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# All about... the metric system

How do we know how long a metre is? And who first decided how we would measure everything? Find out below.

In the 18th century, Europe used a vast range of different measurements. In France alone, there were hundreds of different terms for each type of unit. Some were connected to the human body (thumb, finger, handful, armful, foot, pace), others to transport (donkey-load, load, bag, barrel). The situation was made more complex by the fact that the same word always referred to a range of different values. A foot, for example, corresponded to 20 or so different lengths within the kingdom.

## How long is a metre?

In 1790, a year after the start of the French Revolution, a proposal was put in front of the Assembly that existing measures were to be abandoned and attempts made to find a measurement that was the same everywhere. The Assembly then drafted a decree which was aimed at defining the standard metre.

In March 1791, it was decided, on the recommendation of the Academy of Sciences, that a metre should be one ten-millionth of the length of the distance from the equator to the north pole. The term “metre” was first used in 1790, but was not officially adopted until the Law of 1 August 1793.

## Measuring the metre

It was then necessary to produce the two prototype standards (metre and kilogram) on to which the measurements would be copied for distribution across France. On 22 June 1799, these two prototypes were deposited in the Archives of the Republic, in the iron safe where they are still kept today.

More than a century and a half after it was finally established in France, the metric system has prevailed and spread across the whole world, although we no longer use the prototypes as the standard to judge our metre-sticks by. Scientists have come up with a much more accurate way of measuring distance – wavelengths and light speed. Since 1983, the metre has been internationally defined as the distance travelled by light in a vacuum during a time interval of 1/ 299 792 458 of a second.

## When it all goes wrong

Even though its use seems to be standard, errors can still happen:

•  In 1983 a Boeing 767 jet ran out of fuel in mid-flight because of two mistakes in figuring the fuel supply of Air Canada's first aircraft to use metric measurements.
• In 1999 NASA lost a \$125 million Mars orbiter because one engineering team used metric units while another used US customary units for a calculation.