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

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.

See also

• Electric glow discharge
• Vacuum arc