A new Kilogram? What? How?

A new Kilogram? What? How?

To understand this, let us see how time is defined. We all know the SI unit of time is seconds, and it is defined as the international unit of time, the second, is defined by measuring the electronic transition frequency of caesium atoms. Similarly length (SI unit metre) is defined as the length traveled by light in $\frac{1}{3 \times 10^8}$ seconds or to be more precise $\frac{1}{299792458}$ seconds.
The inspiration to write this blog was derived from Veritasium|How We’re Redefining the kg.
We see that, all these are fundamental quantities are define on a given/fixed standard.
However, the kilogram is set as the weight of a metal cylinder in Paris. That is not that ideal, huh!
So, the NIST is trying to define the unit of mass, i.e., kilogram using some already standardized quantities like the Planck’s constant and the Avogadro’s number. The Planck’s constant takes a dive into advanced applied physics (no, not Quantum physics :p ); therefore, demands my blog’s space and time.
The engineering behind it is pretty simple, to put it in the simplest of the terms;

There is a balancing device, which has a mass unit and a coil
unit. The mass unit is balanced with the magnetic field from
the coil using a motor fixed to it, unit and unless, $F_{magnetic} = F_{gravitational}$. To know more, read Watt Balance or Kibble Balance.

Let’s look into the working now, in this process we will generate some cool equations and in the process will learn some new concepts, as and when required.
First, the watt balance, the principle of operation itself says that,
$$F_{magnetic} = F_{gravitational}$$
Taking into account all the usual scientific nomenclature, we can write,
$$\boxed{m \times g = B \times i \times l} \tag*{(1)}$$
Here, $m$ is the mass, $g$ is the acceleration due to gravity, $B$ is the magnetic flux density (in Tesla($T$)), $i$ is the current in the coil and $l$ length of the conductor in the field.
Equation $(1)$ is for the weighing mode operation of Watt Balance. In which, the weights are matched on both sides.
There is another mode, called the velocity mode in which the mass($m$) is lifted at a height and then the coil is moved back and forth in the magnetic field. This motion induces a voltage, $V$, in the coil. By Faraday’s motional emf expression,
$$\boxed{V = B \times l \times v} \tag*{(2)}$$
Here, $v$ is the velocity of the conductor in the magnetic field.
Now, $(1)$ can be written as,
$$B \times l = \frac{m \times g}{i} \tag*{(3)}$$
and $(2)$ can be written as,
$$B \times l = \frac{V}{v} \tag*{(4)}$$
Equating $(3)$ and $(4)$, we have,
$$\frac{m \times g}{i} = \frac{V}{v}$$
Which can be written as,
$$\boxed{m \times g \times v = V \times i} \tag*{(5)}$$
Interestingly, $m \times g \times v$ is the mechanical power (refer Encyclopedia of Electrochemical Power Sources for more info) and $V \times i$ is the electrical power.
You must all be thinking, how does Planck’s constant come to play. Well, please hold your horses. We are almost there.
To measure $V$ in $(5)$, we go into a concept of superconductivity. Called the Josephson Phenomenon. Please read up on its working here. It is also significant because of being the standard of Voltage.
When DC voltage is applied to a Josephson Junction, the junction experiences an oscillation of frequency(read more),
$$f = \frac{2eV}{h}$$
Here, $f$ is the frequency, $e$ is the elementary charge, and $h$ is the Planck’s constant($6.62607004 \times 10^{-34} m^2 kg \cdot s^{-1}$).
The above equation can be written as,
$$\boxed{V = \frac{h\times f}{2e}} \tag*{(6)}$$
For many Junctions ( say $n$) it is,
$$V = n\frac{h\times f}{2e}$$
The Voltage measure here is accurate to $10^{10}$ parts, refer this.
Now, if we write, $i$ as $\frac{V}{R}$ (where $R$ is the resistance offerered by the junction), then, $(5)$ changes to,
$$\boxed{\frac{h^2\times f^2}{4e^2R} = m \times g \times v} \tag*{(7)}$$
Another question now is how do we measure $R$ ?, for that we will use the idea of Quantum Hall Effect. Quantum Hall effect is the standard for resistance, please refer the paper for more information. But suffice to say, the resistance is defined as,
$$\boxed{R = \frac{1}{k}\frac{h}{e^2}}$$
Here, $k$ is $1, 2, 3, ...$
Please note that, without the integer fraction i.e., $R_k = \frac{h}{e^2}$ is called the von-Klitzing constant, this guy got a Nobel for this. Please read more here.

Using the above equation in $(7)$ we get,
$$\frac{h^2\times f^2}{4e^2 \frac{1}{k}\frac{h}{e^2}} = m \times g \times v$$
Which comes to,
$$h^2\times f^2 = m \times g \times v \times 4e^2 \frac{1}{k}\frac{h}{e^2}$$
$$\implies h\times f^2 \times k = m \times g \times v \times 4$$
$$\implies h = \frac{m \times g \times v \times 4}{f^2 \times k} \tag*{(8)}$$
This is for a single Josephson Junction, for $n$ junctions, it becomes,
$$\implies h = \frac{m \times g \times v \times 4}{f^2 \times k \times n^2}$$
Which can look more elegant if we write it as,
$$\boxed{h = \frac{4}{k \times n^2}\frac{g \times v}{f}m}$$
We have seen that $V$ and $R$ were measured very accurately. Similarly, there is a need that we measure the factors very accurately as well. The $v$ is measured using a Laser Interferometer. The $g$ was measured using a Gravimeter.
So, the scientists in NIST, are just putting in some mass $m$ and get the $h$, and keep tuning the value of $m$ till we get a very accurate $h$.
Cheers!