100 Fun Facts About Measurement

Which of the seven SI base units measures amount of substance?

The mole lost its link to a lump of matter in 2019: once pegged to 12 grams of carbon-12, it is now simply a fixed number of particles.

Which SI base unit measures luminous intensity?

Candela is Latin for candle, and the name is honest: an ordinary wax candle shines with roughly one candela.

Since 1983 the metre has been defined by fixing the value of which constant of nature?

The speed of light in a vacuum is fixed at exactly 299,792,458 metres per second, so a metre is simply how far light gets in one 299,792,458th of a second.

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

The North Pole. To pin the length down, two astronomers spent some seven years surveying the line from Dunkirk to Barcelona.

The gram was first defined in 1795 as the mass of one cubic centimetre of what?

Water, at the temperature of melting ice. That is why a litre of it still weighs almost exactly one kilogram, two centuries of redefinitions later.

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

Its mass had drifted from its replicas by perhaps 50 millionths of a gram in a century, and no one could say why, so in 2019 the unit was tied to a constant of nature instead.

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

Caesium took over from astronomy as the world's timekeeper in 1967: Earth's rotation wobbles and slows, while a caesium clock keeps a steady beat. It counts 9,192,631,770 oscillations to the second.

How many seconds are there in a day?

86,400 is 24 times 60 times 60, but now and then a day officially lasts 86,401 seconds: a leap second is added because Earth's rotation is irregular.

Since 2019 the kilogram has been defined by fixing the value of which fundamental constant?

The Planck constant, the tiny number at the heart of quantum physics, took over on 20 May 2019 from a metal cylinder that had defined the unit since 1889.

Chemistry's famous number, about 6.022 x 10^23, is named after which Italian scientist?

Amedeo Avogadro never knew the number: his 1811 idea about gases was ignored for half a century, and the name was coined in 1909, long after his death.

Anders Celsius's 1742 scale ran backwards, with 0 as the boiling point; which naturalist is among those credited with flipping it?

Carl Linnaeus flipped the scale soon after Celsius's death, for the thermometers in his greenhouses, though he is only one of several people credited with the switch.

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

Centigrade, from the Latin for a hundred steps, was dropped in 1948 partly because in some languages the word already meant a hundredth of a gradian, a unit of angle.

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

Salty ice brine, a slush of ice and ammonium chloride, settles at one steady temperature, which gave him a zero anyone could reproduce. His upper mark was body heat, at 96.

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

Add 273.15: a kelvin is the same size as a Celsius degree, but the scale starts at absolute zero, the coldest anything can get, so it never goes negative.

How many feet are in a mile?

5,280 feet has been the mile since a 1593 Act of Parliament. The Roman original was a round 5,000 Roman feet, stretched to fit the lengths English farmers already used for their fields.

Since 1959 the inch has been defined as exactly 25.4 of which metric unit?

Millimetres: since 1959 the inch has officially been a metric quantity, 25.4 of them exactly, even though its name is Latin for a twelfth of a Roman foot.

Which grain gives its name to an old unit of a third of an inch, the step between English shoe sizes?

Barley: a medieval English statute made three grains of it, dry and round, the length of an inch.

Which unit, an eighth of a mile, still measures horse races?

The furlong is a 'furrow long': the length of one ploughed furrow in a medieval common field, later fixed at 220 yards.

An acre was traditionally the area one man could plough in a day with a team of which animals?

Oxen are hard to turn with a heavy plough behind them, which is why the acre began as a long, thin strip: 220 yards by just 22.

How many square metres are in a hectare?

10,000 square metres, a square 100 metres on a side. The name says as much: a hect-are is a hundred ares, the are being a 10-metre square.

Which unit of 14 pounds is still used for body weight in Britain and Ireland?

The stone owes its 14 pounds to the wool trade: a statute of Edward III fixed it in 1350, when local stones ran anywhere from about 5 to 40 pounds.

Why is the pound abbreviated 'lb'?

From the Roman libra, short for libra pondo, 'a pound by weight': English took the word from pondo but the abbreviation, and the pound sign, from libra.

Which system of weights, with an ounce of about 31.1 grams, is the standard for gold and silver?

