Here is a concise summary of the text:
**The Kilogram's History and Redefinition**
* The kilogram was originally defined as the weight of a liter of water at 4°C (1799)
* Later, a platinum cylinder (Kilogram of the Archives, aka "Le Grand K") was created to embody this mass
* However, measurements revealed that the mass of "Le Grand K" and its replicas diverged over time (up to 50 micrograms)
* To address this issue, scientists are redefining the kilogram:
1. **Silicon Sphere Approach**: Counting atoms in a precisely crafted, ultra-round silicon sphere (2.15x10^25 silicon-28 atoms) to define the kilogram.
2. **Watt Balance Approach**: Fixing Planck's constant to redefine the kilogram.
* If both methods agree within 20 micrograms, the kilogram may be redefined as early as 2014, making it an unchanging, abstract unit.
Here are the key facts extracted from the text, numbered and in short sentences:
**Silicon Sphere Facts**
1. The speaker holds a 1kg sphere of silicon-28 atoms.
2. The sphere contains approximately 2.15x10^25 atoms.
3. The raw material for the sphere was worth 1 million Euros.
4. The sphere is considered the roundest object in the world.
5. The sphere is made of a single isotope of silicon (silicon-28).
**History of the Kilogram Facts**
6. The original base unit of mass in the metric system was called the "grave" (1793).
7. The grave was defined as the weight of a cubic decimeter of water at the melting temperature of ice.
8. The name "grave" was later changed to "kilogram" due to its similarity to the aristocratic title "graf".
9. The kilogram was initially defined as the mass of a litre of water at 4 degrees Celsius (1799).
**International Prototype Kilogram (IPK) Facts**
10. The IPK is a platinum-iridium alloy cylinder created in 1889.
11. The IPK is stored in a climate-controlled vault in the International Bureau of Weights and Measures, Paris.
12. The IPK is the only SI unit still defined by a physical object.
13. 40 identical replicas of the IPK were created, which serve as national standards.
**Measurement and Stability Issues**
14. The masses of the IPK replicas diverged over time (up to 50 micrograms).
15. The mass of the IPK itself is not stable over time.
16. This instability affects not only mass measurements but also other SI units (e.g., Newtons, Joules, Volts, Watts).
**Redefining the Kilogram**
17. A new definition of the kilogram may be based on the number of silicon-28 atoms in a sphere (~2.15x10^25).
18. This approach would redefine Avogadro's constant, decoupling the kilogram from a physical object.
19. An alternative approach involves fixing Planck's constant using a Watt Balance.
20. A potential redefinition of the kilogram might occur as early as 2014 (now historical, as the redefinition occurred in 2019).