System of units

International System of Units (SI)

The key is to define only the minimum number of measures: if you define more definitions become over constrained and could be inconsistent.
Learning the modern SI is also a great way to learn some interesting Physics.

Origins of Precision by Machine Thinking (2017)

Great overview of the earlier history of unit standardization.
Gives particular emphasis to the invention of gauge blocks.

Ampere in the 2019 redefinition of the SI base units

TODO how does basing it on the elementary charge help at all? Can we count individual electrons going through a wire? www.nist.gov/si-redefinition/ampere/ampere-quantum-metrology-triangle by the NIST explains that is it basically due to the following two quantized solid-state physics phenomena/experiments that allows for extremely precise measurements of the elementary charge:

Kilogram

Unit of mass.
Defined in the 2019 redefinition of the SI base units via the Planck constant. This was possible due to the development of the kibble balance.

Kibble balance

Measures weight from a voltage.

Hertz (1857-1894, Hz)

Named after radio pioneer Heinrich Hertz.

Megahertz (MHz)

Mega-Hertz, i.e. a million Hertz.

Caesium standard (1967, 3.26 cm)

Uses the frequency of the hyperfine structure of caesium-133 ground state, i.e spin up vs spin down of its valence electron , to define the second.
International System of Units definition of the second since 1967, because this is what atomic clocks use.
TODO why does this have more energy than the hyperfine split of the hydrogen line given that it is further from the nucleus?

Natural units

A series of systems usually derived from the International System of Units that are more convenient for certain applications.