The only standard international unit of measure that is still defined by a physical object is the kilogram. The kilogram is currently defined by a platinum-iridium alloy cylinder known as the International Prototype of the Kilogram, which is stored at the International Bureau of Weights and Measures in France.

The decision to define the kilogram by a physical object was made in 1889 during the 1st General Conference on Weights and Measures. The International Prototype of the Kilogram was created to serve as the standard for all other kilogram measurements around the world.

However, due to concerns about the long-term stability of the International Prototype of the Kilogram, efforts have been made to redefine the kilogram in terms of a fundamental constant of nature. In May 2019, the kilogram was officially redefined in terms of the Planck constant, a fundamental constant of quantum mechanics.

Despite this redefinition, the International Prototype of the Kilogram continues to be used as a practical realization of the kilogram for calibrating other mass standards. This ensures that measurements of mass around the world remain consistent and accurate.

Overall, the kilogram is a unique unit of measure that is still defined by a physical object, the International Prototype of the Kilogram. While efforts have been made to redefine the kilogram in terms of a fundamental constant, the physical prototype continues to play a crucial role in maintaining the accuracy and reliability of mass measurements worldwide.

In conclusion, the kilogram is the only standard international unit of measure that is still defined by a physical object. Its importance in the world of metrology cannot be understated, as it serves as the foundation for accurate measurements of mass across various industries and scientific disciplines.

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