Measures, units and numbers

Metre

1000 metres make a kilometre.

💡 Did you know? It is defined via the speed of light.

Kilogram

It is the only base unit with a prefix.

💡 Did you know? It was once a metal cylinder in Paris.

Second

60 seconds make a minute.

💡 Did you know? It is defined via a caesium atom.

Watt

A light bulb was often 60 watts.

💡 Did you know? Named after inventor James Watt.

Byte

One byte is eight bits.

💡 Did you know? 1024 bytes make a kibibyte.

voltage

The volt is the unit of electrical voltage: in a sense the 'pressure' that drives current through a wire. It is named after Alessandro Volta.

💡 Did you know? Around 1800 Volta built the first battery, the 'voltaic pile'.

a distance

It is the distance light travels in one year: just under 9.5 trillion kilometres.

💡 Did you know? The nearest star after the Sun, Proxima Centauri, is a good four light years away. We see it as it looked four years ago.

9

1,000,000,000: a thousand million.

💡 Did you know? German calls this number a Milliarde; the German Billion is a thousand times larger. The mix-up has caused million-euro errors in translations.

oscillations per second

50 hertz means fifty oscillations in one second. The European power grid runs at that frequency.

💡 Did you know? The unit is named after Heinrich Hertz, who proved electromagnetic waves in 1886. He thought his discovery useless: radio grew out of it a few years later.

10,000

A square of 100 by 100 metres. A hundred hectares make a square kilometre.

💡 Did you know? A football pitch is clearly smaller at around 0.7 hectares: the rule of thumb that a pitch equals a hectare does not hold.

a constant of nature

Before that a metal cylinder near Paris served as the prototype. Today the unit derives from the Planck constant.

💡 Did you know? The reason for the change: the prototype was measurably losing mass against its own copies: so it was no longer reliably one kilogram.

lines of latitude

Lines of longitude, by contrast, run from pole to pole and meet there.

💡 Did you know? One degree of latitude is about 111 kilometres everywhere. For longitude the spacing shrinks to zero towards the poles.

1.6

Exactly 1.609344 kilometres. The nautical mile is slightly longer at 1.852 kilometres.

💡 Did you know? The nautical mile is no arbitrary figure: it matches one minute of arc on a meridian: which is why sailors and pilots still use it.

-40

The scales cross exactly once, and that is where.

💡 Did you know? Fahrenheit set his zero at the lowest temperature he could reach in the winter of 1708/09 in Danzig: reproducible with a salt solution.

144

The measure comes from trade and was standard as long as people counted in twelves.

💡 Did you know? Twelve was practical because it divides by 2, 3, 4 and 6: ten only by 2 and 5. Hence the dozen, the gross and the twelve inches to a foot.

kilowatt

One metric horsepower is about 0.74 kilowatts. Advertisements often still print the horsepower figure because it sounds familiar.

💡 Did you know? Horsepower goes back to James Watt: he wanted buyers of his steam engines to see how many horses they replaced: one of the first marketing units in history.

loudness

The scale is logarithmic: ten decibels more mean ten times the sound intensity.

💡 Did you know? So going from 80 to 90 decibels is not a bit louder but ten times the energy and from 85 decibels upwards it damages hearing permanently.

the energy content

One kilocalorie is the heat that warms one kilogram of water by one degree. Officially energy is given in kilojoules: 1 kcal is about 4.2 kJ.

💡 Did you know? When people say calories in everyday speech they almost always mean kilocalories. An apple with 80 calories would otherwise carry almost no energy at all.