How We're Redefining the kg

TL;DR

The kilogram, currently defined by a physical object, will soon be redefined based on Planck's constant, with the use of silicon spheres and a Watt balance.

Transcript

What do I have to push, sub-basement?

Woman: Sub-basement. [Buzzing safety alarm] I'm at the National Institute of Standards & Technology in Washington D.C. and I'm going to the sub-basement. It's getting dark down here. We're going to find out how they're going to redefine a kilogram. The kilogram is in trouble. Since 1799, it's been defined as ... Read More

Key Insights

• ⚾ The kilogram is the last base SI unit to be defined by a physical object, and redefining it based on a physical constant ensures stability and accuracy.
• 💪 The two methods used to redefine the kilogram, using silicon spheres and a Watt balance, provide strong agreement between chemistry and physics.
• ⚖️ The Watt balance, also known as the Kibble balance, uses innovative methods like Josephson junctions and the quantum hall effect for precise voltage and resistance measurements.
• 🖐️ Interferometry techniques like laser interferometry and gravimetry play a crucial role in accurately measuring the velocity and acceleration due to gravity in the Kibble balance.
• 🥺 The redefinition of the kilogram based on Planck's constant will lead to a more standardized and reliable unit of mass measurement.
• 😒 The use of silicon spheres and the Watt balance in the redefinition process ensures a high level of accuracy and agreement between different measurements.
• ⚾ The redefinition of the kilogram will bring it in line with other SI base units, which are already defined based on physical constants.

Questions & Answers

Q: Why does the kilogram need to be redefined?

The masses of different kilogram standards have been found to be diverging, which is unacceptable for a standard unit. Redefining it based on a physical constant ensures a more stable and accurate measurement.

Q: How are Avogadro's number and Planck's constant related to the redefinition of the kilogram?

Avogadro's number and Planck's constant are both used to calculate each other. By using both methods to determine these constants simultaneously, it provides a strong agreement between chemistry and physics.

Q: How does the Watt balance work?

The Watt balance balances the weight of a kilogram mass with the electromagnetic force on a coil in a magnetic field. It operates in two modes: weighing mode and velocity mode, and accurately measures voltage and current using Josephson junctions and the quantum hall effect.

Q: What is the significance of using Josephson junctions in the Kibble balance?

Josephson junctions are used to measure voltage accurately. By tuning the frequency and stacking multiple junctions in series, any desired voltage can be created with high precision, which is crucial for the Kibble balance's measurements.

Summary & Key Takeaways

• The kilogram is currently defined as the mass of a metal cylinder in a vault in Paris, but measurements have shown that the masses of different standards are diverging.

• The plan is to redefine the kilogram based on Planck's constant using two methods: using silicon spheres to calculate Avogadro's number and determining Planck's constant with a Watt balance.

• The Watt balance, also known as the Kibble balance, balances the weight of a kilogram mass with the electromagnetic force on a coil in a magnetic field, and also uses Josephson junctions and the quantum hall effect for accurate voltage and resistance measurements.