Planet Analog DesignLine Blog
The quest for ever-better primary standards is a fascinating story
Bill Schweber
12/12/2011 10:45 AM EST
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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SteveRivers
12/12/2011 6:18 PM EST
"Sobel used 1000+ words of description when a single figure or drawing would have been much better..."
That may be true for someone with the requisite engineering background. However the book was written, successfully in my opinion, for the opposite target audience - non-engineers. Which is strangely why it received such acclaim because the vast majority could understand it.
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sharps_eng
12/13/2011 3:54 AM EST
True, I have found that you need pictures and text to communicate to a widest audience, some need one, some the other, some both. (This is old news in psychology).
The key thing with Longitude was the romance and the drama, and the sympathetic story details themselves. I enjoyed it but don't remember any technical details from it.
Fact is, in order for a book to take off, it needs to reach the 'trending' status where it will be promoted and wow-d by those in the media who haven't read it!
If you want a gripping book that may change your thinking read 'Guns, Germs, Steel' by Jahred Diamond, a plant geneticist helping to explain highly current geopolitics, or for one that isn't as widely known as it should be try physicist David MacKay's energy e-book at www.withouthotair.com.
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prabhakar_deosthali
12/13/2011 5:29 AM EST
Whatever may be a human effort in measurement and reproducibility, It is just amazing to see as to what accuracy the nature reproduces itself generation after generation, the knowledge passed from generation to generation - in the form of seeds or sperms and eggs or such forms.
We say that the DIGITAL age has arrived just recently. But the Digital age started when we first started measuring something in terms of a number!
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polson
12/14/2011 9:07 AM EST
These reminds me somewhat of "The Measure of All Things" by Ken Alder. The original standard for the meter was supposed to be 1/10000000 of the distance from the north pole to the equator through Paris (of course). History of science (and math) is really fascinating, and makes me feel quite lazy for the methods I use to solve problems.
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WireMan
12/16/2011 11:04 AM EST
I went to the library yesterday to borrow a copy of Crease's book. Even the introduction has been interesting. Can't wait to get deeper into this book. Happy Holidays.
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PH Eng
12/27/2011 2:13 AM EST
I just reserved online a copy from the library, too. Didn't occur to me at first that I could do that until reading WireMan's post. Just like what often happened during debugging, the most obvious things aren't always that obvious to our brain.
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kalpak
12/18/2011 4:12 AM EST
Now that CERN claims to have found the Higgs boson, maybe they will find a way to created a mass standard just like the new meter standard.
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eembedded_janitor
12/20/2011 9:21 PM EST
Metre please.
If we're all going to use the same measurement then let's start by spelling it properly!
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Jonathan Allen
2/27/2012 2:31 PM EST
Dava Sobel's book on the chronometer has lots of problems. For example, she talks about single and double acting escapements and bimetal temperature compensation but shows no apparent understanding of the underlying mechanics or physics. Her book also spends an inordinate amount of time on the soap opera-like squabbles between Harrison and the Longitude Board. A far better work, especially for those with a technical background is "The Marine Chronometer" by Lt. Cmdr. Rupert Gould. He was the Royal Navy officer who analyzed and rebuilt Harrison's instruments in the early 1920s (They are still "alive" and ticking at the Greenwich Observatory.)
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