What is an "Elementary Charge"?
The elementary charge (abbreviated e) is an estimation of the amount of charge in Coulombs carried by a single electron or proton, currently defined as 1.602176634*10^-19[how to read math] Coulombs1.
How is the Elementary Charge Defined?
The Coulomb is an unit of measurement that measures an amount of electricity, however, Charles-Augustin de Coulomb lived a century before the electron was discovered by Joseph John Thomson in the 19th century2, so it wasn't possible for him to measure electricity in terms of quantities of electrons.
In 1784 he built a special torsion balance. Suspended on a thin piano wire was an arm, which carried an insulated ball at one end and a counterweight at the other. The wire was hanging on a knob, which he could rotate and from which he could read a 360-degree scale.
Near the ball at the arm was a second, stationary one. He charged both of them with the same polarity and the balls moved apart. Now he turned the knob at the suspension point of the wire and determined the angle required to force the two balls closer together. Having previously determined the force it took to twist the wire, he was thus able to measure the minute force electricity exerted.
Excerpts from "Much Ado About Almost Nothing" [http://www.historyofelectronics.com/Sub/Coulomb.htm] (accessed 2025-02-23)
Nevertheless, the unit is defined as:
Coulomb (unit of quantity of electricity) - The coulomb is the quantity of electricity carried in 1 second by a current of 1 ampere.
Resolution 2 of the 41st CIPM (1946) [https://www.bipm.org/en/committees/ci/cipm/41-1946/resolution-2] (accessed 2025-02-23)
The Ampere is in turn (currently) defined as:
[...] one ampere is the electric current corresponding to the flow of 1/(1.602 176 634 x 10–19) elementary charges per second.
https://www.bipm.org/en/si-base-units/ampere (accessed 2025-02-23)
Observe that the standard units do not say the current is carrying an amount of electrons, it says an amount of elementary charges.
The reason for this, I presume, is that if an electron is a tangible thing that exists in the real world, it may be possible for two electrons to be different somehow, meaning that there will be slightly differences between their properties, such as mass and charge, so it's safer to define one unit using an imaginary "elementary charge" unit rather than making assumptions about how an electron works in real life. After all, if it turns out they are wrong in a couple of years they would have to rewrite all of this and release another edition of the Standard Units, and then someone would have to update the Wikipedia articles about the units all over again. It's safer to just ignore the electrons when defining units of measurement.
Nevertheless, the origins of the elementary charge appear to be an experiment that measured the charge of a single electron.
Usually, texts explain that the elementary charge is the common value that can be divided out of (factored from) each of the measured charges. [...]
Millikan’s 1911 article is not only a classic. It is filled with image-provoking language. For example, consider the following fragment “…and supported by evidence from many sources that all electrical charges, however produced, are exact multiples of one definite, elementary, electrical charge, or in other words, that an electrical charge instead of being spread uniformly over the charged surface has a definite granular structure, consisting, in fact, of an exact number of specks, or atoms of electricity, all precisely alike, peppered over the surface of the charged body.” [...]
This is a first estimate of the elementary charge (charge of the electron.) [...]
Revisiting Millikan’s Oil-Drop Experiment [https://websites.umich.edu/~chemstu/content_weeks/leftovers/JCE_2005_851_pearson_Millikan.pdf] (accessed 2025-02-23)
From the text above, we can understand that the elementary charge is the proof that electricity isn't continuous but discrete, that there exists zero electricity and after that we have a minimum amount of electricity and an amount between these two values isn't possible, that electricity always goes up (is incremented) by that value and never by fractions of it.
By measuring the amount of Coulombs in an experiment and trying to calculate the lowest common denominator across measurements, we can arrive at some indivisible constant from which all measurements are multiples of, and that must be the value of one single electron.
I suppose if electrons always moved in pairs, it could be the value of two electrons, but in that case I guess a pair would just be considered to be a single electron by everybody anyway since it behaves like just one thing.
It's worth noting that Millikan's experiment doesn't seem to even talk about charges in terms of Coulombs at all. Instead, it was focused on ions, which aren't subatomic particles, but molecules or atoms that carry a charge. As its title states:
The Isolation of an Ion, A Precision Measurement of Its Charge, and the Correction of Stokes's Law
https://www.jstor.org/stable/1634813 (accessed 2025-02-23)
Since electrons and protons have equal charges that are opposite to each other, the elementary charge may refer either to the charge of a proton or of an electron.
Quantity of charge can be measured in either elementary charges (an elementary charge is the amount of charge on one electron or proton) [...]
http://www.sciencejoywagon.com/physicszone/07electrostatics/coulomb/ (accessed 2025-02-23)
However, since electricity occurs due to the movement of electrons, not of protons, typically we'd be talking about electrons.
Furthermore, it's sometimes said that electrons are "negatively charged" particles while protons are "positively charged" particles. This only means that they are opposite to each other. It certainly doesn't mean that you have to write a negative amount of elementary charge to represent the charge of electrons when talking about electricity, just as you don't write a negative number of Coulombs in such case.
References
- https://www.bipm.org/en/measurement-units (accessed 2025-02-23) ↩︎
- https://web.lemoyne.edu/giunta/ea/THOMSONann.HTML (accessed 2025-02-23) ↩︎