The contribution of British engineers and scientists to the development of the international metric system is well known. What is less well known is that, in addition to the six SI units that are named after British scientists and engineers, there is a seventh unit approved for use with SI. (Article based on contributions by Martin Vlietstra).
As technology develops, so the International System of Units (SI) evolves. The status of SI is published from time to time by the International Bureau of Weights and Measures (BIPM) and is available on their website (home page – http://www.bipm.org).
In the eighth edition the classification of “non-SI units accepted for use with the International System” (such as hours, minute, litres, tonnes, etc) was re-organised. One of the sections that is of interest is the set of units in Table 7 of the SI standard ‘Non-SI units whose values in SI units must be obtained experimentally’. Ten units are listed, three of which are endorsed ‘Units accepted for use with the SI’. These three are:
* Electronvolt (symbol eV),
* Dalton (D) or unified atomic mass unit (u)
* Astronomical unit (symbol ua)
The remaining seven units are included because of their fundamental importance in modern physics.
The choice of the name ‘Dalton’ is of particular interest. John Dalton (1766 to 1844) was born in Cumbria and worked in Manchester. His work laid the basis of atomic theory so it is appropriate that the standard atomic mass (one twelfth of the mass of the nucleus of a carbon 12 atom in its rest state) should be called a “dalton”. Dalton therefore is the seventh British scientist or engineer to be honoured in this way. The other six are:
Isaac Newton – the unit of force is the newton (N)
James Joule – the unit of energy (or work or amount of heat) is the joule (J)
Michael Faraday – the unit of capacitance is the farad (F)
James Watt – the unit of power (radiant flux) is the watt 
William Thomson, Lord Kelvin – the unit of thermodynamic temperature is the kelvin (K)
Louis Gray – the unit of absorbed dose of specific (imparted) energy is the gray (Gy)
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