To measure the quantity of anything, we need a comparison with some precise unit value. Early humans used body parts and natural surroundings to provide suitable measuring instruments.

Rudimentary measures became essential in primitive human societies for tasks such as building dwellings, making clothing, bartering for food and exchanging raw materials:

• Early Babylonian and Egyptian records show that length was first measured with the forearm (cubit), hand (palm and span) and the finger (digit).
• The cycles of the Sun, moon and other celestial bodies were used for time measures.
• Plant seeds were used to establish volume measure, and with the development of scales for weighing, seeds and stones served as standards. For example, the carob seed was the base measure for the carat, which is still used as a mass unit in the gemstone industry.

As trade and commerce expanded, it became necessary to standardise measurement systems, not only within a given country but also between countries. This reduced the likelihood of disagreements arising from measurement system misunderstandings.

## The English system

A Weights and Measures Act (1824) redefined common units of measurement in England. Before this, standardisation was only achieved through royal decree:

• Edward II (1324) defined the inch as 3 barleycorns.
• Henry I (1130) decreed that 1 yard is the distance from the tip of his nose to the end of his outstretched thumb
• Queen Elizabeth I (1575) altered the mile to 5280 feet so it became 8 furlongs (1 furlong = 220 yards).

The 1824 Act standardised the system of units used across the British Empire. It is known as the imperial system based on the foot-pound-second.

## The French system

During the French Revolution (1789–1799), the French Academy of Sciences was asked to “deduce an invariable standard for all the measures and all the weights”.

The Academy decided on two founding principles. The system would be:

• based on scientific observation
• a decimal or base 10 system.

In this system:

• the unit of length – metre = one 10 millionth of the distance from the North Pole to the equator along the meridian running near Dunkirk in France and Barcelona in Spain
• the unit of mass – gram = the mass of 1 cm3 of water at its temperature of maximum density. 1 L of water has a mass of 1 kg at this temperature.

The new calendar consisted of 12 months of 30 days each, with a 5–6 day holiday to complete the 365-day year. Each day was divided into 10 hours, 100 minutes per hour and 100 seconds per minute, but the new calendar was never actually used.

The National Assembly adopted the metric system, based on the metre-kilogram-second, on 7 April 1795. It only became compulsory throughout France in 1840.

Other countries recognised the superiority of the metric system. In 1875, 17 countries signed the Treaty of the Metre. This treaty established the setting up of three separate but linked organisational structures:

• The General Conference on Weights and Measures (CGPM) – a formal body responsible for ratifying any new proposals on metric units.
• The International Committee for Weights and Measures (CIPM) to oversee scientific decisions.
• The International Bureau of Weights and Measures (BIPM) to oversee the activities of the national standards laboratories. The headquarters of BIMP were set up in Sèvres, France.

## The SI system

In 1960, representatives of nations using the metric system attended the 11th General Conference on Weights and Measures. An extensive revision and simplification of the system was agreed to, and six units were established as base units for the system, called Le Système International d’Unités (International System of Units), abbreviated to SI. These units were the metre, kilogram, second, kelvinampere and lumen.

In 1971, another base unit was added – the mole as base unit for the amount of substance.

Standard units like the metre, kilogram and second make it possible to trade and communicate throughout the world without misunderstanding.

An international community of metrologists (people who study measurement) is constantly improving the SI measurement system.

#### Nature of science

Empirical observations made in the course of scientific investigations often involve measurements of some quantity. It is only through a system of standard, agreed units of measure that scientists worldwide can effectively communicate their findings.

Published 17 August 2011