Townsend (unit)

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The Townsend (symbol Td) is a physical unit of $E / N$ , where $E$ is electric field and $N$ is concentration of neutral particles. It is defined by the relation $1 {\rm Td} = 10^{-17}\rm V\cdot cm^2$ .

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 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.