The Metric System

By Earl Hunsinger

Over the centuries, dozens of units of measure have been created to quantify the size, mass, or quantity of a substance. For instance, to measure distance you might have used cubits, spans, rods, furlongs, leagues, or miles. Your unit of choice might have had as much to do with your occupation as where and when you lived. For instance, a sailor might have spoken of traveling so many nautical miles at a speed of so many knots, over water that was so many fathoms deep. Even using the same name for a unit was no guarantee of clarity, since many countries and professions used their own standards for units with the same name.

To simplify matters, in 1790 the French National Assembly commissioned the Academy of Science to devise a system that could be used as an international set of standards for weights and measures. The decimal-based system that they designed became the metric system. If you have mixed feelings about the metric system, or have seen a very slow acceptance of it where you live, think about the fact that it has taken it over 200 years to even get as far as it has. It wasn’t until October of 1960 that the metric system (called Le Systeme international de’Unites or SI for short) was officially adopted as an international standard. While it has been officially recognized and adopted by nearly every country in the world, the degree of its implementation has varied greatly. While most scientific fields were quick to start using it, many long before its official adoption, its acceptance by the general public has been less enthusiastic.

The metric system contains seven basic units of measure, in seven different categories. These cover length, mass, time, electric current, temperature, the amount of a substance, and luminous intensity. Most of the primary units are based on relatively (in the Einsteinian sense) constant physical quantities. For instance, the basic unit for time, the second, is defined as the length of time taken for 9192631770 periods of vibration of the caesium-133 atom to occur. In a similar way, the unit of distance, the meter, is defined as the distance light travels, in a vacuum, in 1/299792458th of a second. The other basic units are the kilogram, for mass; the ampere, for electric current; the kelvin, for temperature; the mole, for substance; and the candela, for luminous intensity.

From these primary units, a multitude of other units can be derived. For instance, the newton is the SI or metric unit for measuring force. It is defined as the force required to give a mass of 1 kilogram an acceleration of 1 meter per second per second. The joule is the unit used to measure work or energy. It is defined as the amount of work done when an applied force of 1 newton moves through a distance of 1 meter in the direction of the force.

One of the benefits of the metric system is that it is decimal. This means that when measuring and recording larger or smaller quantities, new units can be created through the addition of prefixes. This alleviates some of the need for large numbers and decimals. For example, a kilometer is 1,000 meters, a millimeter is 1/1,000 of a meter, and a nanometer is one billionth of a meter.

For some of us, it’s hard to give up our inches, feet, and miles, just as it was probably hard for some to give up furlongs and leagues. Some may be very comfortable with metric quantities in their work, yet have a hard time visualizing so many kilometers per hour or grams of flour. But then, it’s only been a couple of hundred years. Who knows, given time the metric system may yet catch on.