Townsend (unit)

The townsend (symbol Td) is a physical unit of the reduced electric field (ratio E/N), where $E$ is electric field and $N$ 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 $$1\text{Td}=10^{-21}\text{V}\cdot {\text{m}}^2=10^{-17}\text{V}\cdot {\text{cm}}^2.$$

For example, an electric field of $$E=2.5\cdot 10^4\text{V}/\text{m}$$ in a medium with the density of an ideal gas at 1 atm and 0 °C, the Loschmidt constant $$n_0=2.6867811\cdot 10^{25}{\text{m}}^{-3}$$ gives $$E/n_0\approx 10^{-21}\text{V}\cdot {\text{m}}^2,$$ 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 $E/N$ over broad range of $E$ and $N$.

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

See also

• Electric glow discharge
• Vacuum arc