Troy ounces are heavier than kitchen ones, yet a troy pound, at only 12 ounces, is lighter than the ordinary pound of 16.

The carat, a gem weight, is named after the seeds of which tree?

Carob seeds served as jewellers' weights because they were believed to vary very little; the modern metric carat, agreed in 1907, is exactly 200 milligrams.

Which six-foot unit is used for depths at sea?

The fathom gave English a verb: sailors fathomed the sea with a weighted line, and to fathom something still means to get to the bottom of it.

The nautical mile began as one minute of arc of what?

Latitude, and there lay a snag: Earth is slightly flattened, so a minute of latitude is not the same length everywhere. In 1929 the mile was fixed at exactly 1,852 metres.

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

Hour: the name comes from the knots tied along a ship's log line, which sailors counted as it ran out behind them to judge their speed.

Which four-inch unit still measures the height of horses?

The hand counts in quarters, not tenths: a horse of '15.2' stands fifteen hands and two inches, which is fifteen and a half.

Despite the 'year' in its name, a light-year is a unit of what?

Distance: it is how far light travels in a year, about 9.46 trillion kilometres. The nearest star beyond the Sun is some 4.2 of them away, so its light reaches us over four years old.

Which unit, about 3.26 light-years, do astronomers prefer?

The parsec is 'parallax of one second' squeezed into a word in 1913: from that far away, the gap between Earth and the Sun spans just one arcsecond of sky.

The ångström, a ten-billionth of a metre, is roughly the size of what?

Atoms measure one to a few ångströms across, a hydrogen atom about one, which is why the unit suits chemists. It honours Anders Ångström, a Swedish pioneer of spectroscopy.

Which engineer popularised 'horsepower' to compare his steam engines with the draft horses they replaced?

James Watt needed a sales pitch for customers who still relied on horses, so he rated his engines by how many they could replace. The metric unit of power now carries his name.

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

Alexander Graham Bell was honoured by telephone engineers at the Bell System in 1928, yet the bel named for him is seldom used: its tenth part, the decibel, does all the work.

Which SI unit of electric current honours the physicist who showed that current-carrying wires attract or repel each other?

The ampere honours André-Marie Ampère, who heard in September 1820 that an electric current could swing a compass needle and went on to work out the forces between two wires.

NASA lost which Mars probe in 1999 because its builder's software used pound-force units while NASA's navigators expected metric?

Mars Climate Orbiter came in far too low, within about 57 kilometres of the surface, and was lost in the atmosphere after nine months in flight.

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

Pounds and kilograms: the new jet needed its fuel load in kilograms, but the sum was done with a pounds figure, so the tanks ran dry in mid-air. The crew glided to a former air base at Gimli, by then a racetrack.

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

The Harvard Bridge came to 364.4 smoots 'plus or minus one ear', and the joke aged well: Oliver Smoot later chaired the American National Standards Institute and headed the ISO.

The word 'mile' comes from the Latin for a thousand of what?

Paces: the Roman mile was mille passus, a thousand double steps of a marching soldier, counted each time the same foot hit the ground.

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

The Metre Convention was signed by 17 nations on 20 May 1875, and the date still matters: it is now World Metrology Day, and the SI's 2019 overhaul was timed to take effect on it.

Which country's 1988 law calls the metric system 'preferred' for trade, though daily life there still runs on feet and pounds?

The USA made metric its 'preferred system of weights and measures for trade and commerce' in 1988, but switching was left voluntary, so the mile and the gallon carried on.

A litre is equal to how many cubic centimetres?

1,000 cubic centimetres, a cube 10 centimetres on a side. The name comes down from the litron, an old measure for grain, and beyond that from a Greek word for a pound weight.

The old unit of length called a league is thought to have originally represented what?

An hour's walk, roughly three miles, which makes Jules Verne's Twenty Thousand Leagues Under the Seas a journey's length, not a depth: no ocean is remotely that deep.

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

French: SI stands for Système international d'unités, a reminder that the metric system was born in revolutionary France, and the bureau that guards it still sits in a pavilion outside Paris.

The ancient Egyptian cubit was based on which part of the body?

