At atmospheric pressure, air and other gases are poor conductors of electricity. This is because, they do not have free charged particles.
However, electric current may be passed through a gas if by some mechanism, charged particles are produced in the gas. This can be done in many ways, such as
(i) by applying a large potential difference across a gas column at very low pressure and
(ii) by allowing X-rays to pass through the gases.
The study of electric discharge through gases gives valuable information regarding the structure of atoms. This has led to the discovery of electrons by J. J. Thomson and later on, to the discovery of X-rays by Roentgen.