60 Fun Facts About Metric System

How many base units does the modern International System of Units (SI) have?

They are the metre, kilogram, second, ampere, kelvin, mole and candela; everything else, from the newton to the tesla, is derived from them.

The metric prefix "kilo" multiplies a unit by what factor?

"Milli" divides by the same factor, so a milligram is a thousandth of a gram.

What is the process of adopting the metric system called?

The SI has been adopted as the official system of weights and measures in almost every country in the world.

Besides America, which two countries did NIST identify as lacking mandatory metric laws?

Both have since announced moves toward the metric system, with Liberia pledging to adopt it in 2018.

The metric system was formally defined in French law on 7 April of which year?

The same decree fixed the gram as the weight of a cubic centimetre of water at the temperature of melting ice.

On the eve of the Revolution, roughly how many different units of measure were in use across France?

Because each unit could differ from town to town, they had up to a quarter of a million different definitions.

The 1791 definition of the metre made it one ten-millionth of the distance from the equator to what?

In practice the length was fixed by surveying the arc of the Paris meridian between Dunkirk and Barcelona.

Which two astronomers surveyed the meridian arc from Dunkirk to Barcelona to fix the length of the metre?

The job was expected to take two years and took more than six, from 1792 to 1798, amid the chaos of the Revolution.

Which French abbot proposed in 1670 a decimal system of length based on the Earth's dimensions?

His unit, the virgula, could be reproduced anywhere with a pendulum that beat 3,959.2 times in half an hour.

Which British inventor called in 1783 for a global decimal measurement system based on water's density?

He was struggling to communicate with German scientists; five years later a French chemist commissioned decimal brass cylinders for his own experiments.

Which chemist defined the gram with René Just Haüy before the kilo prefix was added in 1792?

The result is that the kilogram is the only SI base unit with a prefix built into its name.

What was Napoleon's 1812 hybrid measurement system with old customary names called?

The metric system was reimposed by the 1837 Assembly and took effect again in 1840.

The Metre Convention was signed in Paris on 20 May 1875 by representatives of how many nations?

The signatories ranged from Argentina and the Ottoman Empire to the United States and Venezuela.

The international prototypes of the metre and kilogram were made of platinum alloyed with which metal?

The 90/10 alloy is exceptionally hard; the London firm Johnson Matthey delivered 30 metre bars and 40 kilogram cylinders.

The prototype metre bar had a special X-shaped cross section, named after which engineer?

The shape minimised torsional strain during length comparisons; the kilograms were simple cylinders.

The International Prototype of the Kilogram, Le Grand K, defined the kilogram from 1889 until which year?

It sits under three bell jars in a vault near Paris that takes three independently controlled keys to open.

How many times was the IPK formally compared with its official copies during its 130-year reign?

The verifications of 1889, 1948 and 1989 showed the copies drifting apart by about 50 micrograms.

The 2019 SI revision defines the kilogram by fixing the exact value of which quantity?

The Kibble balance, called the watt balance until 2016, lets laboratories realise the unit without any artefact.

Since 1983 the metre has been the distance light travels in what fraction of a second?

This definition replaced one based on the wavelength of a spectral line; it fixes the speed of light exactly.

Before the 1983 light-speed definition, the metre was defined by light from which element?

Before that, from 1889, the standard was a platinum-iridium bar kept near Paris.

The SI was launched by the CGPM in which year, initially with six base units?

The mole was added as the seventh base unit later, in 1971.

The second has been defined since 1967 by the hyperfine transition frequency of which atom?

The number is exactly 9,192,631,770 cycles per second, and a good caesium clock can detect a 2 cm change in altitude by gravitational time dilation.

One mole contains exactly how many elementary entities?

That figure is the Avogadro number, and it was fixed as an exact value in the most recent SI revision.

The kelvin was formally added to the SI in 1954 by defining 273.16 K as the temperature of what?

It is now defined instead by fixing the Boltzmann constant.

The Celsius scale was known in English by what name until it was renamed in 1948?

The change honoured Anders Celsius, whose 1742 original ran the other way, with 0 at boiling and 100 at freezing.

In which year did an international convention at an exposition establish the ampere as the unit of current?

It honours André-Marie Ampère, considered a father of electromagnetism alongside the Dane Hans Christian Ørsted.

The candela is the SI unit of what?

A common wax candle gives out roughly one of them.

A litre is exactly equal to which volume?

The word comes from the litron, an old French unit that itself descended from a Byzantine Greek unit of weight, not volume.

From 1901 to 1964 the litre was defined not as a cube but as the volume of what?

That made the old litre about 1.000028 of the modern one, a discrepancy finally abolished in 1964.

A hectare is the area of a square with sides of what length?

It equals 100 ares, the original unit of 100 square metres from the first metric law, and there are 100 hectares in a square kilometre.

