More than two hundred years ago, metrologists determined how much one kilogram weighs: exactly one litre of water.
They soon found that such a criterion was not the most accurate. They made a one kilogram platinum cylinder, which is a more reliable and convenient measure. It was called the International Prototype of the Kilogram.
This cylinder is the only object in the world that weighs exactly one kilogram. It is stored under three glass bells in the vault of the International Bureau of Weights and Measures in Paris.
There were also 67 copies made, which are stored in laboratories around the world. Occasionally they are measured and compared with the original.
It was found that the masses of the cylinder and the copies changed over time. This is a major problem for engineers and scientists, who need to measure extremely accurately.
Metrologists therefore decided to re-determine the kilogram, but this time even more precisely.
Last Friday, at an international meeting, they voted that from May next year the kilogram will no longer be determined by an object, but by a complex physical equation.
A kilogram will, of course, still weigh one kilogram.
»Redefining is similar to renovating the cellar in a house. In the end, the house looks the same from the outside, but it is less likely to collapse due to better foundations,« explains Michael de Podesta, a researcher at the London National Laboratory of Physics.
In the future scientists will be able to more accurately calculate mass, but will not need to use a metal cylinder.
Ponts to Consider
- How do we calculate wight?
- Why do the scientists have to be very precise?
- How do we define the length of a meter?
The original version of this article was published on November 22nd.