When former Rhode Island senator and governor Lincoln Chafee formally jumped into the presidential race on Wednesday, he made a splash. There are good reasons to take Chafee’s bid seriously, but his speech drew the most attention for a less-conventional proposal. “Let’s join the rest of the world and go metric,” he said.

His call was not universally embraced. Labeling it “the worst idea” of the campaign, the National Review’s Jim Geraghty blustered, with perhaps a touch of humor: “You will get my American system ruler when you pry it from my cold dead hand.” Advocates of the metric system are accustomed to such scorn. Back in 1972, Rhode Island senator Claiborne Pell was attacked by his Republican opponent for wasting time on low-priority items like the metric system. The politician leveling that attack? John Chafee.

But Lincoln Chafee, John’s son, is undeterred. “People say it’s expensive, but the economic benefits outweigh the cost,” he told CNN. Many experts agree. Which raises an interesting question: Why, exactly, doesn’t the United States already use the metric system?

To find out, I turned to Stephen Mihm, an associate professor of history at the University of Georgia, and author of the forthcoming book, Mastering Modernity: Weights, Measures, and the Standardization of American Life.

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).

Appelbaum: Chafee's call for the United States to adopt the metric system generated an immediate backlash. Why does a seemingly dry subject like metrology ignite such intense passions?

Mihm: National pride is at stake. The adoption of another country's weights and measures—or in the case of the metric system, the rest of the world's weights and measures—seems an infringement on national sovereignty. That the system in question has a long and distinguished history as a pet project of Francophile, cosmopolitan liberals probably doesn't help make it appealing to American conservatives.