Slide 1
The kilogram moved from a metal object to a constant
The kilogram was once defined by a particular metal object. Since 2019 its definition instead fixes the numerical value of Planck’s constant.
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An artefact can collect contamination or lose material during handling. A definition tied to a constant lets laboratories seek independent realizations rather than treating the condition of one stored cylinder as the ultimate reference.
Slide 2
A Kibble balance links mechanical and electrical measurements
A Kibble balance links the force supporting a mass with electrical measurements. This provides a practical route from a fixed constant to a mass value.
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The experiment uses distinct weighing and motion measurements. Combining them eliminates a difficult magnetic-field factor, allowing electrical quantities and measured motion to connect with mechanical power.
Slide 3
Defining the unit exactly does not make an experiment exact
Fixing a constant defines the unit exactly. Building an experiment that realizes that definition still introduces measurable uncertainty.
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Different laboratories compare their realizations to check agreement. Such comparisons distinguish a universally shared definition from the performance of any particular apparatus or local measurement procedure.
Slide 4
Working weights still pass the definition to everyday balances
Physical weights remain useful for calibrating everyday balances. Their assigned values are now connected to the constant-based definition through measurement chains.
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A laboratory does not need a Kibble balance beside every shop scale. Reference standards transfer the unit through successive comparisons, with uncertainty documented at each stage of the chain.
Slide 5
The change preserved the familiar size of the kilogram
The redefinition was designed to preserve the familiar size of the kilogram. Everyday products did not suddenly acquire different masses.
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The change concerns what ultimately defines the unit, not a new mass scale for ordinary life. Better experiments can improve realization without requiring another replacement of the kilogram’s defining reference.
Sources
- Definitions of SI Base Units
National Institute of Standards and Technology
- Calibration of Mass Standards
National Institute of Standards and Technology