Should you decide to take a jaunt from Philadelphia, PA to Salisbury, MD, you'll wind up traversing the length of the state of Delaware, much of it on a stretch of DE 1 called the Korean War Veterans Memorial Highway. Things go swimmingly enough as you follow Interstate 95 for a few miles, then turn south on DE 1.A few miles further, you'll see this sign. (This is a crop from a Google Street View image.) I recall the first time I drove this way, thinking, "I didn't know Delaware was quite that long." Almost immediately, if you tend to watch for mileposts like I do, you'll notice one that announces "Mile 101". What is going on here?
In 1993, when the route was renamed and made into a freeway, it was measured off in kilometers, according to the then-current federal standard. Km-posts were erected and the exits were numbered accordingly. Starting ten years later, the km-posts were replaced by mileposts, except for a 27 km / 17 mile section near Smyrna, which was only "converted" very recently. Artifacts of partial conversion can be found at this Colorado State U page. Milepost 101 is almost exactly between former km-posts 162 and 163.
This changeable attitude at both federal and state levels toward how to measure our highways has kept the United States as the only major country that clings to standards of measuring that are not based on the metric units used everywhere else in the world except Myanmar (Burma) and Liberia. It exposes a weakness of representative democracy, the weakness of the representatives themselves.
The science, history, sociology and politics of measuring are surveyed in quite entertaining fashion in World in the Balance: The Historic Quest for an Absolute System of Measurement by Robert P. Crease. The need to measure things accurately begins by about age four, when a child realizes that a stack of several blocks is no bigger or smaller than those same blocks all spread out. At the very next dessert time, she is liable to either gloat ("My piece of cake is bigger, tee hee") or complain ("No fair! Johnny's piece of cake is bigger").
It would be easy for such a book to get hopelessly scattered. The author wisely uses just two case studies (Chinese measurement systems prior to 1911, and west-central African gold-measurements prior to the 1800s) to show the gamut of weights and measures and how they were influenced by their social setting.
This is a key theme of the book. Measurement is a social phenomenon, a social action. A hermetic miser may obsessively count and re-count his money, but most of us have no need to count anything until we make a transaction. A wise shopper watches where the butcher's thumbs are when the scale is measuring a cut of meat. Few will go so far as to bring a calibrated weight to check the scale; we trust that an inspector takes care of such niceties. Such trust underlies all commerce. Unfair measuring practices long predate Biblical injunctions not to make the shekel small and the bushel great. In a late chapter the author takes note of a "ruler" that is shorter than the standard, used by crooked lawyers to "measure" a damaged area about which they are suing.
The main historical narrative concerns the gradual conversion of most nations to a system that arose amidst the revolutionary fervor of the French Revolution, culminating in two precious artifacts still kept in a vault in Paris, the 1799 Metre and Kilogram standards, made of platinum. Though these were superseded two generations later by the platinum-iridium standards kept just outside Paris at BIPM (Bureau International des Poids et Mesures or International Bureau of Weights and Measures), they remain unique objects. Yet they and their successors are obsolete.
Starting in the mid-20th Century, the meter was redefined several times, and is now tied only to the speed of light. Now that light velocity in a vacuum is understood to be absolutely constant, unaffected by motion, gravitational potential or any other "environment", it serves as a standard for distance and time measurement that does not depend on the size of Earth, or even on its existence!
Although the Kilogram standard artifact is not yet wholly replaced with an absolute measure, this is expected in 2015, at the next meeting of the appropriate standards setting body, the CGPM (Conférence Générale des Poids et Mesures or General Conference on Weights and Measures). The new Kilogram will be defined based on either Planck's Constant or Avogadro's Number, or perhaps both. Conference members are optimistic that the final details can be worked out by then. Meanwhile, standards-checking still relies on a precious metal cylinder weighing just over 32 troy ounces, and thus worth about $45,000 on the bullion market.
I recall learning the metric system in 1962, when I first took high school Chemistry. I became familiar with the cgs system, for centimeter-gram-second. My first year of college, I was brought up to date with the 1960 world standard, the MKS system, for meter-kilogram-second. Then in graduate school, the units didn't change, except in fiddling detail, but the name did, to SI (Système International or International System). As it happens, it is the most international system we have going! Assuming an absolute Kilogram is defined in 2015, a process that has been going on for 230 years will be nearly complete. It just remains to get one major country to convert to SI … mine!
I am ready. I know my height is 1.83 m and my weight is 98 kg. The second at least is the same the world over, so my age of 64 years and just over a month needs no conversion (and at 10:00 pm tonight, PST, it will be almost exactly 1.802 billion seconds).
In his epilogue, the author returns to the sociological implications of measurement. He had earlier introduced the new words "metrosophy" and "metroscape" to express the philosophy and environment that surround our ever-more-measured life. Will our measurements continue to define and redefine who we think we are? To what extent will it affect us that the standards on which our measurements are made no longer depend on the length of our arm or hand or foot, or the beat of our heart or the speed of a falling apple? As long as the meat I pay for does not include any of the butcher's thumb, I don't need to care. But if one day I decide to vacation on the Moon, it will be very important to know very, very accurately where my landing will be. I'd hate to make an error of 5 parts per million and run out of fuel a couple of kilometers up!
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