Yoni Appelbaum: Let's start at the beginning. What's the utility of having a single, standardized system of measurement?

Stephen Mihm: It permits nations, individuals, or corporations who would otherwise be hampered in their efforts at communicating, trading, or sharing information. Think of it as a common language. If everyone in the world speaks English, it's very easy to do business, travel, and engage in trade. The same is true of a single, standardized system of weights and measures.

Appelbaum: Much of the world started moving to the metric system in the late nineteenth century. You've written that the United States didn't follow, because of “the humble screw thread.” For want of a screw, the metric system was lost?

Mihm: Many factors played a role in frustrating the adoption of the metric system in the United States. But much of the opposition from the 1870s onward came from the manufacturers of high-end machine tools. They had based their entire system—which encompassed everything from lathe machines to devices for cutting screw threads—on the inch. Retooling, they argued, was prohibitively expensive. They successfully blocked the adoption of the metric system in Congress on a number of occasions in the late 19th and 20th century.

Appelbaum: So the people blocking adoption of the metric system weren't backward-looking traditionalists, but cutting-edge industrialists?

Mihm: That's correct. While the anti-metric forces included outright cranks, including people who believed that the inch was a God-given unit of measurement, the most sophisticated and powerful opponents of the metric system were anything but cranks. They were engineers who built the industrial infrastructure of the United States. And their concerns, while self-interested, were not entirely off base. Whatever the drawbacks of the English units, the inch was divided in ways that made sense to the mechanics and machinists of the era: it was built around "2s" rather than "10s," with each inch subdivided in half and in half again—and so forth. This permitted various sizes of screw thread to have some logical correspondence to all the other increments. The same was true of the sizes of other small parts that were essential modern machinery.

Appelbaum: We've arrived at a hybrid system. Most American rulers show inches along one edge, centimeters along the other. Is it possible that the metric system will slowly displace English measurements, not by government fiat, but one inch at a time?

Mihm: Yes, that's right. If history is any guide, government fiats don't work when it comes to weights and measures. The undertow of history and custom is too strong (proponents of the metric system, for example, are often unaware that it took many decades for France to get its citizens to adopt it—there were many, many setbacks and a staggering amount of resistance).