How much is a Kilogram? Read to know about the history and latest changes to kilogram

I raised my hand as high as I could, sitting in the third row of benches in my class, hoping that our chemistry teacher will notice me and ask me for the answer which I believed I had calculated correctly before rest of the class. I strategically chose third row of benches throughout my student life as I believed that third row students got just the right attention from the teachers and were at lower risk of being asked to produce the homework and classwork assignments that teachers could show to the rest of the class as examples of good or bad work.

“27.9”, I said, beaming with confidence when Pandey Sir nodded at me after seeing me raising my hand. Waiting for my appreciation from him, I never anticipated the kind of mockery I was going to become in next minute. Pandey Sir being one of the strictest teacher, yet a fun-loving person who would not miss any opportunity to crack a joke, asked me back, “27.9 Kilometers or 27.9 metre per second square?”. The whole class burst into laughter, forgetting the tension they had a minute before, of being asked the answer by Pandey Sir and not knowing it. Those two units that Pandey Sir mentioned were not at all relevant to the chemistry lecture that we were reading that day.  I understood in a moment the mistake I made, and corrected myself, “27.9 Litres”. Though my answer turned out to be correct, the silly mistake of not mentioning the unit ruined the taste of that small triumph. I made a pledge to myself to always mention units with my answers. A basic lesson taught with a small joke, Kudos Pandey Sir.

A measuring tape clearly mentioning the unit of measurment

The Kilogram

Values mentioned without units are meaningless. There are standard units to help us understand and quantify values based on what quantity they represent. One such unit of measurement in Kilogram. Historically, kilogram was defined as weight of 1000 cubic centimeters (or 1 Litre) of water at temperature of melting ice. However, it is not a very easily reproducible definition since it requires precisely measuring another unit(litre) at a specific temperature. So, in 1799, scientists created a prototype to represent a kilogram. It was a pure platinum cylinder, measuring as close to one litre of water at 4 °C. The prototype was called Kilogramme des Archives and served as standard kilogram for next 90 years. However, in 1889, the prototype was changed to use an alloy, which is still the standard for kilogram. Ask any 6^th grader and he will tell you that “Kilogram is the mass of a Platinum Iridium alloy cylinder, kept at International Bureau of Weights and Measures at Sevres, near Paris in France”. And that is correct answer, as of today.

The International Bureau of Weights and Measures maintains a prototype for kilogram, which is called the International Prototype Kilogram, IPK or Le Grand K(The Big K). There are also various copies of IPK maintained at international and national level. In India, one such copy is maintained at National Physical laboratory of India (NPLI) as New Delhi. The copies are verified against IPK roughly every 40 years, last done in 1989

Image representation of IPK.
Image source : By en:User:Greg L - Originally uploaded to English Wikipedia as CGKilogram.jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2547913

The problem with the physical IPK standard

Well by theory, the IPK’s mass can never deviate from 1 kilogram, as IPK itself defines the kilogram. However, that is not the case practically. The mass of IPK can be compared to other copies periodically to find and deviation in its mass. Since the IPK has its own physical existence, no matter how carefully it is stored and maintained, it can theoretically show variations over longer periods of time, due to real world dependencies. If this happens, the definition of kilogram will need to be redefined to consider this offset as well. Also, the other units which are defined in terms of Kilogram like newton, pascal etc, will change accordingly. The problem stated above is not just theoretical, in last 29 years, since the last verification of prototypes were done in 1989, the prototypes and IPK are likely to have diverged from their standard mass.

The New Definition of Kilogram

However, there is a solution to the problem, which is to represent the Kilogram, instead of a physical object (the IPK), in terms of a constant that will never change. On 16^th November 2018, a new definition was proposed and agreed upon to be the standard for kilogram, which represents the kilogram in terms of Planck’s constant. The new definition will be effective from 20^th May 2019. However, Kilogram is not the first SI Unit to be redefined. Meter was also redefined and is not represented in terms of distance between two marks on a standard alloy bar but is defined in terms of speed of light.

While it doesn’t mean that you will now weigh your groceries using a kibble balance to honor the new definition of kilogram, it certainly means that science has taken a next step in eliminating the variations which are too small for us to notice but could cause errors in many scientific calculations.

Nerd Note 1: None of the copies of IPK maintained have exact same mass as IPK (otherwise they will all be the IPK 😉), but the offset is calibrated and documented precisely to keep these copies effective.

Nerd Note 2: Kilogram is unit of mass and not weight. While mass is the amount of matter in a body and is independent of other factors, the weight is the amount of gravitational pull acting on the body and can change as the gravity changes. For example, a 1 Kg mass on earth will still be a 1 Kg mass on moon but will weigh a lot less than that on earth.

Nerd Note 3: Other approaches like Avogadro project, representing Kilogram in terms of C-12 atoms, ion accumulation methods were also pursued and evaluated before deciding on using Planck’s constant in the new definition. Avogadro project was helpful in providing the accurate value for Planck’s constant which is used in the new definition.

And here is the new definition of kilogram :

The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.

And as always,

Thanks for reading.

Ayush!!!