The forearm, from the elbow to the tip of the middle finger. Egypt's builders used a slightly longer royal cubit: the Great Pyramid was laid out 440 of them along each side and 280 high.

Which scale, devised by a Royal Navy officer in 1805, grades wind from calm to hurricane?

The Beaufort scale began with no wind speeds in it at all: each of its thirteen steps described what the wind did to the sails of a frigate.

The SI added the prefixes ronna and quetta in 2022 to keep pace with what?

Digital data was about to outgrow yotta, the old top prefix, and unofficial names were already circulating, so the SI added ronna and quetta for 27 and 30 zeros.

In 1960 the metre was redefined as a set number of wavelengths of radiation from which element?

Krypton won a contest: American scientists had backed mercury, but in 1960 the metre became a set number of wavelengths from a krypton-86 lamp, retiring a metal bar that had served since 1889.

The Mohs scale ranks minerals from talc to diamond by which property?

Hardness, tested by scratching: each mineral marks the ones below it, so a fingernail, at about 2.5, scratches talc and gypsum but not calcite.

The Scoville scale measures the heat of which food?

Chili peppers were first rated by taste alone: in 1912 Wilbur Scoville diluted pepper extract in sugar syrup until a panel of tasters could no longer feel the burn.

Charles Richter devised his 1935 earthquake scale to compare tremors in which US state?

California's network of seismographs set the scale, on which each whole number marks a tenfold jump in shaking. Scientists now use a newer measure that the news still calls 'Richter'.

Which tradespeople gave their name to a 'dozen' that counts thirteen items instead of twelve?

Bakers who sold short weight in medieval England risked a fine or a flogging, so in one telling they threw in a thirteenth loaf to be safe. Another account makes the extra loaf the retailer's profit.

America's gallon is smaller than Britain's because it began as an English measure for which drink?

Wine was measured by Queen Anne's gallon of 231 cubic inches, which the United States kept as its standard. Britain abolished it in 1824 in favour of a single larger gallon.

The marathon's odd length of 26 miles 385 yards was first run at the 1908 Olympics in which city?

London's course ran from Windsor Castle to a finish in front of the Royal Box at White City Stadium, and that exact length became the world standard in 1921.

To 'prove' a spirit's strength, testers once soaked which substance in it to see if it still burned?

Gunpowder that still flared after its soaking marked a spirit as above proof, a strength later pinned at about 57 percent alcohol.

Which sport's pitch is one surveyor's chain long, a unit of 22 yards devised by Edmund Gunter?

Cricket preserves the chain as the gap between its two wickets. Gunter's original was a real chain of 100 links, made for surveying land.

At which reading do the Celsius and Fahrenheit scales, 32 apart at freezing, show the same number?

−40 is the one reading the two scales share, because a Fahrenheit degree is only five-ninths the size of a Celsius degree and the gap of 32 closes as the readings fall.

Which soft-drink company brought metric shopping to America with the first two-litre bottle?

PepsiCo acted on research by its marketing chief John Sculley, later the head of Apple, and the bottle became one of the few things Americans buy in round metric amounts.

The astronomical unit, the Solar System's yardstick, is roughly how far Earth lies from which body?

The Sun lies about 150 million kilometres from Earth, a gap its light takes roughly 499 seconds to cross.

Which commodity is priced in 'barrels' of 42 US gallons, a size fixed in 1860s Pennsylvania?

Crude oil got its standard barrel when Pennsylvania producers, who had been shipping in whatever casks they could find, settled on one size to win back buyers' trust.

What are the true dimensions, in inches, of a 'two-by-four' plank from an American hardware store?

1.5 by 3.5 inches is all that remains once the board has been dried and planed smooth. The name survives from the rough green timber that once measured a full two by four.

Which Thames-side district names the prime meridian adopted as the world standard in 1884?

Greenwich was chosen at an international conference held in Washington, D.C.; its meridian had been set in 1851 by Britain's Astronomer Royal, George Airy.

What name is given to the coldest possible temperature, which matter can approach but never reach?

Absolute zero stays out of reach because removing the last traces of heat grows ever less efficient. Even there, particles would keep a faint quantum jitter.

Which metric prefix, from the Greek for 'thousand', goes in front of gram, metre and calorie?

