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 density
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.
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