The metric system

Two years ago, France marked the 200th anniversary of its revolution. It’s no coincidence that just one year later we marked the 200th anniversary of another kind of revolution: the birth of the metric system.

Whether this is a reason for celebration or mourning depends on how you feel about the System Internationale d’Units, the official name of the metric system. However, like it or not, the metric system is here to stay, and is almost universally used in science for the simple reason that scientists must have a common system of weights and measurements to be able to communicate their findings effectively–and that’s exactly what the metric system was designed to provide.

Like other European countries, before 1790 France had a system of weights and measurements that had evolved over centuries. The Parisian pouce and pied-de-roi were roughly equivalent to the British inch and foot–but they weren’t identical. In fact, they weren’t even identical to the pouce and pied-de-roi in other French provinces, because every province had its own weights and measurements. The Parisian pied-de-roi was 11 percent longer than the one in Strasbourg and 10 percent shorter than the one in Bordeux. This naturally caused problems, for merchants as well as scientists.

The French Revolution provided the impetus for change in this as in so many other things. The metric system was only one of a number of proposed reforms, including the 10-hour clock, the 400-degree circle, and a Republican calendar, abolished by Napoleon in 1805, that began on September 22 and renamed the months: January became Pluviose, April was Floreal, July turned into Thermidor, etc.

In 1790 the French National Assembly passed an act instructing the Academie des Sciences to prepare a unified system of weights and measures. In March, 1791, the Assembly accepted a proposal that a universal, invariable quantity–one quarter of the Earth’s meridian–become a basis for the new system. (In the original act it was suggested that the basic unit of length be that of a one-second pendulum halfway between the pole and the equator. This “pendule” did not survive. Nor, fortunately, did a suggestion that everything be based on multiples of 12 instead of 10.)

The length of the metre, one 10-millionth of one quarter of the Earth’s meridian, was estimated using the best measurements available. A rod of platinum was produced that was supposed to merely represent this ideal, unchanging length. However, in 1799 the rod itself was declared to be the standard metre.

The kilogram, the basic unit of weight, was defined in 1791. Supposedly a kilogram is equal to the mass of one liter of pure water. But water itself is inconvenient to use as a standard, because it is very hard to purify completely and it has to be contained in something whose weight then also has to be taken into account. Instead, the Academie created a small cylinder of platinum iridium alloy, which is immune to corrosion and has a low expansion as temperature rises. Two centuries later, this cylinder, preserved inside several bell jars, is still the standard kilogram. Copies of it have to be made and carefully tested in Paris for distribution worldwide.

However, the metre is no longer the original platinum rod. In the 1870s an effort began to reproduce the standard metre for international distribution, and in 1889 a new, carefully designed rod was adopted as the standard. Eventually it went by the wayside, as well, as scientists came up with a better definition, based on the wavelengths of the light emitted by atoms–something that could be established cheaply and accurately in any laboratory.

Therefore, when the scientific community established the System Internationale in 1960, the metre was defined as 1,554,164.13 wavelengths of a red line in the spectrum of cadmium, as emitted from a lamp built to a standard specificiation.

Since then the standard metre has been redefined yet again, taking advantage of the fact we’re extremely good at measuring time and that the velocity of light in a vacuum is an absolute constant. The metre is now defined as the distance traveled by light in 1/299,792,458 second.

The metre and kilogram are only two of the units defined in the SI system. It also includes the Kelvin as the basic unit of temperature (one Kelvin equals one degree Celsius), the second as the basic unit of time–and a lot of other units you’ve probably never run across, such as the candela (luminous intensity), the henry (electrical inductance), the poiseuille (viscosity) and the tesla (magnetic flux density).

Many of these are defined in terms of other units within the system. For example, the power consumption of an electric machine can be assessed in watts by measuring amperes and volts (all SI units), and the power output of an athlete running on a treadmill can be computed in the same terms based on the measurements of force (newtons) and speed (metres per second).

All SI units use the same prefixes, denoting fractions or multiples of the base. Thus, a kilogram is 1,000 grams and a kilometre is 1,000 metres. A centimetre is one-100th of a metre and a centigram is one-100th of a gram. Prefixes range from tera- (a terametre is 1,000,000,000,000,000 metres, or a trillion kilometres), to atto- (an attometre would be one billionth of a billionth of a metre).

The importance of measurement to modern civilization can’t be overstated. Humans are the probably the only animals that are able to count, and therefore to measure. Having units of measurement in common allows us to communicate information (how to build a house, for instance), contract for trades or exchanges, and control systems ranging from kitchens to countries.

The need for standard weights and measurements, recognized in France 200 years ago, hasn’t changed; and with the general acceptance of the metric system among scientists and the growing importance of science and technology in the world today, odds are that in 200 more years metres, kilograms and Kelvins will still be the units of choice.

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