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How a metre became a metre

Today, measuring a metre is hardly a major achievement; historically, however, arriving at exactly how long a metre should be has engaged the intellects of philosophers, clergymen and scientists. RISE is designated as Sweden’s National Metrology Centre for the International System of Units (SI) and as such is home to the Swedish standard metre.

Length was one of the first things that people began to measure systematically. These measurements where often based on the length of a given part of the body. The first thoroughly documented example is the Ancient Egyptian cubit, which is based on the distance from the elbow to the fingertips (approximately 52 cm). This became an accepted unit of measurement around 2,500 BC and standardised cubit rods were manufactured in black marble. The cubit was divided into 28 digits which were in turn divided into smaller units, the smallest of which was approximately 1 millimetre in length.

Universal measurement system

A great deal of time elapsed before any standardised units of measurement were adopted in Europe. During the thirteenth century, a number of units were standardised in England, although major variations would continue to exist throughout society for many centuries. By the mid-seventeenth century, there was increasing clamour for a universal system of measurements. It should be mentioned that one of the proponents was the Swedish philosopher Georg Stiernhielm (1598-1672) who, in 1657, proposed a system in which water would define the link between units for mass, length and volume.

The seconds pendulum

Other ideas of note came from the Dutch physicist Christiaan Huygen and the French clergyman Gabriel Mouton. In 1669, Huygen proposed that a unit of length should be defined by a seconds pendulum (a pendulum that completes a single swing per second), although in 1673 a Frenchman named Jean Richer was able to demonstrate that a pendulum’s swing was dependent on the geographic latitude at which it was placed, rendering Huygen’s proposal obsolete.

The meridian arc

Mounton’s proposal implied that one arcminute (1/60 of a degree) should be used as the basic unit for length. This system would be divided into decimals, with the proposed name for the unit being a milliare. Three measurements of the Earth’s meridian arc were carried out – in Peru between 1735-1746, in northern Sweden in 1736 and in South Africa in 1756 – for the purpose of obtaining a universal measurement for a unit of length. However, it took a major societal upheaval in 1789 in the form of the French Revolution for the scientific approach to gain support in terms of the measurement system.

A natural constant

In 1790, Bishop Talleyrand, later to become known as Napoleon's foreign minister, proposed to the French National Assembly that France and England should introduce a common unit of length, to be defined by the length of a seconds pendulum at 45° latitude. This proposal was however never adopted by either the French or the English academies of sciences. Instead, on 19 March 1791, a counterproposal was made by the French Academy of Sciences. The argued that the unit of length should be linked to a natural constant independent of other units. This linked back to Mounton’s idea of a unit based on the Earth’s circumference, and it was recommended that the unit of length be defined as one ten millionth of the distance between the North Pole and the equator through Paris. This proposal was adopted on 26 March 1791.

The metric system

In June 1799, the metric system was officially adopted in France. The stated intention behind this system was that it should be “A tous les temps, a tous les peuples” – for all times, for all peoples. The unit of length in the metric system is the metre (from Greek metron measure). This unit is divided and multiplied in decimals. As previously, the unit was defined as one ten millionth of the distance between the North Pole and the equator along the Paris meridian. It took several years to complete the necessary measurements to create the prototype metre, known as the mètre des Archives, or archive metre.

The archive metre

IT became apparent that the quadrant determination was somewhat wrong, leaving the archive metre short by 0.2 mm. As the standard had already formed the basis for a large number of measuring devices, it was deemed impossible to correct the error and the metre remained defined by the archive metre length. By 1889, over 30 new metre bar prototypes had been manufactured from a platinum-iridium alloy. The example that best matched the archive metre was now adopted as the international metre prototype so, in practice, the metre had once again been redefined; however, it was not until 1927 that this definition was made official by the Seventh General Conference on Weights and Measures (CGPM 7).

Spectral lines

As measurement techniques and instruments evolved, so the demand for accuracy in the metre prototype increased. This led to a search for alternative methods for defining a metre. By using the Michelson interferometer to optically measure the distance between the two lines on the international metre prototype, in 1960, the Eleventh General Conference on Weights and Measures redefined the metre as 1,650,763.73 wavelengths in the vacuum of an orange spectral line in the isotope krypton-86.

The laser

The development did not however end here; it was not long before the first laser was in use. It soon became apparent that frequency-stabilised lasers were superior to other light sources in terms of interferometric measurements. Among other things, this made it possible to accurately record values for the speed of light (c), a crucial constant in the field of physics, by measuring the frequency of the laser (f) and the vacuum wavelength (a) with great accuracy. The speed of light is determined by the relationship c = f • λ.

Current definition

When measuring wavelengths interferometrically, among other things it is important to know the velocity factor of light. It soon became apparent that by obtaining a precise value for the speed of light it would be possible improve the accuracy of the metre. Following this, on 20 October 1983, CGPM 17 redefined the metre thus:

the meter is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.
 

This is the current definition of a metre and there is every reason to believe that this will be the case for the foreseeable future.