50 Fun Facts About Measurement

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

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

Which SI base unit measures luminous intensity?

Its name is Latin for candle, and an ordinary wax candle gives out roughly one of them.

Since 2019, how is the metre defined?

All three distractors were earlier definitions: the pole-to-equator fraction in 1791, the bar from 1799 and krypton light in 1960.

The 1791 metre was defined as one ten-millionth of the distance from the equator to where?

That set Earth's quarter-meridian at exactly 10,000 km, at least on paper.

How was the kilogram originally defined in 1795?

The modern definition, fixed to the Planck constant, still agrees with that litre of water to within 0.003%.

Why was the International Prototype of the Kilogram retired as the standard in 2019?

The kilogram is now defined by fixing the Planck constant, so any lab with a Kibble balance can realise it.

The SI second is defined by a transition frequency of which element's atoms?

The number is 9,192,631,770 cycles of caesium-133 radiation per second.

How many seconds are there in a day?

24 hours times 60 minutes times 60 seconds; the day-based second is still used by the UT1 time system.

On 20 May 2019, four SI base units were redefined in terms of physical constants. Which four?

The date was the 144th anniversary of the Metre Convention; the metre, second and candela had already been redefined earlier.

How many elementary entities are in one mole?

That is the Avogadro number, roughly 602 sextillion.

On Anders Celsius's original 1742 scale, what temperature did 0 degrees represent?

His scale ran backwards; it was flipped after his death, with Linnaeus among those credited.

What was the Celsius scale commonly called in English before 1948?

From the Latin centum and gradus, meaning 100 steps.

According to Fahrenheit's original paper, 0 °F was the freezing point of what?

His upper point was his estimate of human body temperature, first 90 then 96 degrees.

How do you convert a temperature in degrees Celsius to kelvins?

The Kelvin scale starts at absolute zero, so it has no negative values.

How many feet are in a mile?

That is 1,760 yards; since 1959 the mile has been exactly 1,609.344 metres.

Since 1959, the inch has been defined as exactly how many millimetres?

The word comes from the Roman uncia, meaning a twelfth, and the unit was long tied to a thumb's width.

What fraction of an inch is a barleycorn, the unit still behind shoe sizes?

A medieval statute defined the inch as 'three barleycorns, dry and round'.

A furlong is what fraction of a mile?

That's 220 yards; the unit survives mainly in horse racing.

An acre was traditionally the area one man could plough in a day with how many oxen?

Formally it is one chain by one furlong, or 43,560 square feet.

How many square metres are in a hectare?

That is a square with 100-metre sides, roughly 2.47 acres.

How many pounds are in a stone, the unit still used for body weight in Britain and Ireland?

Historic European 'stones' ranged anywhere from about 5 to 40 local pounds.

Why is the pound abbreviated 'lb'?

The £ sign for pounds sterling has the same Roman ancestor.

A troy ounce, the standard for precious metals, weighs about how many grams?

It is heavier than the avoirdupois ounce, but a troy pound of 12 troy ounces is lighter than a normal pound.

A carat, used for gemstones, is equal to how many milligrams?

The metric carat was fixed in 1907 and is divided into 100 'points' of 2 mg each.

How many feet are in a fathom?

Originally the span of a man's outstretched arms, it was the English-speaking world's standard for water depth.

The international nautical mile is defined as exactly how many metres?

It began as one minute of latitude, so the Earth's polar circumference is close to 21,600 nautical miles.

A knot is a unit of speed equal to one nautical mile per what?

A ship doing 1 knot along a meridian covers about one minute of latitude in an hour.

The 'hand', used to measure horses, is standardised at how many inches?

Its subdivisions are in quarters, so 15.2 hands means 15 hands and 2 inches, not 15.2 in decimal.

A light-year is a unit of what?

It is about 9.46 trillion km, the distance light travels in one Julian year of 365.25 days.

A parsec is approximately how many light-years?

It is the distance at which one astronomical unit subtends an angle of one arcsecond; Proxima Centauri is about 1.3 parsecs away.

An angstrom equals what fraction of a metre?

That's 0.1 nanometre, roughly the scale of atoms; it is named for Swedish physicist Anders Ångström.

Which Scottish engineer coined 'horsepower' to compare steam engines with draft horses?

He measured a 'brewery horse' at 32,400 foot-pounds per minute in 1782 and rounded up to 33,000.

The decibel is one tenth of a unit named after which inventor?

The bel itself is rarely used; the decibel came out of early telephone engineering at the Bell System.

The ampere is named after a physicist of which nationality?

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

NASA lost the 1999 Mars Climate Orbiter because one team's software reported thruster impulse in which non-SI unit?

The mismatch put the numbers off by a factor of 4.45 and sent the probe too deep into the Martian atmosphere.

The 'Gimli Glider' 767 ran out of fuel in 1983 because of confusion between which two units?

The new all-metric jet tracked fuel in kilograms, but the crew used a pounds-per-litre density figure; the plane glided to a former air base in Manitoba.

The 'smoot' comes from an MIT student lying down repeatedly across which structure in 1958?

One smoot is Oliver Smoot's height, 5 ft 7 in; the bridge measured 364.4 smoots 'plus or minus one ear'.

How many yards are in a mile?

A yard has been exactly 0.9144 metres by international agreement since 1959.

Which international treaty of 1875 established the body that coordinates the SI today?

It set up the International Bureau of Weights and Measures (BIPM) and the General Conference on Weights and Measures.

Under US law, what has been the 'preferred' system of weights and measures for trade since 1988?

A 1991 executive order required federal agencies to go metric, but conversion was never made mandatory for the public.

A litre is equal to how many cubic centimetres?

It's a 10 cm cube; the name comes from the litron, an old French unit that was actually a weight.

The league, an old unit of length, is thought to have originally represented what?

Its value varied from 2.2 to 7.9 km across Europe, and no country uses it officially today.

The abbreviation SI for the modern metric system comes from its official name in which language?

Système international d'unités is the only measurement system with official status in nearly every country.

Roughly how long was the ancient Egyptian cubit?

Surviving cubit rods measure 523 to 529 mm; the English ell, at two cubits, was standardised at 1,143 mm.

In which year did the 11th General Conference on Weights and Measures adopt the SI?

The 9th CGPM had commissioned a study in 1948 to design a single practical system for all Metre Convention nations.

How many metric prefixes does the SI provide, the newest of which were adopted in 2022?

They span decimal powers from 10 to the minus 30 up to 10 to the 30, and all but four are integer powers of a thousand.

In 1960 the metre was redefined in terms of wavelengths of light emitted by which element?

That replaced the platinum-iridium bar with two marks that had served since 1889; the speed-of-light definition followed in 1983.

Which instrument lets a metrology lab realise the kilogram directly from the Planck constant?

The 2019 redefinition fixed the Planck constant at exactly 6.62607015 x 10^-34 joule-seconds.

What was the 1793 revolutionary precursor of the kilogram, the mass of a litre of water, called?

The 1795 decree renamed the grave the kilogramme and its thousandth part, the gravet, the gramme.

Who proposed in 1670 a universal length unit, the 'virga', of 1/1000 of a minute of arc of latitude?

His virga would have been about 1.852 metres, essentially a thousandth of what became the nautical mile.