Units and Standards

In an effort to bring some rationality to systems of measurement, the French National Assembly established a committee in 1790 to propose a new system of measurement, with new units and new standards. That system has come to be known as the metric system and is now the only system of measurement used by all scientists and in every country of the world except the United States and the Myanmar Republic. The units of measurement chosen for the metric system were the gram (abbreviated g) for mass, the liter (l) for volume, the meter (m) for length, and the second (s) for time.

A specific standard was chosen for each of these basic units. The meter was originally defined as one ten-millionth the distance from the north pole to the equator along the prime meridian. As a definition, this standard is perfectly acceptable, but it has one major disadvantage: a person who wants to make a meter stick would have difficulty using that standard to construct a meter stick of his or her own.

As a result, new and more suitable standards were selected over time. One improvement was to construct the platinum-iridium bar standard mentioned above. Manufacturers of measuring devices could ask for copies of the fundamental standard kept in France and then make their own copies from those. As you can imagine, the more copies of copies that had to be made, the less accurate the final measuring device would be.

The most recent standard adopted for the meter solves this problem. In 1983, the international Conference on Weights and Measures defined the meter as the distance that light travels in 1/299,792,458 second. The standard is useful because it depends on the most accurate physical measurement known—the second—and because anyone in the world is able, given the proper equipment, to determine the true length of a meter.