Dennis Brownridge wrote in USMA 8874: >Gustaf has raised an important question. When the English-speaking countries >decided to metricate, why did they generally suppress the prefixes hecto, >deka, deci, and centi? Can anyone give us a documented historical answer? In >the United States, for example, multiples like hectogram, deciliter, and >centiliter are not legal for retail trade. The centimeter is legal, but most >products are labeled in millimeters instead. The prefix deci is legal only >in square decimeter (rarely used). Let me summarize this whole prefixes >issue, which seems to elicit such strong emotions: > >FIRST ARGUMENT > >These prefixes (not powers of 1000) are clearly simpler for everyday >lengths, masses, areas, and volumes--especially when people are not doing >technical calculations. In ordinary life, people very rarely measure things >to the millimeter, and they NEVER measure to the gram or milliliter. Nothing >is packaged to gram or milliliter precision, and household measuring devices >aren't that accurate. Even a laboratory graduated cylinder can't measure a >liter to 1 mL precision. Mandating only g, mL, and mm means that folks must >deal with useless zeroes and ridiculously false-precise numbers. Unnecessary >digits and false precision are the bane of our existence--by far the biggest >complaint of ordinary folks who otherwise support metrication. In the 1999 >survey by the UK National Federation of Consumer Groups, 67% supported >metrication, but 90% OPPOSED prefixes that weren't "user friendly." A >typical respondent's comment was, "I was brought up on the Continent and >looked forward to the conversion, but that stupid insistence on millimetres >put me right off!" > >OPPOSING ARGUMENT > >These prefixes are "irregular" and violate the normal pattern. They are >little used in science and technology, or in everyday life for quantities >other than length, mass, volume, and area. In science and technology, they >are NOT convenient and make calculations more complicated. Science and >technology deal with a wide range of values requiring many prefixes, and >the "irregular" prefixes add needless complexity without any advantage. >Values are precise, not rounded nominal sizes, so "useless zeroes" are not a >concern. Even for rounded product sizes, there is usually no advantage. For >example, 60 W or "sixty watts" [3 syllables] is easier than 6 daW or "six >dekawatts" [4 syllables]. Most importantly, a calculator (set to ENG >display) automatically gives answers in powers of 1000, but it cannot be set >to automatically give answers in powers of 10^-1, 10^2, 10^1, or 10^2. So >recording an answer with an "irregular" prefix requires bothersome manual >manipulation of the decimal point and the possibility of a mistake. This is >especially true for volume units like centiliter and deciliter that are not >coherent. When you calculate the volume of an object from its dimensions, >you get an answer in cubic meters or one of its multiples. It is hard enough >to remember that the prefix must be cubed along with the unit and that >several multiples have old-metric "nicknames" (liter, milliliter, and >microliter). But at least you don't have to move the decimal point. What you >see is what you get: > >E-3 m3 = dm3 = L >E-6 m3 = cm3 = mL >E-9 m3 = mm3 = �L > >But deciliter, centiliter, and hectoliter aren't ANYTHING cubed. You have to >move the decimal point. > >dL = 100 cm3 = 100 mL >cL = 10 cm3 = 10 mL >hL = 0.1 m3 = 100 L > > >PRACTICES THAT VIOLATE BOTH ARGUMENTS > >Curiously, Europeans don't seem to use the convenient prefixes for large >quantities. My otherwise very bright and capable European students >(including many from Sweden and Germany) will write long rows of useless >zeroes instead of a simple hm3, Mg, or Tg. (To our European friends: is this >typical of everyone?) > >No one seems to use dekagram, even though it would be convenient for the >range of small masses encountered in everyday life. No doubt that's because >it's harder to both write and say than grams: 2 dag or "two dekagrams" [4 >syllables] is more work than 20 g or "twenty grams" [3 syllables]. > >The hectometer (hm) would be convenient for flight altitudes, if aviation >ever metricates. > >The irregular prefixes are used in science and technology as a subterfuge to >avoid giving up obsolete, non-SI, non-coherent metric units: > >(1) dekanewton (daN) is sometimes used for forces because it is close to the >obsolete kilogramforce (kgf) or kilopond (kp), even though most values could >be more easily expressed in kilonewtons (kN). This practice seems to be >declining. > >(2) dekapascal (daPa) is used by audiologists because it is close to the >obsolete mmH2O they formerly used, even though the values involved could be >more simply expressed in kilopascals (kPa). This practice has recently >become firmly entrenched. > >(3) hectopascal (hPa) is used for atmospheric pressure because it is the >same as the obsolete millibar (mb), even though the values could be >expressed as easily, or more easily, in kilopascals (kPa). This practice >seems to be growing. > > >I don't know the definitive answer to Gustaf's original question, but two >explanations seems likely: > >(A) Those in charge of metrication were scientists and engineers with little >or no practical experience using SI in everyday life. > >(B) A public-relations rationale: "English-speakers are unfamiliar with >prefixes, so we don't want to confuse them with too many. If we don't >introduce these four, they will eventually wither away and die." This >rationale seems to have backfired and turned the public against SI. And in >reality, the irregular prefixes (along with kilo and milli) have long been >taught to every American student whereas the other prefixes (e.g., mega, >giga, micro, and nano) are not routinely taught, even though they are far >more common than deci, deka, and hecto. Students are taught centimeters from >an early age and are much more comfortable with them than millimeters. >Getting rid of the irregular prefixes in areas and volumes would require >very long, impractical rows of non-significant zeroes (e.g., hm3 = 1 000 000 >m3 or 0.001 km3, hm2 = ha = 10 000 m2 or 0.01 km2). When Britain started to go metric in 1965 the Metrication Board on the advice of the British Standards Association took the corect engineering position that centi, milli, deka, and hecto should not be used. That led the Metrication Board to publish a picture of a fashion model with her dimensions in millimetres, which led to much laughter and derision. Since then Anne, Countess Attlee (widow of the son of Clement Attlee) has been waging what she calls "The Metric Sense Campaign". Before marriage she had spent several years in France as a journalist where she learned how the metric sustem is used in all situations in the country that invented the system.
