# Today’s Factoid

TODAY’S FACTOID….I am learning new things about the metric system today thanks to Measuring America, which has a chapter about how the metric system came about.

The first part was familiar to me: the metric system’s revolutionary inventors defined the meter as one ten-millionth of the distance from the equator to the pole. What wasn’t familiar was that this was actually a last minute choice.

It turns out that the idea of standardizing weights and measures based on “scientific” principle had already been a hot topic for a while in England, France, and America, and by 1793 the frontrunner for the unit of length was based not on the size of the earth but on the length of a pendulum with a period of one second. Not only did this strke the scientists of the day as an eminently rational choice, since the period of a pendulum depends solely on its length, but it also had a big practical advantage: anyone could create an accurate meter stick anywhere in the world even if they didn’t have access to the vacuum-sealed brass reference rod buried in a vault in Paris.

(OK, not 100% accurate, since the length of the second’s pendulum varies slightly from latitude to latitude. But close enough, especially compared to the crappy fifth generation copies of yardsticks that most people ended up using in those days.)

So why didn’t the French stick with the second’s pendulum, which coincidentally is almost exactly the length of a meter anyway?

Jean-Charles Borda…persuaded the commission to select the meridian because it would enable them to achieve the goal of French science for more than a century, that of establishing the size of the earth. Since the meridian would have to be measured with the superb repeating theodolite that he had invented, the credit for completing the work…would finally go to him.

….It was backed by another still more compelling motive, one familiar to generations of scientists and funding bodies since then: establishing the length of the meridian was a bigger, more expensive research project than establishing the length of the second’s pendulum. On these grounds their choice was shrewd, for the National Assembly did indeed appropriate 300,000 livres for the project, a sum that kept many of the Acad?mie’s scientists in work when the institution was abolished in 1793.

And that was that. We ended up with the meter because it provided more work for some underemployed scientists.

The rest of the system fell into place easily, of course. A liter is a cube one-tenth of a meter across, and a kilogram is the weight of one liter of water.

And a second is just a second. They never did manage to decimalize that, though not for lack of trying….