Townsend (unit)

The townsend (symbol Td) is a physical unit of the reduced electric field (ratio E/N), where is electric field and is concentration of neutral particles.

It is named after John Sealy Townsend, who conducted early research into gas ionisation.


It is defined by the relation


For example, an electric field of


in a medium with the density of an ideal gas at 1 atm, the Loschmidt constant




which corresponds to  .


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   over broad range of   and  .

The concentration  , which is in ideal gas simply related to pressure and temperature, controls the mean free path and collision frequency. The electric field   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.

See alsoEdit


  • A Bankovic´, S Dujko, R D White, J P Marler, S J Buckman, S Marjanovic´, G Malovic´, G Garc´ıa and Z Lj Petrovic, Positron transport in water vapour. 2012 New J. Phys. 14 035003.