Volta's invention of the battery and thus an electric current made many scientist in the nineteenth century carry out research into electricity. They researched how electricity behaved in different gas mixtures and at different gas pressures. They used glass tubes that could be nearly evacuated (vacuum).
Two almost touching wire filaments are horizontally attached to the two rods in the glass tube. When a high voltage is put on the rods, sparks are generated between the wire ends, even when the gas pressure in the tube is very low.
Conduction in vacuum
1825 - 1849