Valuable insights
1.Global Mass Equivalence is Essential: Accurate mass measurement equivalence across the globe is crucial, especially for international trade and sensitive sectors like pharmaceuticals dealing with minute quantities.
2.The IPK's Fundamental Instability: The International Prototype Kilogram (IPK) is inherently unstable because all its copies show relative drift, yet no higher standard exists for comparison.
3.Transitioning to Physics-Based Standards: The scientific community is moving away from physical artifacts to define constants, basing the new kilogram standard on physics, likely involving Planck's Constant.
4.Silicon Sphere Method for Definition: One proposed method involves manufacturing near-pure, single-crystal silicon spheres and measuring lattice spacing and volume for high accuracy.
5.High Cost of Atomic Counting: While the silicon sphere method can achieve accuracy of roughly one part in $10^{-8}$, the cost for manufacturing such a sphere reaches approximately €2.5 million.
6.The Watt Balance Electrical Alternative: An alternative approach utilizes the Watt Balance, an electrical method that measures mass by relating weight to the current required to restore a position.
7.Future Standards Remain Physical: Even with a new absolute definition based on physics, physical reference kilograms will continue to exist globally, serving as tangible benchmarks.
The Physical Standard: The Kilogram Prototype
The concept of the kilogram is currently defined by a physical artifact located at the National Physical Laboratory in Teddington. This artifact, a lump composed of a platinum-iridium alloy, is maintained under extremely secure conditions. Its primary function is to serve as the tangible representation of one kilogram, the fundamental unit of mass used worldwide.
The International Prototype Kilogram
This specific artifact is known as the International Prototype Kilogram (IPK). It resides in a highly secure safe, acting as the ultimate reference point for mass measurement. Historically, all other national standards and calibrated equipment ultimately trace their accuracy back to this single object housed in Paris.
It is one kilogram, no matter if it has gained a tiny amount of mass from adsorbing air contaminants or lost a tiny amount when it was cleaned.
Instability of the Physical Artifact
A significant issue arises because the IPK is known to be unstable over time. Since all its copies exhibit instability relative to the prototype, there exists no higher, more stable reference point against which the IPK itself can be compared to confirm any drift.
The Imperative for Global Measurement Equivalence
The necessity for mass measurement equivalence around the world is paramount for consistent commerce and industrial application. When a commodity is purchased in America, for instance, whether it is a milligram or one hundred tonnes, the recipient must be certain that the measurement is equivalent to the same quantity in the United Kingdom.
Critical Areas of Uncertainty
The areas where measurement uncertainty becomes critically important include the pharmaceutical industry. This sector manages very small quantities of active ingredients that demand extremely precise measurement to ensure product efficacy and safety. The accuracy of consumer devices, such as bathroom scales, traces back through several calibration chains to the IPK.
Redefining Mass via Physics: Silicon Spheres
Scientists globally are working to establish the 21st-century standard for the kilogram, shifting the formal definition away from a physical object toward a definition rooted in physics, with Planck's Constant being the likely foundation. The challenge then becomes translating this constant into a reliable physical realization.
Counting Atoms with Silicon
One primary option involves creating a sphere made of almost-pure, single-crystal silicon. This manufacturing process is relatively achievable. The key measurement involves determining the lattice spacing—the distance between atoms—and the overall volume of the sphere. These measurements allow for the generation of a sphere with an accuracy approaching one part in $10^{-8}$.
- Manufacturing a sphere of nearly pure, single-crystal silicon.
- Precise measurement of the atomic lattice spacing.
- Accurate determination of the sphere's total volume.
The downside of that is it costs you €2.5 million for a sphere.
The Electrical Measurement: Watt Balance
An alternative method exists that could prove both cheaper and easier than the silicon sphere approach, although it is less intuitive. This method utilizes the Watt Balance, which provides an electrical means of measuring mass. This technique demonstrates how electrical forces can be used to quantify weight.
Demonstrating the Watt Balance Principle
The principle can be illustrated using a loudspeaker: passing a current through its coil causes the cone to move. If a mass is placed on the cone, the specific current needed to restore its original position serves as a direct measure of that mass's weight. Although the magnet field strength and coil turns are unknown, a second experiment involving movement and measuring the produced voltage allows for the elimination of these unknown factors, yielding the mass.
- The current required to maintain the mass at a fixed position.
- The velocity of the cone movement during the second experiment.
- The voltage produced during the movement experiment.
Conclusion: The Absolute Future of Mass
Regardless of which approach is ultimately chosen to redefine the kilogram, physical reference kilograms will not disappear from the world. However, for the first time, scientists will possess an absolute physical constant against which these physical artifacts can be compared, ensuring stability and accuracy across all future measurements.
Acknowledging the National Physical Laboratory
Gratitude is extended to all personnel at the National Physical Laboratory for their contributions to this endeavor. Further details and insights into their work are available through their official YouTube channel and website, which occasionally announces open days for public viewing.
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