Redefining the kg

In mid-November, the General Conference on Weights and Measures met in Versailles, France to make a heavy decision in the world of measurements. The origin of the measurement originally came from France in the 1700s in an effort by King Louis XVI to make trade measurements more fair. The Sun King created a new unit known as the grave, weighing in at exactly the mass of a frozen liter of water. Following the French Revolution, the standard was changed to the gram, which matched the mass of one cubic centimeter of water at 4 degrees Celsius; however, it was changed again when the impracticality of carrying around a pea-sized gram weight was realized. At the turn of the 19th century, France created their own kilogram weight to be kept as an artifact and used as the new standard. As world trade became more frequent in the new century, the need arose for a worldly amount of mass. Soon, 17 countries adopted the kilogram and created their own matching artifacts, one of which resides six hours away from Deerfield in Gaithersburg, Maryland. On May 20, 2019, this standard that has been used for so long will finally be changed after 129 years.

• 1795 The gram was defined as “the absolute weight of a volume of pure water equal to the cube of the hundredth part of the metre, and at the temperature of melting ice”
• 1875 The Metre Convention was signed and further formalized the metric system (a predecessor to the SI)

The International Prototype of the Kilogram has many names, including “Big K” and “Le Grande K”, but they all describe the same cylinder of platinum-iridium alloy that resides just outside of Paris. Each of the copies distributed around the globe must be carefully handled. For example, scientists in charge of them never touch the kilograms without gloves because even the slightest amount of oil can add mass and change the standard.

Since the kilogram’s standardization in 1889, scientists have noticed that the mass of Big K has decreased: the International Prototype has deviated by 50 micrograms since its creation. That’s only equivalent to about one grain of salt, but it demonstrates the fallibility of physical standards such as this one. Along with it is the upsetting notion that the meaning of kilogram has changed along with Big K; a kilogram now is not exactly what a kilogram once was.

The conference decided that these flaws were unacceptable and unanimously voted to change the basis of the kilogram to Planck’s constant, the speed of light in a vacuum. With just simple conversions and a tool called a Kibble balance, the mass of an object in kilograms can be consistently derived. There will be deterioration of this stationary number.

Kibble balance works by comparing mechanical energy to electrical energy, the latter of which depends on an object’s mass. It then compares these energies using a complex equation, the root of which is Planck’s constant. With this tool, Big K will become obsolete, allowing the standard for the world’s kilogram to remain constant.

Despite the formal meetings and meticulous science surrounding this conversion, the death of Big K will not affect the average person directly. While we won’t have to throw away our scales, changing the standard kilogram will have an impact on processes like manufacturing car parts and creating pharmaceuticals. There is also the added bonus of not needing to calibrate each and every copy to the International Prototype of the Kilogram, which will save them time. Everyone will have access to the constant need to calculate the standard--the exact measurement will no longer be available to just a select few, everyone will be able to have that scientific precision. Access to standardization has suddenly become more democratic.

• 1889 The magnitude of the kilogram has been defined as the mass of an object called the International Prototype of the Kilogram
• 2010 The CIPM voted to submit a resolution to “take note of an intention” that the kilogram be defined in terms of the Planck constant.