In order to start a discharge, some charged particles are needed to start the
process of ionization and excitation of gas particles. This is provided to a large extent
by weak background radiation from radioactive minerals where some ionized
molecules and free electrons exist, and to a less extent from cosmic rays. By applying
an electric field these electrons gain enough energy for further ionization and
generation of secondary electrons, which in turn contributes to creating a so-called
Townsend discharge with a relatively low, but avalanching, current (Fig. 2).