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Early history

The first units of length were based on the human body, the most obvious example being the foot. Other body-based units are the cubit, which is the length of the arm from the elbow to the tip of the middle finger (roughly 18 to 22 inches), and the span, which is the distance between the tip of the thumb and little finger when the fingers are extended. The hand is still used to measure the height of a horse. Needless to say, these units can vary widely from person to person, and an example of allowing for this variance can be seen in the 12th century Scottish definition for the inch, which was taken to be the thumb width from three men - one big, one medium, one small. Attempts to standardise the units of length date back many hundreds of years - for example this 16th century engraving shows the determination of the average foot. Surveyors would wait outside a church on Sunday, and line up the first sixteen men that came out.

1795

The metre was initially defined as one ten millionth of the distance along the Earth's meridian from pole to equator. The meridian chosen for this definition in 1795 was that passing through Paris and Barcelona. The subsequent Physical embodiment of this metre, known as the international prototype metre, was the distance between two lines engraved on a platinum bar held at the International Bureau of Weights and Measures (BIPM) in Paris.

1960

There have been two recent redefinitions of the metre, which have moved away from the artefact-based standard. First, in 1960, the General Conference of Weights and Measures (CGPM) redefined the metre in terms of the wavelength of a particular orange-red coloured light (spectral line) emitted by a krypton lamp. This was a major advance, because, for the first time, the metre could be experimentally reproduced anywhere in the world, without reference to the platinum metre bar in Paris.

Unfortunately, the krypton lamp was not a particularly convenient light source for length measurement by interferometry.

1983

During the 1960s, the world was presented with a new type of optical technology that was referred to in jest as "a solution in search of a problem". This device was the laser - an acronym for Light Amplification by Stimulated Emission of Radiation. The laser's characteristics are that it is a source of intense, coherent light - well suited to use in interferometry. This development prompted the second redefinition of the metre, made by the CGPM in 1983, which defined the metre as follows:

"The metre is defined as the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second"

In essence there is no longer a primary standard of length! This apparently strange definition actually defines the metre more accurately, because the time interval can be measured with an accuracy of better than 1 in 10¹¹. By comparison, stabilised lasers are 100 times less accurate.

When the CGPM redefined the metre in 1983, they also defined a list of recommended radiations for the practical realisation of the metre. Each of these is essentially an optical frequency standard, traceable to the caesium primary frequency standard. Internationally, the most commonly used of these radiations is the visible light produced by an ordinary red helium-neon laser, frequency stabilised by saturated absorption in iodine:

At frequency f = 473 612 214.8 MHz

And vacuum wavelength l = 632 991 398.1 fm

with an estimated overall uncertainty of ± 1x10^-9. This frequency applies to the radiation of a stabilised He-Ne laser containing an iodine cell, subject to certain operating conditions.

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