The definitions of four SI base units – the kilogram, ampere, kelvin and mole –
have been changed.
The new definitions are based on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (k) and the Avogadro constant (N
A) respectively. They came into force on 20th May.
The seven SI base units are:
second (s) length:
metre (m) mass:
kilogram (kg) electric current:
ampere (A) thermodynamic temperature:
kelvin (K) amount of substance:
mole (mol) luminous intensity:
You can read more about the units and the change on the website of the
Bureau international des poids et mesures (BIPM) and on Wikipedia.
Royal Institution Christmas Lectures are popular science talks given around Christmas each year. The first one was in 1825.
They were started by the scientist
Michael Faraday. They have been televised since 1936.
You can watch the more recent lectures online: for example, Carl Sagan’s
six lectures on the planets (1977) and last year’s lectures on genetics.
physics.org is a guide to physics on the web, published by the Institute of Physics in Britain.
You can search its database of 4,000 physics websites or do some experiments, like the one pictured. There is also information about courses and careers.