Townsend (unit)
The Townsend (symbol Td) is a physical unit of the ratio E/N, where 
is electric field and 
is concentration of neutral particles. It is defined by the relation 
.
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 
. 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.