October 12, 2012

Image of the Week: Le Grand K of the Metric System

by Christine Hoekenga

The weight known as Le Grand K

“Le Grand K,” a cylindrical weight made of platinum and iridium that serves as the international prototype of the kilogram, is kept in a vault in France. Image: Bureau International des Poids et Mesures

In honor of National Metric Week (which wraps up tomorrow), our image of the week shows “Le Grand K,” a cylindrical weight that is the international standard barer for the kilogram.  Made of platinum and iridium and created in 1889, this special weight literally defines the mass of a kilogram. There are six official copies held at the Bureau International des Poids et Mesures (International Bureau of Weights and Measures) and a number of other copies around the world that serve as the national standard for the kilogram in their home countries.

The kilogram is the only metric measurement that is still defined by a physical object, rather than a universal physical constant. (A meter, for example, used to have a platinum-iridium standard as well but is now defined as the distance that light travels in a vacuum in 1/299,792,458 of a second.)  In recent years, scientist have determined that Le Grand K is losing small amounts of mass, and some have been advocating for a new kilogram standard that is based on a physical constant.

Check out our module on the Metric System to learn more about the various metric units and how scientific notation helps us describe things that are very large or very small

Visit the Bureau International des Poids et Mesures to read more about the International Prototype for the Kilogram

Christine Hoekenga

Written by

Christine is a freelance writer, editor, and content strategist, specializing in science and nature. She holds an Bachelor's degree in Environmental Science and Media Studies and a Master's of Science Writing. She has been working in science communication and education for nearly a decade as a journalist, an organizer for conservation groups, and a museum educator. Before joining the Visionlearning team, she served as the New Media and Online Community Manager for the Webby award-winning Smithsonian Ocean Portal. Christine is assisting Visionlearning with developing new modules and glossary terms, managing the blog, and outreach through social media.