{{one source |date=March 2024}} The '''townsend''' (symbol Td) is a physical unit of the reduced electric field (ratio ''E''/''N''), where <math>E</math> is electric field and <math>N</math> is concentration of neutral particles.
It is named after John Sealy Townsend, who conducted early research into gas ionisation.
==Definition== It is defined by the relation <math display="block"> 1\ \text{Td} = 10^{-21}\ \text{V} \cdot \text{m}^2 = 10^{-17}\ \text{V} \cdot \text{cm}^2. </math>
For example, an electric field of <math display="block"> E = 2.5 \cdot 10^4\ \text{V}/\text{m} </math> in a medium with the density of an ideal gas at 1 atm and 0 °C, the Loschmidt constant <math display="block"> n_0 = 2.6867811 \cdot 10^{25}\ \text{m}^{-3} </math> gives <math display="block"> E/n_0 \approx 10^{-21}\ \text{V} \cdot \text{m}^2, </math> which corresponds to 1 Td.
==Uses== 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 <math>E/N</math> over broad range of <math>E</math> and <math>N</math>.
The concentration <math>N</math>, which is in ideal gas simply related to pressure and temperature, controls the mean free path and collision frequency. The electric field <math>E</math> 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 also==
* Electric glow discharge * Vacuum arc
==References== {{reflist}} * {{cite journal |author1=A. Banković |author2=S. Dujko |author3=R. D. White |author4=J. P. Marler |author5=S. J. Buckman |author6=S. Marjanović |author7=G. Malović |author8=G. García |author9=Z. Lj. Petrović |title=Positron transport in water vapour |year=2012 |journal=New J. Phys. |volume=14 |article-number=035003 |doi=10.1088/1367-2630/14/3/035003|hdl=1885/61498 |hdl-access=free }}
Category:Electrical breakdown
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