Vaccum Discharge, which will become the basis of discovery of "electron"
William Watson (1715-1787) [Wikipedia]
Johann Heinrich Wilhelm Geißler (1814-1879) [Wikipedia]
Julius Plücker (1801-1868) [Wikipedia]
1752 William Watson (1715-1787)

Aurora! in a glass tube.

Researchers believed that electricity escapes from charged substance through air. Watson examined evaculation of the environment around an electrode of the charged substance to reduce the reduction of the electricity. What he observed was not what he expected. He encountered a phenomenon known as "electrical discharge" instead of the reduction of escaping electricity. He explained the phenomena as "Aurora" in his paper. This observation may be the starting point of researches on "vacuum discharges."

An account of the phænomena of electricity in vacuo, with some observations thereupon
Philosophical Transactions of the Royal Socociety of London, Volume 47, pp. 362-376 (1752).
1855 Johann Heinrich Wilhelm Geißler (1814-1879)

Geißler acceralated researches on vacuum discharge with his novel vacuum pumps.

In 1855, Geißler designed a mercurial vacuum pump based on moving a column of liquid mercury instead of mechanical pistons, which allowed him to eventually reach absolute pressuresof about 0.1 mm Hg.

Simón Reif-Acherman
Heinrich Geissler: Pioneer of electrical science and vacuum technology
Proceedings of the IEEE, Volume 130, pp. 1672-1684 (2015).

I have not yet found the primary source(s) on Geißler's works.
1858 Julius Plücker (1801-1868) [Wikipedia]

Plücker was studying vacuum discharge at the University of Bonn. With the cooperation of Geißler, Plücker investigated the state of vacuum discharge using a glass tube with a higher vacuum than before, and made the following discoveries.
- The wall of the glass tube facing the cathode glows green.
- The green light emitting part moves by holding the magnet.
Plücker considered that the material that made up the cathode was jumped from the cathode in the form of particles. In fact, this idea was wrong. However, this discovery by Geißler and Plücker triggered further research on vacuum discharge. And finally it leads to the discovery of electrons.

J. Plücker
On the action of the magnet upon the electrical discharge in rarefied gases
London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 4th Serires, Volume 16, pp. 119-135 (1858).
Translated by F. Guthrie, from Poggendorff's Annalen, Vol. 103, 88-106 (1858).

J. Plücker
Ueber die Einwirkung des Magneten auf die elektrischen Entladungen in verdünnten Gases
Annalen der Physisk und Chemie, Volume 179, pp. 88-106, pp. 151-157 (1858).
(AKA. Poggendorffs Annalen Volume 103)
[DOI] [WikiSource] -> [Gallica] [Google] [Archive]