In 1834, Michael Faraday used the expression 'atoms of electricity', generally taken now as the earliest reference to what we today call the electron.
George Johnstone Stoney coined the name 'electron' for the unit of electric charge, in 1874. Later, this name was transferred to the cathode ray particles.
If scientists could measure the charge/mass ratio (e/m) for an electron, that was proof that there was really something fitting the name 'atom of electricity'.
In 1890, Arthur Schuster measured the e/m ratio for electrons, and found the value was about 1000 times the value for a hydrogen ion. He dismissed it as wrong.
In 1895, Jean Perrin showed that cathode rays are negative particles, rather than being a form of electromagnetic radiation, as German scientists believed.
Jean Perrin showed that cathode rays had negative charge, leading the way for J. J. Thomson to measure the ratio e/m, and prove that electrons were particles.
In 1897, both Walter Kaufmann and J. J. Thomson carried out separate measurements of the electron charge to mass ratio by deflection of cathode rays.
When J J Thomson measured the charge/mass ratio of the electron, e/m, this proved once and for all that electrons were particles, not electromagnetic radiation.
R A Millikan succeeded in studying the behaviour of charged oil drops in an electric field, and so deduced the charge on the electron, and that it was uniform.
In 1924, Louis de Broglie more or less suggested that electrons might be in some ways like waves. Actually, he said that the particles were guided by waves.
In 1927, Clinton Davisson, Lester Germer, and G. P. Thomson demonstrated electron diffraction by a crystal, showing that electrons have wavelike properties.