Name: How We're Redefining the kg
Uploaded: 2020-03-28T18:28:17.000Z
Duration: 9 min 49 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

What do I have to push, sub-basement? >>Woman: Sub-basement.

I'm at the National Institute of Standards & Technology in Washington D.C.

We're going to find out how they're going to redefine a kilogram.

Since 1799, it's been defined as the mass of a metal cylinder,

in a locked vault in a basement in Paris.

But over the last century, careful measurements of this international prototype kilogram

and in-theory-identical national standards from around the world,

have shown that their masses are diverging.

The spread has grown to around 50 micrograms, or 50 parts per billion.

And having a standard of mass that changes is unacceptable.

Plus, the kilogram is the last of the base SI units to still be defined by a physical object.

The metre, for example, used to be defined as the length of a platinum bar in Paris,

but in 1983 it was redefined as the distance light travels

This definition means that the speed of light is set to exactly 299,792,458

Note how this works: first, you take the existing definition,

and you measure as carefully as you can how it relates to a physical constant of the universe:

Then you set the exact value of that constant

and use *it* to redefine how long a metre is.

I know this might seem circular, but, importantly, it moves the point of truth

off of the physical object, and onto the unchanging constant of the universe.

So, naturally, the thought is to do the same thing with the kilogram.

Well, there are a number of different strategies that were attempted

but the two that achieved the greatest success were:

1) using a silicon sphere to determine and set Avogadro's number

and 2) to use a Watt balance to determine and set Planck's constant.

>>DEREK: Hi, how ya' doin'? I'm Derek. >>JON: Pretty good. >>DEREK: Nice to meet you.

>>STEPHAN: The Watt balance is behind these closed doors, and...

>>STEPHAN: It's correct, and right now the problem is that...

We are in a crunch to get a number by the end of May.

>>STEPHAN: The Planck's constant. This is what we measure with the Watt balance.

In 2011, the General Conference on Weights and Measures

decided that the kilogram should be redefined based on Planck's constant,

but that doesn't mean that the Avogadro approach was futile.

I mean, you can use Avogadro's number to calculate Planck's constant and vice-versa.

So, ultimately, both approaches are going to be used to redefine

Planck's constant and Avogadro's number simultaneously.

>>STEPHAN: One good thing about having silicon spheres,

is that you only want to redefine if you have agreement between different numbers, right?

And the silicon sphere method is a method in my mind that comes out of chemistry.

You measure Avogadro's constant, which is a constant that comes out of chemistry.

This method comes out of physics, we measure Planck's constant.

So if they both agree, it's a pretty strong sign, right? Because you know chemistry and physics agree.

Now, since I've already discussed the Avogadro approach in a previous video,

here I want to focus on the Watt balance.

It's actually now called a Kibble balance in honor of its inventor, Bryan Kibble,

You know, traditional balances work by equating the gravitational forces on objects in two pans.

The Kibble balance looks kind of similar, but all of the balancing happens on the left-hand side,

where a mass pan is attached to a coil of wire in a magnetic field.

The whole apparatus is sealed and operated in vacuum.

and both are required to determine Planck's constant.

In weighing mode, a kilogram mass standard is placed on the mass pan

and then current is passed through the coil in the magnetic field

and adjusted until the weight of the kilogram is equal and opposite

to the electromagnetic force on the coil.

The equation for this is Mass times the local gravitational acceleration

is equal to the Magnetic field, times the length of wire in the coil, times the current flowing through it

In this equation the variables that are difficult to measure exactly are the magnetic field strength, and the length of wire in the coil

But luckily the Kibble balance allows us to get around this problem using velocity mode

In velocity mode the kilogram mass is lifted off the mass pan and now the motor on the other side of the balance is used to

Move the coil back and forth at constant velocity through the magnetic field.

This motion induces a voltage in the coil which is equal to the magnetic field,

times the length of wire in the coil, times its velocity.

Now we have two equations which we can solve for B times L and so we can set them equal to each other and

eliminate these variables without having to know precisely what their values are

and if we rearrange a little bit you get voltage times current equals mass times gravity times velocity.

on the left hand side, there is electrical power and on the right hand side, mechanical power,

and that's why this was called the Watt, the unit of power, balance

But how do you go from this to Planck's constant the number that relates a photon's frequency to its energy?

Well it turns out there's actually a way of measuring voltage accurately using a macroscopic quantum effect that involves Josephson junctions

so a Josephson junction consists of two superconductors separated by a thin piece of insulator

Now if you apply a microwave radiation to that junction, you create a voltage across the device and its value is precisely known to be

hf over 2 e. Where h is Planck's constant, f is the frequency of the radiation, and e is the charge on an electron

Now by tuning that frequency and stacking as many of these Josephson junctions as you want in series

you can create virtually any voltage you like very very precisely.

The way this is used in the Kibble balance is a stack of hundreds of thousands of Josephson junctions

are put into the circuit with the coil as it is moved through the field

and so you exactly balance the voltage which is induced in the coil using those Josephson junctions

So you can measure that voltage very very accurately.

Well it turns out this voltage measuring method is so good that instead of trying to measure current directly

we instead measure V on R which is the same thing

So this current is passed through a resistor, and we measure that voltage again using Josephson junctions