National Physical Laboratory

History of Length Measurement

Introduction

This is a brief overview of the history of length measurement in the United Kingdom, outlining some of the more important and interesting aspects.

Measurement has been important ever since man settled from his nomadic lifestyle and started using building materials; occupying land and trading with his neighbours. As society has become more technologically orientated much higher accuracies of measurement are required in an increasingly diverse set of fields, from micro-electronics to interplanetary ranging.

The Cubit
The cubit
(fingertip to elbow)

Body Parts

One of the oldest units of length measurement used in the ancient world was the 'cubit' which was the length of the arm from the tip of the finger to the elbow. This could then be subdivided into shorter units like the foot, hand (which at 4 inches is still used today for expressing the height of horses) or finger, or added together to make longer units like the stride. The cubit could vary considerably due to the different sizes of people.

The Yardstick
The yardstick
(Henry I, thumb to nose)

As early as the middle of the tenth century it is believed that the Saxon king Edgar kept a "yardstick" at Winchester as the official standard of measurement. A traditional tale tells the story of Henry I (1100-1135) who decreed that the yard should be "the distance from the tip of the King's nose to the end of his outstretched thumb".

Measuring the British Isles
Measuring the British Isles

Assize of Measures

It was not until the reign of Richard the Lionheart that the standardisation of units of measurement was first documented. In the Assize of Measures in 1196 it was stated that "Throughout the realm there shall be the same yard of the same size and it should be of iron". The Magna Carta (1215) also attempted to standardise measurements throughout the kingdom, although it concentrated on measures of wine and beer!

Barleycorns and the Rod

In Edward I's reign (1272-1307) the yard (or Ulna) and its sub- and aggregated divisions were defined.

Barleycorns
Barleycorns

"It is remembered that the Iron Ulna of our Lord the King contains three feet and no more; and the foot must contain twelve inches, measured by the correct measure of this kind of ulna; that is to say, one thirty-sixth part [of] the said ulna makes one inch, neither more nor less... It is ordained that three grains of barley, dry and round make an inch, twelve inches make a foot; three feet make an ulna; five and a half ulna makes a perch (rod); and forty perches in length and four perches in breadth make an acre."

Measuring Feet
Measuring feet

The perch or rod, as it was also known, was a traditional Saxon land measure and survives in twentieth century. It had originally been defined as the total length of the left feet of the first sixteen men to leave church on Sunday morning.

Fire in London
Fire in London

The Fire

It is thought that Henry VII (1485-1509) went back 350 years to obtain his standard, and it is likely that it may have been a direct copy of the old standard of Edgar, 'the yardstick', one of the earliest Anglo-Saxon standards.

In 1588 Elizabeth I issued a new standard yard which remained the legal British yard for over 300 years until 1824, when it was superseded by an Act of Parliament under George IV. This act attempted to introduce systems of measures more widely into British society and remove inaccuracies associated with measurement.

The new yard became the first imperial standard and was actually a standard that had been commissioned by the Royal Society in 1742, which in turn had been based on an earlier Elizabethan standard.

In fact this yard had a very short official life (9 years and 198 days) as in 1834 it was damaged in a fire that burned down both Houses of Parliament. A new standard was eventually legalised in 1855 and was based on unofficial standards that had been compared to the Imperial Yard before it was damaged.

France
France

The Polar Quadrant Survey

It had long been realised that a universal standard of measurement was needed, and that it should be a natural constant. In 1791 the French National Assembly decided in favour of a standard that would be one ten millionth part of a quarter of the earth's circumference. The survey which established the length was made from Dunkirk, in France, to Barcelona, in Spain. The work was long and difficult and was carried out during a time when France and Spain were heading to war. On a number of occasions the surveyors were arrested as spies and nearly lost their heads! From this survey a platinum 'end bar' was produced in 1799 which was known as the 'Mètre des Archives' and was the master standard for the world's new measuring system, the so-called metric system.

Initially this new system was rejected by the French people and it only became rigorously enforced from 1837 onwards: by 1870 it was being widely used in continental Europe.

Metre Rules
Metre rules

BIPM

In 1875 the Metre Convention was signed by participating nations and the International Bureau of Weights and Measures (BIPM) was established just outside Paris. In 1889 a number of platinum-iridium metre bars were produced and one of these (number 6) replaced the Mètre des Archives to become the International Prototype Metre. The remaining bars were distributed to the representative nations, lots were drawn and Britain received bar number 16. The alloy from which the bar was made proved to be exceptionally stable. The same cannot be said of the Imperial Standard Yard of 1885, which was made of base metal and shrank at the rate of one part per million in about 20 years.

Britain did not sign the Metre Convention until 1884 and even then was not willing to implement the clause that referred to the introduction of metric measures into signatory countries. The use of metric weights and measures in trade only became lawful in Britain in 1897.

Scientist Measuring Wavelength
Scientist measuring wavelength with a ruler

Krypton

The convenience and stability of the metre in terms of the International Prototype Metre led to its world-wide adoption in preference to the earlier meridian definition. However the advantage of the permanence of a 'natural' standard had not been forgotten and it was suggested that the wavelength of light could be used to establish a natural standard of length. Once the metre had been defined in terms of the wavelength of light from an atomic discharge lamp it could be reproduced in any well equipped laboratory.

Between 1892 and 1940 nine determinations of the metre bar in terms of the wavelength of light were made in various laboratories (including two at NPL between 1932 and 1935). The mean of these nine results became the basis of the new definition of the metre as "the length equal to 1 650 763.73 wavelengths in a vacuum of the radiation ... of krypton-86." This was the way that the metre was defined in 1960 and at the same time the International Prototype Metre was replaced.

Lasers

It was also in 1960 that the first laser was constructed and by the mid 1970s lasers were being used as length standards. In 1983 the krypton-86 definition was replaced and the metre was defined as "the length of the path travelled by light in a vacuum during a time interval of 1/299 792 458 of a second" and it is realised at NPL by iodine-stabilised helium-neon lasers which have a reproducibility of better than ± 3 parts in a hundred thousand million.

Historically the industrial and trade needs of man have led not only to greater accuracies of measurement but also changes in definitions, culminating in the world-wide acceptance of the metre and the metric system.

Length Measurement Today

Today length measurement is used in every sphere of life to enable fair trading conditions and to develop new and improved products and processes that enhance our standard of living.

This ranges from the production of microscopic electronic devices with circuit dimensions made to accuracies of some ten thousand millionths of a metre, to millimetre accuracy in distance measurement in construction over many kilometres, for example to enable the channel tunnel works from France and England to meet in the middle. But this also extends to everyday life where we rely on accurate length measurement to ensure, for example, that our clothes fit or our self assembly furniture goes together.

Lasers are now used as length standards, they work because:

Light produced from the electronically excited Helium and Neon atoms is
Amplified by the
Stimulated
Emission
Radiation produced by housing the laser gain tube between two highly reflective mirrors

There are always a whole number of wavelengths in the round trip between the two mirrors. The wavelength (λ) can be controlled by moving one of the mirrors a minute amount. Iodine molecules absorb a fixed and definite wavelength of light. This is detected by the electronics which maintain the mirror position so that the wavelength of light is held at this value. This process is called laser stabilisation.

Laser Length Measurement
Laser length measurement

The examples of length measurement are too numerous to mention, but we can be sure that the importance of what was started (in our story) some 900 years ago, will continue to grow in the next millennium.

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