Townsend (unit)

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The Townsend (symbol Td) is a physical unit of the ratio E/N, where E is electric field and N is concentration of neutral particles. It is defined by the relation

1 \, {\rm Td} = 10^{-21} \, {\rm V\cdot m^2} = 10^{-17} \, {\rm V\cdot cm^2}.

For example, an electric field of E = 2.5 \cdot 10^{4} \, {\rm V/m} in a medium with density N = 2.5 \cdot 10^{25} \, {\rm m^{-3}} gives E/N = 10^{-21} \, {\rm V \cdot m^{2}}, which corresponds to 1 \, {\rm Td}.

This unit is important in gas discharge physics, because the mean energy of electrons (and therefore many other properties of discharge) is a function of E/N. It means that increasing the electric field intensity E by some factor q has the same consequences as lowering gas density N by factor q.

It is named after John Sealy Townsend.

See also[edit]