Nested Units

Newton-Not-In-A0By The Metric Maven

Bulldog Edition

My friend Kat once told me this joke:

Einstein, Newton and Pascal decided to play hide and seek. Einstein put his head against a nearby tree and began counting. Newton only traverses a couple of paces, then reaches into his coat and produces a piece of chalk. He draws a perfect one meter square on the pavement, and then steps into it.

Einstein finishes counting, looks up, and immediately sees Newton standing near him. Einstein says with surprise, “Newton, you really suck at hide and seek, I immediately found you.”

Newton replies “No, no, no you haven’t, you found one Newton per square meter!—-you found Pascal!”

Longtime readers may recall that I’m very much against the adoption of unit identifiers which are the names of persons. If memory serves, Isaac Asimov argued that the names of units should provide a clue as to what they might be. I’m very much of the same mind. As you might imagine I have a first order aversion to the “nesting” of units named after famous scientific persons. When I was taking some long forgotten class in engineering mechanics, I recall a number of problems which defined pressure in pascals. I didn’t question the pascal, but it always seemed a bit remote as far as gaining an intuitive understanding of the amount of pressure present.

I had not really thought much about those ancient exercises in engineering until recently. I was visiting my father in my small hometown, and he was working with another person installing a new Japanese printing press. A technician was installing air for the pneumatics, but was familiar with using pounds per square inch (PSI). He asked me “what is the conversion between pascals and PSI?” to which I could only reply that I could not recall it off the top of my head. The conversion is 1 PSI = 6894.757 pascal. The PSI is so removed in magnitude from a pascal, that one would need to deal in Kilopascals to obtain 1 PSI = 6.895 Kilopascals. But at that moment I was at a loss and could only blurt out what I thought was a useless statement: “well, a pascal is a newton per square meter.” The countenance of the technician brightened. It was clear that my statement actually helped him to understand that the metric system was not somehow creating a mysterious and esoteric alternative to force over area, but that a pascal could actually be related to a pound per square inch in terms of a newton per square meter.

What struck me was that SI, in its quixotic rush to further fete scientists who will never be forgotten as long as the scientific endeavor and humanity continues, have obscured meaning. When I was a boy and first heard pounds per square inch, I understood the concept immediately. The Ye Olde English unit expressed itself within its name. If I had a small one inch square of wood, and I stood on it and weighted 100 pounds, it would be 100 pounds per square inch. If the cross-section of the wood became smaller and smaller the pressure in pounds per square inch would increase. When the area is reduced to a small point it can puncture objects with little applied force. The spear, and arrow rely on an understanding of this principle, and they are some of the first technological tools used by humans. Understanding force over an area, allows one to comprehend why women in high heels attempt to avoid walking on grass, and when they do, they ramble across it on their toes. The pounds per square inch of their heels will easily puncture the sod and form a vacuum that might capture their shoes in place. When neighborhood boys taught me how to patch the inner tubes of my bicycle tires, there was no confusion when they told me how many pounds per square inch were needed for proper inflation. The concept was very intuitive.

The use of the word newton to describe a kilogram-meter per second squared makes as much sense as the pound, and has a name which cannot claim to be a superior nomenclature. The cgs unit of force, the dyne, at least used a word which was not that of a person, and also attempted to use a word which is similar to dynamic. It attempted to describe in words what the unit describes mathematically. In my view SI then doubles down on anthropomorphism at the expense of explanation by calling a newton per square meter a pascal. If a newton per square meter was abbreviated as NSM for newton per square meter, and dual scale gauges found in the US had PSI and KNSM a person who was transitioning to metric could understand that metric was at least on the same planet as the Ye Olde English units. A pascal is an abstract notion by comparison and only serves to conceal information, and not express it.

When I did EMI testing in a GTEM years ago, the amount of noise generated by electrical equipment (often horrible tones) were measured with a device which required the computation of dBspl (decibels Sound Pressure Level) and I recall immediately converting to newtons per square meter for the math used to process the data. The pascal was never really expressive in a way that attracted its direct use. In my view for SI to become more intuitive and useful, questions like this need to be examined, and possible simplifications should be considered, and if they make sense, instituted. If Einstein could not see an obvious relationship between a Newton per square meter and Pascal—why should we?

The International Unit of Mystery

By The Metric Maven

I have had the pleasure of visiting a number of technology and manufacturing firms over the years. But whenever I question them about if and how they use the metric system, I often feel I’ve been thrust into an Austin Powers movie. Because Austin was frozen in the 1960s and thawed in the 1990s he had not kept up with the times. His use of groovy is clearly anachronistic.  Dr. Evil’s plan to hold the world hostage for ONE MILLION DOLLARS shows that he has not been out to purchase a Bob’s Big Boy Burger in a while. His cryogenic nap has left him  culturally temporally challenged. Here is a version of the Austin Powers conversation I have about metric:

MM: “So, do you use metric here at your company”

Proud Company Representative: “Yes we do, we use microns and millimicrons.”

