History Of Calibration
Assize of measures
It is widely believed that one of the oldest units is that of length measurement used in the ancient world. The 'cubit' was formed which was the length of the arm from the tip of the finger to the elbow. The cubit 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.
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"
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!
In Edward I's reign (1272-1307) the yard (or Ulna) and its sub- and aggregated divisions were defined. "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."
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.
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.
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.
Since 1990, measurements of electrical resistance and voltage at NPL have been traceable to the hall effect and the Josephson effect respectively. Research is also taking place into single electron tunnelling which it is hoped will lead to a quantum standard of electrical current in the longer term.
NPL can provide NAMAS accredited calibration of standard resistors in the range 100 µ? to 1 G? and calibration of voltage standards at 1 V, 1.018 V and 10 V. Higher voltages are catered for by the calibration of voltage ratio devices at voltages of up to 1 kV. The linearity of long scale digital multimeters can now be evaluated directly against our Josephson effect voltage standard.
Measurement of electrical resistance over such a wide range requires a variety of techniques and systems. Cryogenic current comparators are now used in a number of measurement systems at NPL and a project has just been completed which extends the range of these comparators to 100 A.
NIS would like to thank the National Physical Laboratory (www.npl.co.uk) for their help in aiding the research of the history of calibration.