# Townsend (unit)

The Townsend (symbol Td) is a physical unit of the reduced electric field (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, where it serves as scaling parameter because the mean energy of electrons (and therefore many other properties of discharge) is typically a function of $E/N$ over broad range of $E$ and $N$. The concentration $N$, which is in ideal gas simply related to pressure and temperature, controls the mean free path and collision frequency. The electric field $E$ governs the energy gained between two successive collisions.

Reduced electric field being a scaling factor effectively 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.