Tag Archives: seconds pendulum

The Metre


By T. V. Antony Raj

The Meter is a metric measurement slightly longer than a yard; thus, a 100-meter dash might take you a second longer than a 100-yard dash. – Definition of Meter by Merriam-Webster.

1 metre ≈ 1.0936 yard or 39.370 inches.


A seconds pendulum is a pendulum whose period is precisely two seconds; one second for a swing in one direction and one second for the return swing, a frequency of 1/2 Hz. Christiaan Huygens had observed that length as 38 Rijnland inches or 39.26 English inches; that is, 997 mm.


The second pendulum, with a period of two seconds so each swing takes one second (Original simulation by Wolfgang Christian and F. Esquembre)


In 1660, Christopher Wren suggested the use of the seconds pendulum to define length to the Royal Society. In 1668, John Wilkins, an English cleric and philosopher in an essay proposed the adoption of a decimal-based unit of length using the universal measure or standard based on a seconds pendulum. However, the Royal Society took no official action on these suggestions.


The metre was originally defined in 1791 as being 1/10,000,000 of the distance from the North Pole to the Equator through Paris, making the kilometre 1/10,000 of this distance. (Source: Globe Atlantic.svg)

During the French Revolution that lasted 10 years from 1789 to 1799, the French Academy of Sciences charged a commission with determining a single scale for all measures. On October 7, 1790, that commission advised adopting the decimal system, and on March 19, 1791, advised adopting the term mètre (Greek “measure”), a basic unit of length, which they defined as equal to one ten-millionth of the distance between the Earth’s equator and the North Pole through Paris, thus making the kilometre 1/10,000 of this distance.

In 1793, the French National Convention adopted the proposal. The use of metre in English began at least as early as 1797.

The metre (British spelling and BIPM spelling) or meter (American spelling) from the French unit mètre, derived from the Greek noun μέτρον (“measure”) is the base unit of length in some metric systems, including the International System of Units (SI). The SI unit symbol is m.

In 1799, the metre was redefined in terms of a prototype metre bar. However, it was later determined that the first prototype metre bar was short by about 200 micrometres because of miscalculation of the flattening of the Earth, making the prototype about 0.02% shorter than the original proposed definition of the metre. Regardless, this length became the French standard and was progressively adopted by other countries in Europe.

The main problem with defining the length standard by an artefact such as the meter bar is that there is no sure way to determine if it has changed length due to age, deterioration, or misuse. It can be compared to other bar standards, but these may have changed length themselves.

In the 1870s and in light of modern precision, a series of international conferences were held to devise new metric standards. The Metre Convention (Convention du Mètre) of 1875 mandated the establishment of a permanent International Bureau of Weights and Measures (BIPM: Bureau International des Poids et Mesures) in Sèvres, France. This new organisation was to construct and preserve a prototype metre bar, distribute national metric prototypes, and maintain comparisons between them and non-metric measurement standards.

The BIPM made 30 prototype standard bars of 90% platinum–10% iridium alloy. One of the bars was selected as the International Meter. In 1889 at the first General Conference on Weights and Measures (CGPM: Conférence Générale des Poids et Mesures), the International Prototype Metre was established as the distance between two lines on a standard bar composed of an alloy of 90% platinum and 10% iridium, measured at the melting point of ice.

The Prototype Metre bars had a modified X cross-section named for the French scientist, Henri Tresca, who proposed it.

After selecting the bar for use as the International Prototype Meter, the other bars were calibrated relative to it and were given to nations to serve as their national standards.


Closeup of National Prototype Meter Bar No. 27, made in 1889 by the International Bureau of Weights and Measures (BIPM) and given to the United States, which served as the standard for defining all units of length in the US from 1893 to 1960.


The United States received the National Prototype Meter Bar No. 27, and No. 21 in 1890. The US adoption of the metric system in 1893 made the meter the fundamental length standard of the US, and No. 27 became the primary national standard for all length measurements.

Now, this original international prototype of the metre is now in the collection of the NIST Museum, Gaithersburg, Maryland, USA, because in 1960 the SI changed the standard of length to define the meter by the wavelength of light of a spectral line of krypton 86.