Why is this bar triangular, with two gold plugs in it? In 1820 British army officer Captain Henry Kater made precise measurements (to a millionth of an inch) to compare this bar with others, with the aim of creating an official length standard for the British Empire. He chose a triangular rather than flat bar to minimise thermal expansion. He chose gold for the plugs because it is soft and inert, and he used a microscope to position the lines he inscribed on the plugs 40 inches apart.
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