Kilo comes from the Greek chilioi, and the English scientist Thomas Young found the coinage so barbarous that some 19th-century writers followed him in spelling it 'chilio'.

Which paper size, 8.5 by 11 inches, is standard in American offices in place of A4?

Letter became the norm for US federal forms only in the early 1980s, under the Reagan administration; before that the government used a smaller sheet of its own.

Which unit of computer storage did IBM's Werner Buchholz coin by misspelling a common word?

The byte was respelled from 'bite' so that a slip of the pen could never turn it into 'bit'. Its size varied from machine to machine before eight bits became the norm.

Which dashboard instrument, first described by the Roman Vitruvius, records how far a car has gone?

The odometer Vitruvius described dropped a pebble into a box for every Roman mile a chariot's wheels turned, so a journey was tallied by counting pebbles.

Britain's Weights and Measures Act of 1824 gave which name to its unified pints, gallons and miles?

Imperial units replaced the older Winchester standards, which is why American pints and gallons, inherited before the change, no longer match Britain's.

The quoted size of a television or phone screen, such as 55 inches, is measured along which line?

The diagonal became the rule with the first televisions, whose picture tubes were round, so the picture was sized from corner to corner.

Which unit of 500 sheets of paper takes its name from an Arabic word for a bundle?

A ream reached English from the Arabic rizmah by way of Spain, where the Moors had introduced papermaking. An older 'short' ream held only 480 sheets.

Which ancient people, heirs to Sumerian base-60 counting, gave us the 60-second minute?

The Babylonians counted in sixties, a base that divides cleanly because 60 is the smallest number divisible by every number from one to six.

Which unit of frequency honours the German physicist who first proved electromagnetic waves exist?

The hertz replaced the plain phrase 'cycles per second' in 1960, though the popular press only began to make the switch in the late 1960s.

Which electrical unit honours the Italian whose 'pile' of stacked metal discs was the first battery?

The volt honours Alessandro Volta, whose pile so impressed Napoleon Bonaparte that he was invited to demonstrate it at the Institute of France.

Which unit of energy honours the English brewer who showed that mechanical work turns into heat?

The joule honours James Prescott Joule, who managed his family's brewery and first turned to physics to see whether electric motors could replace its steam engines.

Which unit of pressure honours the author of the Pensées, who built an early mechanical calculator?

The pascal honours Blaise Pascal, a child prodigy so precocious that René Descartes was convinced one of his early works had been written by his father.

Which measure of acidity did the Dane Søren Sørensen devise at the Carlsberg brewery's laboratory?

pH holds a small mystery: Sørensen never explained what his lowercase 'p' stood for, and scholars dispute its exact meaning.

Which term for speed relative to the speed of sound comes from the name of an Austrian physicist?

Mach 1 is the speed of sound itself; the name was proposed in 1929 by the aeronautical engineer Jakob Ackeret, in honour of the physicist and philosopher Ernst Mach.

Doctors quote blood pressure in 'mmHg', a unit based on the height of a column of which element?

Mercury takes its symbol Hg from the Latin hydrargyrum, and the pressure beneath a short column of the heavy metal once defined the unit medicine has kept.

Which name is given to a champagne bottle of 1.5 litres, twice the standard size?

Magnum bottles are thought to make better champagne, because there is less oxygen inside for the amount of liquid. The bigger sizes mostly borrow the names of biblical figures.

Which old word for a group of twenty appears in the opening of Lincoln's Gettysburg Address?

Score opens the speech in 'Four score and seven years ago', Lincoln's way of saying 87. The word probably began with shepherds cutting a notch in a stick for every twenty animals counted.

What name, from a medieval phrase meaning 'large dozen', is given to a count of 144 items?

A gross multiplied by another dozen makes a great gross of 1,728, while ten dozen, or 120, was once known as a small gross or a great hundred.

Which metric prefix meaning one billionth takes its name from the Greek word for 'dwarf'?

Nano was officially endorsed as a prefix in 1960, and it suits the machinery inside living cells: a ribosome is about 20 nanometres across.

Which weather instrument, credited to Galileo's secretary Evangelista Torricelli, measures air pressure?

