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How the definition of the kilogram changed over time

Here, in America, we use the customary system while most of the rest of the world uses the metric system. The SI unit (International unit) for mass is the kilogram but how is the kilogram defined?

Measuring is something humans have done since the beginning of time. It has been used to count, to measure the length of things, to weigh things. It has helped human civilizations have an economy. It is the fundamental base of the world economy. GPSs find our position because of measurements. It is the foundation for different fields such as agriculture, engineering, medicine, commerce and general science.

The kilogram has been defined by the exact weight of a platinum-iridium cylinder that resided in Paris. It is called the International Prototype Kilogram, or IPK. It is also called “Le Grand K” or “Big K.” Then, the definition of a kilogram changed to mean almost the mass of one liter of water at about 4 degrees celsius. At the National Institute of Standards and Technology (NIST) laboratory in Gaithersburg, Md., there is a special collection of small shiny metal cylinders that are kept and preserved with utmost care forty feet underground. The masses of these cylinders are supposed to be the standard for the weights and measures in our country.

But, on Nov. 16, 2018, the definition of the kilogram changed. The reason for the kilogram’s definition changing was that the mass of Big K was inconsistent and systematically measured less than what it should have been. This change in definition that depends on constants makes the kilogram more reliable. The seven base units are being defined by constants found in the natural world. The seven base units are meter (distance), second (time), kilogram (mass), mole (amount of a substance), ampere (electrical current), kelvin (temperature), and candela (luminosity). All these bases are interconnected so changing what defines the kilogram changes all the other bases as well.

This new definition is based on three constants: the speed of light, the cycle of radiation for the element cesium, and the Planck constant. The Planck constant describes the behavior of atomic particles and the energy it emits and absorbs.

Planck’s constant is 6.626069934 x 10-34 kgm2/s. It was so precise that it would work “for all times and for all people.”

“For all times and for all people” was the hope that the SI units would bring, that they would be for everyone and be used by everyone to make everyone’s lives easier.
While doing research, experiments involving microscopic and sub-microscopic particles need very precise measurements. For medicine, the composition of different elements has to be very specific in order for it to be effective and have the intended results.

When we did not have scales for the measurements, sellers and buyers used to hold the items in their hands and had to estimate, and it used to be pretty close. The important factor behind this was people-to-people trust for such measurements. When manual scales evolved, stones or metals of same weight were used. Keeping such weights of different sizes and different denominations, sellers were able to weigh for the requested amounts.

We still use pounds and feet as the forms of measurement in the USA. One pound is defined as 0.45359237 kilograms and one foot is 0.3048 meters.

The change in the definition of the kilogram will affect applications that need very meticulous measurements and researchers’ lives but it won’t shake things up in most people’s lives.

Vaageesha Das is a sophomore grader at Morgantown High School. Today’s information comes from: Kaplan, S., & Washington Post. (n.d.). It’s Official: The Definition of a Kilogram Has Changed. Retrieved from; (2020, Feb. 6). For All Times, For All Peoples: How Replacing the Kilogram Empowers Industry. catRetrieved from; Steer, G. (2018, Nov. 16). What Is a Kilogram? The Answer Just Changed, and Here’s Why. Retrieved from cat