A Staff Report from the Straight Dope Science Advisory Board

What's the origin of the meter and the metric system?

March 14, 2001

Dear Straight Dope:

Just wondering if you could tell us a bit about where the "meter" originally came from? I've heard about the French "meter of the archives" and how it was pretty much the same compared to the meter we use today; is this true? Also, what's the whole deal with the metric system anyway?

Both the meter and the metric system have their origins in France. While it is hard to explain "the whole deal" with a lot of French things, this topic is relatively straightforward. The length of the meter hasn't changed much since it was established in the late 18th century, but the precision by which it is measured has improved dramatically.

The idea of a metric system of units is older than most people think. Some authorities credit Gabriel Mouton, a French vicar, for originating of the metric system in 1670. Other note that the concept of a decimal-based system of measurement was advocated in Simon Stevenius's book De Thiende, written back in 1584. But for a long time, the metric system was just an idea.

Then, in 1789, the French had themselves a revolution. Among other things, the French saw this as an opportunity to improve upon the awkward traditional units of measurement. In 1791 the French Academy of Sciences was instructed to create a new system of units. It was decided that this new system should be based on powers of ten and that the fundamental measuring units of this system should be based on natural values that were unchanging.

To this end, FAS decided to figure out the distance of an imaginary line that began at the North Pole, ended at the equator (AKA a quadrant) and ran through Paris. They would then divide this line into exactly ten million identical pieces. The length of one of these pieces would be the base unit for the new system of measurement.

After a six-year survey ending in 1795, this unit was determined to be 39.37008 inches in length. The name "metre" was chosen for this unit based on metron, the Greek word for "measure." Yes, my fellow Americans, the correct, official, internationally-approved spelling is "metre," not "meter." Technically speaking, a "meter" is not a unit of measurement but a device that measures stuff, like a parking meter. But seeing as how "meter" is pretty much the way all of us Americans spell it, that's the way I'm spelling it from here on in.

Interestingly, there were other ideas about how to define the standard unit of length. More than a century before the French Revolution, Dutch astronomer and founding member of the FAS Christiaan Huygens suggested that a good standard of length would be that of a pendulum having a period of one second. The problem with this definition is that the period of a simple pendulum depends not only on its length but also the force of gravity. Earth's gravitational pull is not uniform in different locations. The acceleration of gravity varies slightly with changes in elevation, latitude, and other factors. For this reason the FAS turned down this suggestion.

The meter became the basis for other metric units. A gram was defined as the weight of a cubic centimeter of water at maximum density. The liter was defined as one thousandth of a cubic meter. In 1799, a one-meter platinum bar (as well as a one-kilogram platinum weight standard) was placed in the Archives de la République. The meter was defined as the distance between the two polished ends of the bar at a specific temperature.

A lot has changed since 1799. For example, the gram is no longer considered a unit of weight but a unit of mass. This distinction was made official in 1901. Additionally, both technology and scientific theory have made considerable strides since the French first archived slabs of platinum. As a result, various international conferences have been held over time to further clarify these base units, including the meter.

The problem with the first meter prototype was that it was a bit too small. It seems that when the French computed the quadrant they based the meter on, they didn't compensate enough for the earth's tendency to flatten out due to its rotation. In 1872, the International Commission of the Meter recognized this discrepancy, but declared that it really didn't matter. The 1799 meter remained the standard. By 1899, the International Bureau of Weights and Measures had crafted and distributed new prototypes for the meter based on the 1799 standard. This new prototype was an X-shaped graduated standard made out of a platinum-iridium alloy. The meter was then defined as the distance between the two graduation lines at 0 C.

In 1960 the metric system was officially named "Système International d'Unités" or SI. At this time, it was also decided that there was a more precise way to define a meter than to rely on hunks of metal in France. Light waves could now be measured with great precision. Many elements give off light waves at specific wavelengths when their atoms make what is called a transition. Since these transitions are uniform for a given atom, it was thought that the meter could be based on one of these wavelengths. It was finally decided to use atoms of the isotope krypton-86. Specifically, one meter was defined as being equal to 1,650,763.73 wavelengths of the orange-red line (corresponding to the unperturbed atomic energy level transition between levels 2p10 and 5d5) in the spectrum of the krypton-86 atom in a vacuum. Got that?

Most recently, the meter was defined using yet another unchanging value, one that is known with great precision: the speed of light in a vacuum. As of 1983, the meter is officially defined by the General Conference on Weights and Measures as "the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second." This not only standardizes the length of a meter, but it also sets the speed of light in a vacuum at exactly 299,792,458 m/s.

It's a credit to the FAS that their system of measurement, created more that two centuries ago, is not only still around, but the dominant system in the world. It's even crept its way into the stubborn American frontier. With that kind of longevity, it makes one wish that the French entertainment industry was under the control of the FAS. They probably would have advised against that whole Gérard Depardieu thing.

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