Lightning arresters

The lightning arrester is a device used to protect electronic circuits from atmospheric electricity. It is a tube with two electrodes, filled with a low-pressure gas. It could be connected e. g. to the input of a telephone line into a building. In case of a voltage surge, an electric discharge would form inside the arrester and this would short the surge to ground. To ensure good discharge ignition even with low voltages, some types contained a small amount of radioactive substances that would ionise the gas inside.