Behaviour fully described by quantum electrodynamics.
Experiments to measure it:
The 2019 redefinition of the SI base units defines it precisely and uses it as a measure of charge: en.wikipedia.org/wiki/2019_redefinition_of_the_SI_base_units#Ampere
As mentioned e.g. at QED and the men who made it: Dyson, Feynman, Schwinger, and Tomonaga by Silvan Schweber (1994) chapter 1.6 "Hole theory", Dirac initially wanted to think of the holes in his hole theory as the protons, as a way to not have to postulate a new particle, the positron, and as a way to "explain" the proton in similar terms. Others however soon proposed arguments why the positron would need to have the same mass, and this idea had to be discarded.
Clear experiment diagram which explains that the droplet mass determined with Stoke's law:
American Scientific, LLC sells a ready made educational kit for this: www.youtube.com/watch?v=EV3BtoMGA9c
Here's some actual footage of a droplet on a well described more one-off setup:
This American video likely from the 60's shows it with amazing contrast: www.youtube.com/watch?v=_UDT2FcyeA4
First observed directly by the Cowan-Reines neutrino experiment.