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Defining the Gram

The Economist:

Science moves on. Time is now defined by readily available clocks that use caesium atoms as their pendulums, while distance is specified in terms of the speed of light in metres per second (the caesium atoms having already provided the value of a second), and these days that speed can be measured with great accuracy using easily purchased equipment. Mass, however, remains stubbornly stuck in Paris, under three concentric glass lids (pictured) strangely reminiscent of cheese covers, which are intended to stop it either absorbing or shedding matter and thus changing in value.

Unfortunately, the cheese covers have not worked.

★ Monday, 14 January 2013