Â鶹´«Ã½

From Jon Crawley

I really enjoyed your article on basic units (22 February, p 32). Contrary to what some of your readers’ letters have said (8 March, p 29), it is possible to arrive at definitions of the ampere, the candela and the mole that are independent of the kilogram.

A single ampere represents 1 coulomb moving past a single point in one second. The coulomb can be expressed as the charge of 6.24 × 10¹⁸ electrons.

Until 1979, the candela was defined as the luminance of a Planckian black-body radiator at the melting temperature platinum (a black body being a theoretical object that completely absorbs all incident radiation and also emits electromagnetic radiation at all wavelengths, the emitted radiation being related to its temperature by Wien’s constant).

The mole is related to a constant whole number. Although defined as the number of atoms in 0.012 kilograms of carbon-12, it can also be defined as the amount of substance in which the number of elementary entities is equal to Avogadro’s constant, which is approximately 6.022 × 10²³. This can be applied to any elementary particle, so we can have a mole of atoms, a mole of molecules, a mole of electrons, and so on.

So why is it not possible for the kilogram to be defined as the mass of the number of carbon-12 atoms equal to (1000 ÷ 12) × Avogadro’s constant?

Partridge Green, West Sussex, UK