Ciro Santilli $$Sponsor Ciro$$ 中国独裁统治 China Dictatorship 新疆改造中心、六四事件、法轮功、郝海东、709大抓捕、2015巴拿马文件 邓家贵、低端人口、西藏骚乱

# System of units

split "Physics" words: 344

## International System of Units (SI)

split toc "System of units" words: 344
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? https://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

split toc "Kilogram" words: 5
Measures weight from a voltage.

### Time

split toc "International System of Units" words: 173

#### Frequency

split toc "Time" words: 10
##### Hertz (1857-1894, Hz)
split toc "Frequency" words: 10
Named after radio pioneer Heinrich Hertz.
###### Megahertz (MHz)
split toc "Hertz" words: 6
Mega-Hertz, i.e. a million Hertz.

#### Clock

split toc "Time" words: 163
##### Quartz clock
split toc "Clock" words: 51
##### Atomic clock
split toc "Clock" words: 112
###### Caesium standard (1967)
split toc "Atomic clock" words: 28
Uses the frequency of the hyperfine structure of caesium-133 ground state () to define the second.
International System of Units definition os the second since 1967, because this is what atomic clocks use.

### Length

#### Gauge block

split toc "Length" words: 4

### Natural units

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