Q 01/05

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

A) The mole

B) The gram

C) The litre

D) The dalton

Answer · why

A) The mole

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.

Q 02/05

Which SI base unit measures luminous intensity?

A) Lumen

B) Steradian

C) Lux

D) Candela

Answer · why

D) Candela

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

Q 03/05

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

A) The speed of light

B) The gravitational constant

C) The Boltzmann constant

D) The charge of an electron

Answer · why

A) The speed of light

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 se...

Q 04/05

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

A) The Tropic of Cancer

B) The Arctic Circle

C) The North Pole

D) The 45th parallel

Answer · why

C) The North Pole

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

Q 05/05

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

A) Mercury

B) Alcohol

C) Air

D) Water

Answer · why

D) Water

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.