Analog Angle Blog

The quest for ever-better primary standards is a fascinating story

     I just finished “World in the Balance: The Historic Quest for an Absolute System of Measurement” by Robert Crease; it’s a history of measurement systems and metrology, especially metric. What a pleasure to slow down, lie back, and read about those challenges, and how we got to where we are now.

     I suspect most people will not be interested in such historical material, but we should: how do we know that the 5½ digit DVM reading is correct? What does “correct” even mean? What about those femtosecond readings, or even the long-baseline time bases for experiments? We need primary standards, of course, and we need a way to describe them (such as the metric system). Given how casually we now measure with extraordinary accuracy, precision, and repeatability—well, it hasn't been an easy path.

     Even if you are not interested in the historical aspects, you'll find the last part of the book very interesting, as it details on the quest for a reproducible mass standard. As most of you know, the kilogram is the only primary standard which is still represented by a tangible physical artifact rather than a reproducible standard. And apparently, the primary kg is losing weight, for various possible reasons—or maybe the “copies” are gaining? [And before we used wavelengths of light to define the meter, we used the primary meter stick as the standard—can you imagine the basic challenges of comparing your secondary meter rod to the primary one with its “scratchmarks”?]

     For mass, they are looking for a standard which has accuracy and reproducibility in the range of 1 part in 108 . The techniques being investigated, but have so far fallen short, are an ultrapure mass of silicon measured via a relationship to Avogadro’s number; and a watt force-balance electromechanical scheme. Both are very, very good, but not good enough—all sorts of 2nd and 3rd -order sources of error to worry about at these levels of performance.

     The book also discusses the types of meetings and concerns that the international societies have, to decide and plan next steps—it’s a real subculture, of course. And at these levels of precision, some of the questions and issues are the equivalent of “how many angels can dance on the head of a pin?”, so to speak—they are in the philosophical “what does this really mean, anyway?” realm, as well as “just” scientific.

     There's also a great quote in the book from an MIT science dean, c. 1940: “No single tool has contributed more to the progress of modern physics than the diffraction grating”—and the book explains the critical role of the grating in metrology standards development.

     BTW, this book is much, much better in my opinion than “Longitude” by Dava Sobel (about the quest for the highly accurate clock in the 1700s, needed for sea-faring navigation) which received much praise but I did not like at all: Sobel used 1000+ words of description when a single figure or drawing would have been much better—that’s what I call the “New Yorker” school of writing!?

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