Still the kilogram: All you need to know about the new and improved unit of mass

The redefinition in 2019 would be the first time since 1983 that the metric system gets an update.

On 16 November, a field of scientists and diplomats from 60 different countries packed themselves into an auditorium in Versailles, France and voted on a treaty.

This modern-day treaty to re-define the metric system would be passed later that day, fundamentally changing trade and technology forever, for the better.

This would be the first time since 1983 that the metric system got an update.

Units 'for all times, for all people'

In a unanimous vote, the international General Conference on Weights and Measures (CGPM) ditched the old method of using physical objects to measure four quantities — mass (kilogram), current (ampere), temperature (Kelvin) and concentration (mole).

The new units were defined using constants in physics (like the Planck constant for mass and Avogadro constant) which are true, unchanging constants in nature. The standards to measure distance (metre) and light-intensity (candela) already use natural constants of the speed of light and the motion of the atoms in the element cesium to be quantified.

Out with the old and in with the new – the new quantities used to measure the standard (SI) units are all constants in nature. Image courtesy: BIPM

Out with the old and in with the new – the new quantities used to measure the standard (SI) units are all constants in nature. Image courtesy: BIPM

With this 2019 redefinition of the international system of units (SI), the metric system is, for the first time in its history, derived completely from natural phenomena.

What’s more, they also comply with the metric system’s “motto”: For all times, for all people.

Putting away Le Grand K

Being the most widely used of the measurements, the update to the kilogram will likely be the most significant of the lot in day-to-day use.

This big change to mass is the last remaining unit measured using a physical artifact: the International Prototype Kilogram, also called Le Grand K.

This great-great-granddaddy of all the world’s kilograms till date is a lemon-sized metal cylinder locked in a secure vault in France that requires three separate keys to access.

A prototype kilogram replica on display at a Paris Museum, featuring three layers of protective glass. Image courtesy: Jabs 88

A prototype kilogram replica on display at a Paris Museum, featuring three layers of protective glass. Image courtesy: Jabs 88

For 129 years, this hunk of platinum-iridium alloy has been the one true kilogram based on which all the world’s others were adjusted periodically. Even after its scientific significance is replaced next year by the Plank constant, Le Grand K will not be discarded.

Now, it takes on a new role: that of a cylinder with a lot of history.

Years of weight-loss

The alloy of platinum and iridium that makes up the Grand K is corrosion-resistant and rarely sees the light of day.

Over time, while its many copies around the world seemed to be weighing as expected, the Grand K appeared to be gradually dropping. Or were the copies getting heavier? It’s nearly impossible to say which of the two is true.

“A material object will not be for all time,” Schlamminger says.

With glasses that shatter, clothes that rip and pipes that rust, the Grand K that appears to have lost 50 micrograms (as heavy as a grain of salt) compared to its copies by sitting still in a locked vault, is neither accurate nor “for all people,” Schlamminger adds.

“The system will be intrinsically correct by reference to the laws of science, the laws of nature," Martin Milton, the BIPM director, told The Associated Press.

"We won't have to depend on just assuming that one particular object never changes…everything else has been recycled and replaced and improved."

The artisans who forged, polished and perfected the International Prototype Kilo more than 130 years ago worked with mind-boggling precision and skill to craft the platinum/iridium cylinder. “If we did this work today, we’d probably come up with exactly the same composition. It has turned out to have really stood the test of time very well,

The artisans who forged, polished and perfected the International Prototype Kilo more than 130 years ago worked with mind-boggling precision and skill to craft the platinum/iridium cylinder. “If we did this work today, we’d probably come up with exactly the same composition. It has turned out to have really stood the test of time very well," Milton said. AP

What is the kilogram changing to?

For all intents and purposes, what we call a kilogram today will be called a kilogram for the foreseeable future.

What changes is the new instrument used to reset all the balances in the world to match a single standard: the Kibble balance.

The Kibble balance is similar to a classic beam balance in almost every way: it has a bar in the middle, with a pan hanging on either side. In the beam balance, one pan is occupied by a standard, and the second pan by the object being weighed, and gravity tips the scale based on the difference in weights.

One of the pans in a Kibble balance is replaced with a coil in a magnetic field, and the difference in masses is balanced using electromagnetic force instead of gravity. This shift makes it possible to make very tiny measurements of the Planck’s constant, which is 0.000000000000000000000000000000000662607015 metre-squared kilograms per second. (That's 33 zeroes after the decimal or 6.626176 x 10^-34 joule-seconds.)

Kibble for Kilogram

From an uncertainty of 50 micrograms, the Kibble balance can now give exact measurements of a kilogram having a margin of error weighing as much as a quarter of an eyelash (1/10,00,00,000 a kilogram).

The change from the current system to the updated unit of a kilogram will officially begin on 20 May 2019.

"On that day, you won't see any change in our daily lives," Richard Davis, emeritus research physicist at the International Bureau of Weights and Measures, told The Associated Press.

The Kibble balance, which began full operation in early 2015, can measure Planck's constant. With an accuracy of close to 13 parts per billion, the balance has been declared accurate enough to assist with the redefinition of the kilogram in 2019. Image courtesy: Wikimedia Commons

The Kibble balance, which began full operation in early 2015, can measure Planck's constant. With an accuracy of close to 13 parts per billion, the balance has been declared accurate enough to assist with the redefinition of the kilogram in 2019. Image courtesy: Wikimedia Commons

While the upgrade technically affects all the weighing scales in the world, as the clock strikes 00.01 am on 20 May next year, groceries, atta and gold coins will still cost just the same per gram.

The impact of minute inaccuracies won’t be felt in bakeries or hypermarkets but in research and industries. In fields like computing, pharmaceuticals and climate science, precise measurements may make or break consequences.

It may take a lot longer than a year from now for the redefined kilogram to take full effect in Indian industries, according to reports. The National Physical Laboratory (NPL) in Delhi, tasked with producing the Kibble balances, will spend Rs 60 cr over the next 3 to 4 years building a facility to create and calibrate the weighing instrument.

Given that everything in the world is in a state of flux, the good-old kilogram will (in a year or few) join seven other units that will remain constant and forever unchanging.

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