Wonderful Engineering

The Standard Kilogram Has Changed. Here’s Why

A metal cylinder sits inside three nested bell jars which are placed in a temperature-controlled vault of International Bureau of Weights and Measures in Sevres, France. This metal cylinder, also known as Big K, is an alloy of platinum and iridium and has been used as a standard for weight around the world. But this will change soon. Recently, during the 26th meeting of the General Conference on Weight and Measures in Versailles, France, representatives from more than 60 countries voted to redefine the kilogram. It was suggested that rather than using a physical object, it would be based on the Planck’s Constant, a fundamental factor in physics. This infinitesimally small value holds 33 zeros after its decimal point and defines the packets of light known as photons in every part of the universe.

The leader of the National Institute of Standards and Technology, Stephen Schlamminger, who worked with a team of international scientists on the redefinition of weight in accordance to Planck’s constant, said: “That fundamental constant is woven into the fabric of the universe.” The best part of this redefinition is that it will remain constant for ages and even location won’t affect it. The kilogram is one of the seven base units defined by the International System of Units. These units help us in our daily life be it manufacturing, commerce or scientific innovation.

The metric system was introduced in the late 1700’s and has become the International System of Units ever since. It was presented in hopes to simplify day-to-day life and as “something for all times, for all people,” says Schlamminger. Most of the metric units were based on natural things, Richard Davis explained, who is a physicist at the International Bureau of Weights and Measures, like meter was 10,000,000th part of the distance from equator to the North Pole passing through Paris and kilogram was defined as the mass of one liter of distilled water at its freezing point.

Davis said: “They just didn’t have the technology or the science to succeed,” In June 1799, two standards were introduced made of platinum, a rod defining meter and a cylinder defining the kilogram. For increased stability, these standards were reforged out of an alloy of platinum and iridium and kept securely. Even then this dependency on a physical object is not for all times as it is prone to wear over time. And being under secured lock and key, these prototypes were indeed not for everyone.

Big K has been doing just fine in the lockup. Scientists have made multiple copies of it around the world for use. Only three times in its 130 years of captivity has it been released to be compared with its copies. With each comparison, the scientists realized that it might be losing weight. A total of 50 milligrams were lost in all these years. This may seem small, but it is a considerable difference in fields like medicine. It also affects many other units like Newton which are defined about mass.

To overcome this issue, the General Conference of Weights and Measures passed a resolution in 2011 to redefine kilogram, ampere, Kelvin, and mole. Scientists around the world worked hard to find a solution. Two possibilities were suggested that could be considered and they both were tied to Planck’s constant. The first suggestion was based on the Kibble balance, something similar to a classic beam balance with pans on both ends. To measure the weight of something, a known mass is placed on one side and the unknown mass on the other side. The beam balance works due to Gravity, but the Kibble balance has a little different system.

The Kibble balance uses a coil in a magnetic field instead of one of the pans. By comparing the mass with its electromagnetic properties, scientists can make an exact measurement of Planck’s constant. The second solution was to craft another spherical object of crystalline silicon-28. Then by calculating the precise number of atoms in the sphere, scientists can figure out the value of Avogadro’s number and then convert it into Planck’s constant. The value of Planck’s constants turned out to be infinitesimally small: 0.000000000000000000000000000000000662607015 meter-squared-kilograms per second. With the help of these two methods, the scientists can measure the kilogram with an uncertainty of 1/100,000,000. The new changes will take effect on May 20, 2019. Even though measuring basic things at home might not be affected, the redefinition will undoubtedly impact the industrial processes like drug manufacturing