# Townsend (unit)

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.