If, between two electrified conductors, there is some potential difference, than there will be the spark- over between that conductors. This occurrence is explained this way: In the air there is some number of ions. In the electric field they move with some acceleration colliding with air particles. If they are fast enough, they can percipitate electrons out of air atoms when colliding. These, new created ions collide again etc. Ions and molecules are stimulated to luminosity by collisions. Transporting quite a big electric charge they are strong heated until there is a strong decompression- the spark.
To provocate the spark- over one has to increase potential difference or to rarely the gas. Thanks to it ions' paths are getting longer when accelerating so the ions get bigger velocity. The gas rarefies in the special tube with electrodes attached at both sides. While rarefying the look of the spark changes, until the moment when the gas stops illuminating. But the glass in front of the cathode starts to illuminate. That is because of invisible rays coming out the cathode. They are called cathode rays.
The experimental discovery of electron