Lepton

Can be contrasted with baryons as mentioned at baryon vs meson vs lepton.

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

Given the view of the Standard Model where the electron and quarks are just completely separate matter fields, there is at first sight no clear theoretical requirement for that.

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

Hypothesized as the explanation for continuous electron energy spectrum in beta decay in 1930 by .

First observed directly by the Cowan-Reines neutrino experiment.

Because they interact weakly with matter and mostly just escape out of nuclear reactors, you can likely locate all nuclear reactors on Earth by measuring neutrino flows:The CIA must love that shit, they must have had it years prior to this public paper.