The tonne, or metric ton, corresponds exactly to which SI unit?

The symbol t was adopted by the BIPM in 1879, and NIST accepts it in the United States too.

Why do metric prefixes attach to the gram, even though the kilogram is the SI base quantity for mass?

So a thousandth of a kilogram is a gram, never a "millikilogram".

Which four metric prefixes were adopted in 2022?

R and Q were the only Latin letters left unused, and the large ones were added in anticipation of data-science needs.

In the 19th century, which prefix from the Greek for ten thousand meant 10,000?

It fell out of use and is not part of the SI.

Before the 1935 extensions broke the pattern, how were the languages of the prefixes originally divided?

Nano- and micro- both have Greek roots despite being fractional prefixes.

Which SI derived unit for catalytic activity, equal to one mole per second, was added in 1999?

Its addition shows how the governing body extends the system as new needs arise.

CGS had two energy units, the erg and the calorie. Which single unit of energy did the coherent SI define?

In a coherent system, Einstein's E = mc² needs no extra conversion constants.

The Metric Act of 1866, legalising metric use in US commerce, is also named for whom?

President Andrew Johnson signed it on 28 July 1866, and each state was supplied with a set of metric standards.

Which 1893 directive made metric standards the fundamental American standards for length and mass?

American customary units have been legally defined in terms of the metre and kilogram ever since.

Which 1975 US law set up a national board to plan a voluntary changeover to SI?

The 1988 Omnibus Trade Act later amended it to name SI the preferred system for trade and commerce.

NASA's Mars Climate Orbiter was lost in 1999 because its builder Lockheed Martin supplied data in what?

The probe descended to about 57 km instead of the planned 150 km and presumably burned up.

Confusion between which two units helped cause the 1983 'Gimli Glider' fuel-exhaustion incident?

A failed fuel-quantity sensor forced the crew to calculate the load by hand, and the aircraft was underfilled.

A formal UK government policy to support the changeover to metric units was agreed by which year?

Mandatory metric units for packaged goods only arrived in 1995, and for loose goods in 2000.

As of 2024, British law mandated imperial units alone for which of these?

Draught beer and cider, returnable milk bottles and precious metals were the other legal holdouts.

Which set of measures, with a pint over twice the English size, did the Weights and Measures Act 1824 outlaw?

Both systems had been in use in Scotland despite sharing many of the same names.

A 1790 proposal floated by France to Britain and America would have based the metre on what?

Both the British Parliament and the US Congress rejected it, a position that lasted nearly two centuries.

Where is the headquarters of the International Bureau of Weights and Measures (BIPM)?

France offered the treaty members the Pavillon de Breteuil, built in 1675 on the estate of the Château de Saint-Cloud.

What became of Joseph Dombey, sent from France in 1793 with a standard kilogram for the US?

Thomas Jefferson had requested the equipment to evaluate the metric system; the kilogram never arrived.

Which US president's 1991 executive order required all federal agencies to adopt the metric system?

Conversion was not made mandatory for the public, so customary units remain in everyday use.

Which Swiss state adopted the metric system by law in 1801, prompted by Johann Georg Tralles?

Swiss immigrant Ferdinand Rudolph Hassler carried copies of the French metre and kilogram to the United States in 1805.

Which 1999 failure was caused by a subcontractor reporting customary units on a metric NASA project?

NASA had specified metric units in the contract, but Lockheed Martin's software reported thruster data in pound-force seconds.

Which engineer designed the X-shaped cross section of the 1889 prototype metre bar?

The shape reduced flexing when measurements were taken; Deville cast the platinum-iridium ingot in 1873.

What broke out in July 1870, two weeks before the International Metre Commission was due to meet?

Delegates met without Germany and agreed to reconvene once all nations, including a German delegation, could attend.

How many members did the General Conference on Weights and Measures have as of October 2024?

It also had 37 associate states and economies, a category created in 1999.

How many metrologists sit on the International Committee for Weights and Measures (CIPM)?

Originally fourteen, they come from different member states and advise the CGPM, much like a board of directors.

The authoritative language of all official Metre Convention documents is which language?

The original 1875 treaty was written in French, and France communicates with the BIPM via its foreign minister.

In which year was the Metre Convention amended to extend beyond units of mass and length?

The sixth CGPM broadened its scope to other fields of physics; the SI itself followed at the eleventh meeting in 1960.

The MTS unit system, used in France and the USSR from 1933 to 1955, was based on the metre, second and what?

Its unit of force was the sthène and its unit of pressure the pièze.

Which Italian physicist proposed in 1901 a coherent system built on metre, kilogram, second and one electrical unit?

The idea fed into the MKS-based proposal the CIPM put to the 10th CGPM in 1954.

Which numbered provision of Magna Carta in 1215 required a single standard of measure throughout England?

It declared there was to be a single measure of wine, ale, corn and cloth across the realm.