# 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 $1 {\rm Td} = 10^{-17}\rm V\cdot cm^2 = 10^{-21}\rm V\cdot m^2 = 1\rm zV\cdot m^2$.
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 field intensity E by some factor q has the same consequences as lowering gas density N by factor q.