In the latest episode of the video series “Measured World”, dedicated to the 150th anniversary of the Metre Convention, a classic riddle is brought into focus: which is heavier—a kilo of cotton or a kilo of gold? The answer is explained by Allar Pärn, national standard custodian and measurement specialist at Metrosert.
“All scales are actually calibrated to weigh steel—that is, objects with a density of around 8000 kg/m³,” Pärn explained. “As a result, all scales, regardless of price, tend to show slightly lower values when weighing objects with lower density—such as water, wood, or cotton—and slightly higher values for denser materials like gold, silver, or uranium.”
Pärn added that the density of the object being weighed determines how much the surrounding air can lift it. “Air has a very low density—around 1.2 kg/m³—and it would lift everything off the ground if only gravity allowed it,” the specialist noted.
In everyday life, mass differences caused by density are usually negligible, as the discrepancies are quite small. Still, there are exceptions—for example, in the pharmaceutical industry, where accuracy down to the milli- or microgram is critical for measuring active substances. As science progresses, so do the requirements for weighing precision. “We are gradually moving towards optical measurement solutions and quantum technologies even in mass measurement,” Pärn remarked.
150 Years of the Metre Convention
The historic Metre Convention was signed in Paris on May 20, 1875, by representatives of 17 countries, laying the foundation for international cooperation in the field of measurement. It marks a key milestone in the history of metrology.
In essence, the agreement ensures that all member countries operate according to the same measurement standards. It guarantees a globally unified system of measurement units—meaning a meter is the same length no matter where in the world it is measured.
Importantly, only national metrology institutes can sign the Metre Convention on behalf of a country. In Estonia, this role is fulfilled by Metrosert, which maintains and develops the country’s national measurement standards and has operated under various names since 1919.
Estonia became a full member of the Convention on January 14, 2021, when President Kersti Kaljulaid signed the decision at Kadriorg Palace. This gave Estonia the opportunity to participate in international metrological cooperation and to more easily develop its national standards in collaboration with metrology institutes worldwide.
Currently, Estonia maintains national standards in four areas: mass, length, temperature, and electrical quantities. Based on the needs of Estonian businesses, new fields are also being developed. One such project, the development of the national standard for time and frequency, is underway and will continue until the end of this year.
Read more about the 150th anniversary of the metre convention HERE.