The Interconnected Nature of the Metric System

If you think back to your initial instruction regarding the metric system, you probably remember sitting in an elementary classroom, staring at a series of colorful posters on a wall.  Each one denoted a different prefix used as metric units of measure magnified by powers of 10. Your teacher likely had you practice metric conversions from centimeters to meters to show the value of the base-ten structure and its relationship to decimals. You learned how to read the metric side of that standard wooden ruler with the metal edge on one side. You may have poured deciliters of water into a large beaker to prove that there really were ten to a liter. But in all of the metric exposure you had, if your experience was similar to mine, there was one key fact that was never mentioned: The metric system was designed to be interconnected.

The origins of the metric system can be traced back to France during the late 18th century. Units of measure before its creation were derived from lengths of portions of the body and from natural surroundings, making it difficult for people on opposite sides of the world, or continent, to use the same measurements. As the world became more connected, the need to standardize measurements became apparent. In 1790, the French Academy of Sciences set out to create a system of measure that was both scientific and easy to use. To that end, the metric system was designed to link the measure of length, liquid volume, and weight together.

The basic unit of measure for mass, the gram, is defined as the weight of a cubic centimeter of water at its temperature of maximum density (around 39 degrees Fahrenheit). The basic unit of liquid measure, the liter, was defined as the fluid volume of a cubic decimeter. So altogether:

1 cubic decimeter = 1 liter = 1 kilogram

This means that a cubic decimeter (I use the one from a clear plastic geometric solids set), filled with water (room temperature works well as its density only decreases by around 0.2% from the optimal temperature), will amount to a liter when poured into a metric beaker and will weigh one kilogram (plus the initial weight of the beaker). The simplicity of this relationship blows students’ minds. After the discovery, I like to ask them to figure out what dimensions would hold a gram of water. After some calculation, they arrive to the following conclusion:

1 cubic centimeter = 1 milliliter = 1 gram

At this point, I ask students if they have ever heard of the term “cc” used in a movie or TV show in regards to a measure of liquid medicine administered by a nurse/doctor. After several raised hands, I point out that a “cc” is simply the abbreviation for a cubic centimeter. Since this is equivalent to using milliliters, the two abbreviations have become interchangeable. Students usually agree that saying “I need 500 cc’s” is easier than “I need 500 mL’s.”

Standard kilogram

A national prototype kilogram standard mass, number K4, kept by the National Institute of Standards and Technology, USA, one of two which serve as the official standard for defining all units of weight and mass in the United States.

Many students think that the metric system is not a part of current conversation. More than 200 years past its formation, the metric system is still maintained and updated regularly. The National Institute of Standards and Technology, outside Washington, D.C., is home to several official kilograms. Companies and organizations in the U.S. travel to NIST to calibrate weights against it.  NIST scientists, in turn,  travel to France with them periodically to calibrate theirs against the official kilogram that is housed at the International Bureau of Weights and Measures.  Over time, the weights of the official kilogram and its copies have drifted apart and the kilogram appears to weigh slightly less than the copies.  A project is underway at NIST to redefine the kilogram, based on a constant number instead of a physical object.

To learn more about this exciting project, check out this virtual field trip to NIST and hear from the scientists who are working on this new method and see the “kilogram room” where the U.S. Copies are kept under lock and key!

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