Townsend (unit)

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==