All seven base units in the metric system are based on constants of nature. This means that they can never be destroyed. The seven SI base units are the metre, kilogram, second, ampere, kelvin, mole and candela. All other SI units are derived from these base units. Hence, they are called SI derived units. Anyone with the right laboratory equipment and expertise can reproduce any of these base units. The problem with tying units to physical objects is that they can be destroyed, and they tend to degrade over time.
Just 10 years after the Weights and Measures Act 1824 introduced the British imperial system, a fire destroyed the Houses of Parliament in 1834. In the fire, the Imperial Standard Yard and Pound were lost. These two physical objects defined the length of the imperial yard and avoirdupois pound respectively.
The last metric unit to be tied to a physical object was the kilogram, the metric unit of mass. Until 2019, the kilogram definition was tied to a physical object called the International Prototype Kilogram, also known as Le Grand K. Every few decades, it was compared with its physical copies that were distributed to the nations that were members of the BIPM. The comparison tests revealed that its mass was drifting compared to its copies. The differences in mass were measured in micrograms. Even though these differences are very small, it could be significant for some applications.
The kilogram definition was replaced by a definition based on the Planck constant. A Kibble balance can be used to reproduce a kilogram to a very high standard of precision based on the current definition.
The metre, the metric unit of length, is defined as the length of the path travelled by light in a vacuum during a time interval of 1/299 792 458 of a second.
The second is the familiar unit of time that we count with our watches and clocks. It is defined by the natural beats of a highly stable caesium atomic clock.
The ampere, informally called the “amp”, is the unit of electric current. It is the fundamental base unit for electricity and magnetism. The ampere is defined in terms of the amount of electric charge on one electron.
The kelvin is the unit of thermodynamic temperature. It measures temperature as we understand it (i.e., how hot something is). The Celsius temperature scale is closely related to the kelvin scale. The difference is that kelvin starts from absolute zero, i.e., zero kelvin is where there is no heat at all whereas the Celsius scale uses 0 degrees for the freezing point of water and 100 degrees for the boiling point of water. The Celsius scale is derived by adding 273.15 to convert kelvins to degrees Celsius. The kelvin is based on the fixed numerical value of the Boltzmann constant.
The mole defines the amount of substance and is used in chemistry and physics. It represents a fixed number of “elementary entities” of a substance. One mole contains exactly 6.022 140 76 × 1023 elementary entities. This is the fixed numerical value of the Avogadro constant. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.
The candela is the unit of luminous intensity in a given direction. Essentially, this measures the “brightness” of radiation. It is defined in terms of the intensity of a very pure yellow-green light source with a strength measured by its power spread over a cone shaped beam. The candela definition is based on the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency.
The constants of nature are always there and do not change. When physical objects are lost, destroyed or decayed that measurement unit definitions depend on, it causes a lot of problems. The use of constants of nature for defining metric units overcomes these problems.
Sources:
Metric Views 
Thank you. A very clear explanation. I’ve often wondered, “Whosoe foot?”
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Even though the BIPM still insists the kilogram is a base unit, the 2019 definitions make it a derived unit. The same with the ampere.
Base units are those units defined directly from nature. The metre is defined directly from nature, so it is a base unit, as is the second, the kelvin. the mole and the candela.
The coulomb has in reality replaced the ampere as a base unit. The coulomb is defined by fixing the elementary charge at 1.602176634 ⋅ 10^-19 C exactly, then the coulomb becomes equal to 1/ 1.602176634 ⋅ 10^-19 or 6 241 509 074 460 762 607.7762409809304… elementary charges. The ampere is then defined as exactly one coulomb per second.
The kilogram is now a derived unit. It is defined using the kibble balance, The Kibble balance equates the mechanical joule to the electrical joule such that one volt coulomb equals one newton metre. In this relationship we have two nature defined units (the metre and the coulomb) and two that aren’t. To make the equation and the Kibble balance work, one of the two have to be defined from nature. In reality, the volt is now a defined from nature unit.
Since 2019, the Josephson Junction definition defines the volt from nature. You can read about it here:
https://en.wikipedia.org/wiki/Josephson_voltage_standard
Now we have three nature defined units and the newton via the Kibble balance is derived. Working backwards we define the kilogram as being one newton second squared per metre.
Thus, the volt and the coulomb replace the kilogram and the ampere as base units.
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The SI brochure states the following about the status of base units:
“Prior to the definitions adopted in 2018, the SI was defined through seven base units from which the derived units were constructed as products of powers of the base units. Defining the SI by fixing the numerical values of seven defining constants has the effect that this distinction is, in principle, not needed, since all units, base as well as derived units, may be constructed directly from the defining constants. Nevertheless, the concept of base and derived units is maintained because it is useful and historically well established, noting also that the ISO/IEC 80000 series of Standards specify base and derived quantities which necessarily correspond to the SI base and derived units defined here.”
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m quoted from the SI brochure:
“the concept of base and derived units is maintained because it is useful and historically well established, …”
That is exactly the same logic that opposers to metrication use as a means to preserve pre-SI units and some use to continue to use older, deprecated metric units (cgs based). Even for not renaming the kilogram to a non-prefixed word, etc. Because it is historically well establish.
History should never be a reason to justify moving backwards or being stuck in a rut.
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@Daniel: Your explanation as to why the kilogram is a derived unit does not hold water. The definition of the kilogram depends on measuring the metre, the second and the Planck constant by whatever means you see fit. The “mise and practique” identifies the Kibble Balance technique to demonstrate that there exists at least one way to measure the kilogram using the “Planck constant definition” and therefore that the definition is realisable.
You are free to use any other technique to generate a prototype kilogram and if you can achieve a greater precision in your measurements than can be achieved using a Kibble Balance, I am sure that the BIPM would love to hear from you and would invite you to publish your technique in the journal Metrologia. (See https://phys.org/journals/metrologia/).
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Martin,
The definition of the kilogram post 2019 depends on the coulomb, the metre, the newton, the second and the volt. The Planck Constant only comes into play in defining the volt in the same way that the speed of light defines the metre.
I am completely satisfied with the Kibble Balance method to ultimately define the kilogram and don’t need to offer the BIPM an alternative method to define the kilogram directly from nature. You may wish to, however, since you feel the need to have the kilogram as a base unit. You may wish to offer a suggestion in support of the silicon sphere that was being developed parallel to the Kibble Balance.
The silicon sphere would restore the kilogram’s base unit status, but then we would be back to the problem of an artifact that decays over time.
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@Daniel: You are confusing the terms “definition” and “realization”.
The preface to the SI Brochure (https://www.bipm.org/en/publications/si-brochure) contains the text “Any valid equation of physics relating the defining constants to a unit can be used to realize the unit thus creating opportunities for innovation, realization everywhere with increasing accuracy as technology proceeds.”
Appendix 4 of the SI Brochure says “For the kilogram, the unit whose definition has undergone the most fundamental change, realization can be through any equation of physics that links mass, the Planck constant, the velocity of light and the caesium frequency.”
Appendix 2, Section 3 of the SI Brochure (https://www.bipm.org/en/publications/mises-en-pratique – Document Mise en pratique for the definition of the kilogram in the SI) states “There are currently two independent primary methods that are capable of realizing the definition of the kilogram with relative uncertainties … “.
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Clearly, metrology is not a walk in the park!
But the idea that we should use something related to human anatomy (feet? whatever?) is nothing short of laughable.
The discussion in this post is fascinating and clearly in an entirely different league from what advocates of Imperial units have to say about measurement. 😉
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