Engine created the first computer program

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Future Technology

The World Cup will bring us together and not just to talk football

From apples to gravitational waves: a brief history of gravity © Getty Images

Everyday science

From apples to gravitational waves: a brief history of gravity