I've been traveling with a friend of mine who has diabetes and uses an insulin pump. I've come to be very familiar with the numbers he should see on his machine, but he tells me this reading would be very different in America and that the pumps are even incompatible with one another. It led me to wonder what this conversation sounded like when America was first industrializing in the late 19th century and early 20th century. Were there political and/or business movements inside America to keep the Imperial system? Were there movements of people who found the imperial system ineffective or even stupid in comparison? Wouldn't business leaders have seen the huge potential in standardizing with the rest of the world, if for no other reason than to sell their widgets globally (or as globally as the market was back then)? And if so, did they try to push the Metric system (even if only for their own financial benefit) to deaf ears?
I've heard arguments today that switching [to metric] would be economically disastrous (though this may be incorrect in itself), but surely there existed a time when this wouldn't have been such a big deal. Finally, have there been any semi-successful efforts to switch over to Metric within America?
So, to begin, the US does not use the Imperial system. The Imperial system was created in 1824 to standardize the units used in the Empire at the time. The US maintains its own system ("US Customary Units"), which, until 1959, were not the same as Imperial units. As the US units were derived from English 'customary' units in use in 1776, they share the same name (which, in some cases, can be traced back to Roman units). The UK was forced to redefine their length and mass units in 1855, after a fire in 1834 destroyed the original standards. Copies were made, and sent to the US, but materials of the time meant there were slight differences.
These small differences in length and weight between the two systems were generally negligible, except in map surveys over many miles. The US actually has two "miles", the standard mile, and the "statute mile", as a result of the 1959 international yard and international pound agreement slightly changing the definition. The US and Imperial gallons, however, remain significantly different (1 US gallon is 0.833 Imperial gallons).
Before the metric system was accepted, there was a complete mish-mash of units, which not only varied from country to country, but city to city. Thus a complete overhaul in continental Europe was welcome. France, where the metric system originated, initially mandated it's use in 1798, but Napoleon rescinded that edict in 1812, and it wasn't until 1840 that it was again forbidden to use earlier systems. By 1880, the metric system had been adopted in most of Europe and South America. Meanwhile, the US and UK, as far back as 1790, were attempting to reconcile the various unit definitions. George Washington mentions the "Uniformity in the currency, weights, and measures of the United States" in his first address to Congress. The initial investigations fell to Thomas Jefferson. While he was aware of the French proposals, he objected to the use of the 45th meridian in the definition of the meter, meaning only France could define the meter.
Instead of concurring . . . in a measure which, like the pendulum, may be found in every point of the 45th degree, and through both hemispheres, and consequently in all countries of the earth lying under that parallel, either northern or southern, they adopt one which can be found but in a single point of the northern parallel, and consequently only in one country, and that country is theirs."
In 1795, the US received copies of the meter and kilogram, and Washington urged Congress to adopt the metric system. However, relations with the French were strained by the US refusal to take sides between the French and British. The problem of standardized unit in the US languished until John Quincy Adams' 1821 Report Upon Weights and Measures, in which he weighed the pros and cons of both the metric and customary units. He praised the divisibility of the base 12 system. He also noted Napoleon's 1812 revival of the old standards, which called into question the future of the metric system.
Meanwhile, many states had enacted their own standards, and in the first half of the 19th century, the federal government was hesitant to override the states' laws. In 1836 and 1838 (before France reinstated the metric system), resolutions were passed by Congress to require the Treasury to adopt a weights and measures standard for customs houses based on the current ones and provide copies to the states. The British 1824 and 1855 efforts to standardize their Imperial system also contributed to the US avoidance of the metric system. Therefore, as the industrial revolution took hold in America, they drove on with the customary units.
This did not mean that US manufacturers ignored the metric system. The Metric Act of 1866 protected the use of the metric system in commerce, and provided an official conversion table to US Customary Units. One of the arguments for the bill was that US manufacturers were already making metric instruments for export. Melvil Dewey, of library decimal system fame, founded the American Metric Bureau to advocate for the metric system, and to sell metric measurement devices. In 1875, the US was one of the first signatories to the Metre Convention. In 1893, US Customary units were defined in terms of metric units,
Until the middle of the 20th century, the UK and Canada, America's largest trading partners, used Imperial units. Metric units weren't legal in the UK until 1897. Throughout the late 19th and early 20th century, debates raged in favor of one or the other system. The scientific community generally favored metric, while manufacturing usually favored US units. Since manufacturers who wanted or need to use metric had already done so, the argument of the other manufactures was one of cost. And sometimes the argument boiled down to the idea that metric units just sounded a bit too foreign. Fredrick Halsey and Samuel Dale summed up the feelings (if not strictly the facts) of metrication in his 1928 book The Metric Fallacy: An Investigation of the Claims Made for the Metric System and Especially of the Claim that Its Adoption is Necessary in the Interest of Export Trade.
Interchangeable parts began to be used in the middle of the 19th century, but for the most part this required the tooling to be the same, and the tooling resided at the manufacturer. Thus a locomotive made in the US would require spare parts from the US manufacture, regardless of the system used. Bolt threads had many different standards in other countries, so unless your export had a single country destination, there was little point in trying to use anything other than your own standard. The ISO standard threads weren't universally adopted until 1947 (bolt thread standards proved a major problem in WW1 and WW2) There was no compelling reason, then, for US manufacturers to switch to metric unless they saw a need. In typical US fashion, industry was allowed to proceed as they saw fit, without government mandates. In short, everyone was OK with the status quo.
Interchangeable parts between the two competing systems are not necessarily a problem. Unit conversions allow a European manufacturer to produce a part that fits a US-made part, and manufacturers do this all the time. Furthermore, US manufacturers are free to make metric parts (and a good chunk of US industry, including automotive, use metric). Customary units persist in metric countries as well. Tire sizes are a strange mix of inches and millimeters. European pipe sizes are in inches. Go over to an Airbus parts bin and grab a 6.35mm Hi-Lok (a type of bolt used in aircraft manufacture). The Airbus part number will reveal it to be an HL8. Now grab an HL8 fastener from a Boeing parts bin. This one has a nominal diameter of 8/32", or 6.35mm...it's the same part!
A History of the Metric System Controversy in the United States, US Department of Commerce, August, 1971.