The barometer's inventor Torricelli, Galileo's secretary for the last three months of the old man's life, argued that it was the weight of the atmosphere that held up the liquid in his tube.

Which English charter, sealed by King John, also ordered standard measures for ale, corn and cloth?

Magna Carta is best known for limiting royal power, but its clause 35 turned to the marketplace and ordered uniform measures throughout the kingdom.

Which Greek librarian of Alexandria made the first known calculation of Earth's circumference, using a noon shadow?

Eratosthenes knew that at noon on the summer solstice upright rods cast no shadow at Syene, far to the south, while those at Alexandria still did, and the difference gave him the planet's curve.

Le Grand K, the cylinder that defined the kilogram until 2019, sits in a vault near which city?

Paris is the nearest city: the vault lies in suburban Saint-Cloud and can be opened only with three independently controlled keys.

Which fruit names an informal 'equivalent dose' used to explain tiny radiation exposures?

Banana flesh contains potassium-40, a naturally radioactive form of the mineral, so eating one delivers a tiny dose that makes a handy teaching yardstick.

In Japan, the size of a room is traditionally given as the number of which floor mats it holds?

Tatami mats pair up to form a square, and that two-mat square, the tsubo, is itself a traditional Japanese unit for the floor area of houses.

The ohm, the unit of resistance named after Georg Ohm, is written with which Greek symbol?

Omega was suggested in 1867 by William Henry Preece because the word sounds so much like 'ohm'. Conductance was long measured in the 'mho', which is ohm spelled backwards.

Which dry measure for crops is paired with 'a peck' in a love song from Guys and Dolls?

The bushel in 'I love you, a bushel and a peck' holds four pecks. In the grain trade it has become a set weight that differs from crop to crop.

Which English joiner solved the problem of longitude at sea by inventing the marine chronometer?

John Harrison chased a reward of up to £20,000 that Parliament offered after a naval disaster off the Scilly Isles in 1707, yet political rivalries meant he never received it in full.

Which future president, as secretary of state, proposed decimal weights and measures to Congress?

Thomas Jefferson tied his 1790 plan to the swing of a one-second pendulum; a Senate committee endorsed his decimal units, but Congress never made them law.

Which country changed every speed limit sign from miles to kilometres over one weekend in 1977?

Canada made the switch over the Labour Day weekend under an agreement among its provinces, and new cars had to carry speedometers marked in kilometres.

Which railway pioneer gave his name to the standard track gauge of 4 feet 8½ inches?

George Stephenson's gauge won Britain's 'gauge war' against a rival seven-foot track because the lines already built to it ran eight times as far.

Which US agency, once the National Bureau of Standards, keeps the country's measurement standards?

NIST took its present name in 1988, and the atomic clocks at its Boulder, Colorado, laboratories help keep official US time.

Which system of weights, built on a pound of 16 ounces, was developed for the medieval wool trade?

Avoirdupois began as an Anglo-Norman phrase for 'goods of weight', a class of merchandise sold by the pound, before it came to name the weights themselves.

Lord Kelvin, namesake of a unit of temperature, took his title from a river by which university?

Glasgow's River Kelvin flows near the laboratory where the physicist William Thomson worked, and he took its name when he was made a baron in 1892.

Which tiny unit of area did nuclear physicists of the 1940s jokingly name after a farm building?

The barn, about the cross-section of a uranium nucleus, was named over a cafeteria dinner in 1942 by two Manhattan Project physicists, who joked that it was 'really as big as a barn'.

To fix the first metre, Delambre and Méchain surveyed an arc from Dunkirk to which Spanish city?

Barcelona's Montjuïc castle marked the southern end of the survey and a belfry in Dunkirk the northern, and the work took the two astronomers from 1792 to 1798.

Which unit of angle, the SI's rival to the degree, fits just over six times into a full circle?

The radian is the angle made when an arc as long as a circle's radius is laid along its edge, so a full turn holds two pi of them, or about 6.28.

Which unit on a spectacle prescription gives a lens's power as the reciprocal of its focal length?

The dioptre turns prescriptions into simple arithmetic, because the powers of lenses roughly add together. The relaxed human eye has a power of about 60 dioptres.