My mind halts and is transported back to the time many moons ago, when I was employed at my first engineering position. This position is also where I produced my first drawings in metric, and was told in no uncertain terms that they would not be accepted. Not long after I had been told that metric was not acceptable in aerospace, I had a dimension described in microns. “Micron?, what the hell is a micron?” I thought. I’d never heard of it when I was in college or doing amateur projects. I opened up my copy of Reference Data for Radio Engineers, which was published in 1982. There was no entry for micron in the index. Another engineer who also had a copy said, “look on page 1-1″ which is the very first page of the reference. When I did, the book did not exactly define the micron, but it did have this footnote: (Note that the term “micrometer” has superseded the term “micron.”). Micron was only mentioned as a term which should not be used in engineering work, which I was now told to use in my non-metric aerospace engineering environment. Now that’s groovy baby! Just as groovy, was the use of the term mil or 1/1000th of an inch. This was the aerospace default measurement “unit,” but the way micron and mil were used, one might never realize that one was Ye Old English, and the other a poor usage of the metric system. Microns were welcome, but no micrometers please, this is American aerospace, only proven “heritage” is welcome.

In fact I seldom heard micron used, so mil became the preferred default feral non-unit of choice. Then when discussing some coatings with a researcher one day, he said “yeah, these are really thin” and then quoted a value in millimicrons. Millimicrons? Ok, so we’ve proudly added a metric prefix to a non descriptive archaic metric moniker of French origin which has lost its prefix. In other words it’s a millimicrometer,  which is of course a nanometer for those of you out there who are numerate.

Wikipedia confirms my experience with a micron in industry:

The term micron representing the micrometre, was officially accepted between 1879 and 1967, but officially revoked by the International System of Units (SI) in 1967.[2]

Nevertheless, in practice, “micron” remains a widely used term in preference to “micrometre” in many English-speaking countries, both in academic science (including geology, biology, physics, and astronomy) and in applied science and industry (including machining, the semiconductor industry, and plastics manufacturing).[citation needed] Additionally, in American English the use of “micron” helps differentiate the unit from the micrometer, a measuring device, because the unit’s name in mainstream American spelling is a homograph of the device’s name.

In engineering it is common to use the micron as a replacement for the traditional thou or mil, each of which represent a thousandth of an inch. So a bin bag may be originally specified as 0.35 mil thick, but stated as 8.89 microns.

Part of this Wikipedia entry is laughable: “the use of “micron” helps differentiate the unit from the micrometer, a measuring device.” I used a micrometer when I worked as a printer, and experienced a bit of machining. Every US technical person I’ve met calls the measurement device a My-crom-et-er (accent on the first syllable). The measurement unit is pronounced mike-Crow-meter with an emphasis on the second syllable. The word micron is pronounced mike-ron, and the instrument for measurement, the micrometer, is proverbially called a “mike.” When I worked as an offset pressman, all other printers called a micrometer (the instrument) a mike. We generally shared one mike, and I never recall a single pressman requesting that he borrow the “micrometer.” This explanation appears to be yet another strange after-the-fact rationalization for strange versions of US metric non-usage. The micrometer-micron usage explanation also appears similar to folk etymology. There is no excuse other than the irrational urge for US engineers to continue Living in The Past.

Wikipedia also “weighs in” (I could not help myself) on the millimicron:

The nanometre was formerly known as the millimicrometre – or, more commonly, the millimicron for short – since it is 1/1000 of a micron (micrometre), and was often denoted by the symbol or (more rarely) µµ.[1][2][3] In 1960, the U.S. National Bureau of Standards adopted the prefix “nano-” for “a billionth”.[4] The nanometre is often associated with the field of nanotechnology. Since the late 1980s, it has also been used to describe generations of the manufacturing technology in the semiconductor industry.

I’m not knowledgeable enough of the current workings of the semiconductor industry in the US to know if they use nanometers or not. I do know one of the largest  semiconductor corporations uses it as a company name. The irony of using micron is that it appears it was coined by the French in 1880 for one millionth of a meter, and became accepted in 1892. Apparently the tonality of a French term for a micrometer is so alluring that Americans just can’t give up sonorous franophonic sound of the word micron.

One would generally never get a clue that the micron is French, or a metric measurement when looking through a US technology catalog. Here is an advertisement for a micromachining product:

One of the most curious of articles I have come across, is in a trade magazine. The article has five authors, and is about a new glass material which allows one to create through glass vias (TGV). Vias are just methods of connecting electric circuits together. The entire article is nothing but a discussion of micron dimensions. The  caption for figure 2 is all microns:

The next figure has descriptions using microns, but also reminds the reader that it is a millimeter wave circuit.

The final figure in the paper is even more “descriptive,” it has millimeters, microns, and micrometers:

The mad desire for microns is not confined to engineers. Caleb Scharf in his 2012 book Gravity’s Engines on page 52 describes interstellar dust:

This is not the same kind of dust you find under your bed. This is far finer and very different in composition. A typical grain of interstellar dust is only about 0.001 millimeters (one micron) across….

It’s not Austin Powers fault that he had not kept up with the times, he never experienced them. We in the US have no excuse when it comes to metric usage, other than we refuse to experience change, and try to preserve the perfected world of 1789 forever.


I thought the use of the micron was exclusively a US problem, but apparently it exists in Europe and the UK. A blog from the UKMA describes the problem: The Reports of the Death of The Micron are Greatly Exaggerated.