It is defined by the relation

${\displaystyle 1\,{\rm {Td}}=10^{-21}\,{\rm {V\cdot m^{2}}}=10^{-17}\,{\rm {V\cdot cm^{2}}}.}$ 

For example, an electric field of

${\displaystyle E=2.5\cdot 10^{4}\,{\rm {V/m}}}$ 

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

${\displaystyle n_{0}=2.6867811\cdot 10^{25}\,{\rm {m^{-3}}}}$ 

gives

${\displaystyle E/n_{0}\approx 10^{-21}\,{\rm {V\cdot m^{2}}}}$ ,

which corresponds to ${\displaystyle 1\,{\rm {Td}}}$ .

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 ${\displaystyle E/N}$  over broad range of ${\displaystyle E}$  and ${\displaystyle N}$ .

The concentration ${\displaystyle N}$ , which is in ideal gas simply related to pressure and temperature, controls the mean free path and collision frequency. The electric field ${\displaystyle E}$  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.

• Electric glow discharge
• Vacuum arc

• 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.