Q 01/05

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

A) Three

B) Seven

C) Five

D) Ten

Answer · why

B) Seven

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

Q 02/05

Which SI base unit measures luminous intensity?

A) Lumen

B) Steradian

C) Photon

D) Candela

Answer · why

D) Candela

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

Q 03/05

Since 2019, how is the metre defined?

A) By a platinum-iridium bar kept near Paris

B) By wavelengths of krypton-86 light

C) By the distance light travels in a vacuum in a fraction of a second

D) By one ten-millionth of the equator-to-pole distance

Answer · why

C) By the distance light travels in a vacuum in a fraction of a second

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

Q 04/05

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

A) The Tropic of Cancer

B) The Arctic Circle

C) The North Pole

D) Paris

Answer · why

C) The North Pole

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

Q 05/05

How was the kilogram originally defined in 1795?

A) As the mass of a specific gold coin

B) As the mass of one litre of water

C) As one thousandth of a French tonne

D) As the mass of a cubic foot of iron

Answer · why

B) As the mass of one litre of water

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