LONDON — Scientists say they are close to achieving a 200-year-old goal of creating a universal system of measurements based on stable quantities, as they progress toward changing how the kilogram is defined.

The kilogram is the only base unit in the International System of Units (SI) that is still defined by a physical object — a prototype of platinum-iridium kept in the vaults of the International Bureau of Measurements (BIPM) in France.

The stability of the kilogram is crucial as it forms the basis from which many other units are derived.But measurements made over more than 100 years suggest that the mass of the international prototype may have changed by about 50 micrograms — the size of a small grain of sand — prompting the BIPM to try to develop a new definition based on a fundamental physical property.

Scientists will gather at the Royal Society, Britain’s national academy of science, on Monday to present their progress on redefining the kilogram according to something called “the Planck constant,” a fundamental constant of quantum physics.

“International consensus has been achieved, that in the near future the kilogram shall be redefined, based on a fixed value of the Planck constant,” Michael Stock, a physicist at the International Bureau of Weights and Measurements (BIPM), said in a statement.

Stock said researchers have been conducting experiments that establish a link between mass and the Planck constant by comparing measurements of electrical and mechanical power.

But the new definition of the kilogram can’t take place until the results of tests, conducted in laboratories across the world, are in agreement, he explained.

The International System of Units is the world’s most widely used system of measurements for commerce and science. It is made up of seven base units — meter, kilogram, second, ampere, kelvin, candela and mole — each of which represents a different physical quantity.

Its origins can be traced back to 18th century France and it has been recognized internationally as the standard metric system since the 1960s.

The meter was once defined as the distance between two lines on a platinum-iridium prototype, but is now defined by the speed of light.

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