Incoming links: Electrical resistance
Starting in the 2019 redefinition of the SI base units, the elementary charge is assigned a fixed number, and the Ampere is based on it and on the second, which is beautiful.
This choice is not because we attempt to count individual electrons going through a wire, as it would be far too many to count!
Rather, it is because because there are two crazy quantum mechanical effects that give us macroscopic measures that are directly related to the electron charge. www.nist.gov/si-redefinition/ampere/ampere-quantum-metrology-triangle by the NIST explains that the two effects are:
- quantum Hall effect, which has discrete resistances of type:for integer values of .
- Josephson effect, used in the Josephson voltage standard. With the Inverse AC Josephson effect we are able to produce:per Josephson junction. This is about 2 microvolt / GHz, where GHz is a practical input frequency. Video "The evolution of voltage metrology to the latest generation of JVSs by Alain Rüfenacht" mentions that a typical operating frequency is 20 GHz.Therefore to attain a good 10 V, we need something in the order of a million Josephson junctions.But this is possible to implement in a single chip with existing micro fabrication techniques, and is exactly what the Josephson voltage standard does!
Those effect work because they also involve dividing by the Planck constant, the fundamental constant of quantum mechanics, which is also tiny, and thus brings values into a much more measurable order of size.
The Kibble balance is so precise and reproducible that it was responsible for the 2019 redefinition of the Kilogram.
It relies rely on not one, but three macroscopic quantum mechanical effects:How cool is that! As usual, the advantage of those effects is that they are discrete, and have very fixed values that don't depend either:One downside of using some quantum mechanical effects is that you have to cool everything down to 5K. But that's OK, we've got liquid helium!
- atomic spectra: basis for the caesium standard which produces precise time and frequency
- Josephson effect: basis for the Josephson voltage standard, which produces precise voltage
- quantum Hall effect: basis for the quantum Hall effect, which produces precise electrical resistance
- on the physical dimensions of any apparatus (otherwise fabrication precision would be an issue)
- small variations of temperature, magnetic field and so on
The operating principle is something along:Then, based on all this, you can determine how much the object weights.
- generate a precise frequency with a signal generator, ultimately calibrated by the Caesium standard
- use that precise frequency to generate a precise voltage with a Josephson voltage standard
- convert that precise voltage into a precise electric current by using the quantum Hall effect, which produces a very precise electrical resistance
- use that precise current to generate a precise force on the object your weighing, pushing it against gravity
- then you precisely measure both:
- local gravity with a gravimeter
- the displacement acceleration of the object with a laser setup
How We're Redefining the kg by Veritasium
. Source. The Kibble Balance, realizing the Kilogram from fundamental constants of nature by Richard Green
. Source. Presented in 2022 for a CENAM seminar, the Mexican metrology institute. The speaker is from the Canadian metrology institute- youtu.be/ZfNygYuuVAE?t=854: they don't actually use the Quantum Hall effect device during operation, they only use it to calibrate other non-quantum resistors
The Watt balance and redefining the kilogram by National Physical Laboratory
. Source. Nothing much, but fun to hear Kibble talking about his balance in beautiful English before he passed.