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Thetownsend (symbol Td) is a physical unit of the reduced electric field (ratioE/N), where${\displaystyle E}$ iselectric field and${\displaystyle N}$ is concentration of neutral particles.

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

It is defined by the relation$${\displaystyle 1\text{Td}={10}^{-21}\text{V}\cdot {\text{m}}^2={10}^{-17}\text{V}\cdot {\text{cm}}^2.}$$

For example, an electric field of$${\displaystyle E=2.5\cdot {10}^4\text{V}/\text{m}}$$in a medium with the density of an ideal gas at 1 atm, theLoschmidt constant$${\displaystyle n_0=2.6867811\cdot {10}^{25}{\text{m}}^{-3}}$$gives$${\displaystyle E/n_0\approx {10}^{-21}\text{V}\cdot {\text{m}}^2,}$$which corresponds to 1 Td.

This unit is important in gas-discharge physics, where it serves asscaling 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 inideal gas simply related to pressure and temperature, controls themean free path andcollision 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 intensityE by some factorq has the same consequences as lowering gas densityN by factorq.

• Electric glow discharge
• Vacuum arc

• A. Banković; S. Dujko; R. D. White; J. P. Marler; S. J. Buckman; S. Marjanović; G. Malović; G. García; Z. Lj. Petrović (2012). "Positron transport in water vapour".New J. Phys.14: 035003.doi:10.1088/1367-2630/14/3/035003.hdl:1885/61498.

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