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The world’s most common unit of mass cannot be trusted. That is the conclusion after new check weighing revealled that the 214-year-old original ”kilogramme cylinder” has gained weight. So now physicists seek to define the weight of the kilogram once and for all by creating the most circular sphere in the world… and counting its atoms.

The kilogram must lose weight. Physicists have discovered that the original, cylinder-shaped piece of metal, which is used to define the unit of mass, has added at least 10 microgram. The kilogram is kept in a showcase near Paris, but the problem is the same in all 40 countries that possess a copy of the original kilogram.

The metal cylinders were made in the late 1800s, and they are used as the standard measure of one kilogram. However, check weighing demonstrates that they all have slightly different weights. Ten microgrammes mightn’t seem like much, but the difference could ruin scientific experiments, which require 100 % accurate measurements, or result in incorrect measurements, when for instance radioactive materials, of which only very small amounts can be sold, are traded.

The kilogram weight problems have caused headaches for physicists, who must solve two problems in the short and long term: They need to rid the original kilogram cylinders of impurities and define a kilogram based on a physical constant such as gravity or the speed of light. For instance, physicists use the speed of light to define one metre as “the distance travelled by light in vacuum in 1/299,792,458 of a second”.

The kilogramis locked up The problem is a result of a rather abstract phenomenon: nobody has ever defined what a kilogram really is – it just weighs what the original kilogram cylinder weighs.

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When the measuring systems of the French Revolution were introduced in the late 1790s, the world began to use centimetres and metres plus the new kilogram measure. In 1799, French scientists made the first kilogram prototype: a platinum cylinder with a 39.17 mm diameter, which has been kept in a top-security box on the outskirts of Paris ever since. That is the prototype on which all other kilogram cylinders are based. The problem of a metal cylinder being a standard measure is that external materials can pollute the kilogram and thus upset the scientists, who base their measured values on the weight of the 214-year-old platinum cylinder.

When the kilogram was created, physicists were aware that it could gain weight. Consequently, they provided the standard kilogram with  a cylinder shape, whose surface area is smaller than that of for instance a box, and in the past decades, it has been kept in filtered air at a constant temperature.

Mercury pollutes the cylinder
But unfortunately, even the most carefully thought-through safeguards cannot prevent minute particles from settling on the surface of the kilogram over time. Using a technique called X-ray spectroscopy, physicists have studied the platinum cylinder and observed, how minimal amounts of mercury and carbon slowly accumulate on the surface, just from the air in the room and the box the cylinder is kept inside. The carbon probably derives from car exhaust, whereas the mercury particles presumably spread in the air when lab thermometers are broken by accident. The accumulated material can be removed, but the problem is that physicists are cleaning the original kilogramme and the 40 copies at different times, using different methods. And so, none of the kilograms in the world ever weigh precisely the same at any time.
Physicists have tried to keep the kilogram in ultraviolet light, which breaks down the carbon atoms’ bindings and continously removes the pollution, but the method is not 100 % efficient yet, and it does not solve the problem with mercury.
The only method which can guarantee a permanent, common measure of mass will simply be to define the kilogram based on a physical constant. Then scientists will always be able to adjust their scales accurately.

sphere

Scientists count atoms
It is most obvious to describe a kilogram based on the number of atoms of a particular material in a given mass. That is the method used by an international team of scientists right now. For two years, they have been engaged in making a silicon sphere of exactly one kilogram based on the sphere’s mass and weight, by calculating the precise number of silicon atoms, which make up an exact kilogram of silicon. The sphere shape was chosen because the surface has no edges which could be damaged, and because the shape minimises the amount of waste products, which could stick to the sphere. At the same time, the sphere’s volume can be calculated based on only one known measure: the diameter.
What’s more, silicon atoms are extremely stable. They usually collect into cubic shapes of eight atoms each. And thus, physicists can in reality measure the sphere and subsequently calculate how many atoms it contains. However, the job is more difficult than it sounds. According to the scientists, the project is like looking at a mountain of six pack soft drink cans and figuring out the number of cans.

Gravity may hold the answer
And so for two years, the scientists have laboriously counted atoms and fine-polished the sphere to approach the weight of the original kilogram. Right now, it is not perfect, but the atom count is so close to being completed that the uncertainty involves as little as 30 parts per billion. In comparison, physicists calculate with an uncertainty of 20 parts per billion when they measure based on the original kilogram.
However, the atom count is not the only project. Other scientists are trying to measure gravity’s effect on a mass and defining the kilogram as the force needed to counteract gravity. But in spite of different methods, everybody agrees that a breakthrough is imminent. Within 5-10 years, we will probably have a scientific definition of the kilogram – only 215 years late.

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