Dear Tim, Bill and All, Thanks for your thoughts on the decimetre. I agree with you almost entirely, but with a few small disagreements and minor clarifications that I have included by interspersing them in red.
On 2007 01 24 12:53 PM, "Bill Hooper" <[EMAIL PROTECTED]> wrote: > On 2007 Jan 21 , at 9:48 AM, [EMAIL PROTECTED] wrote: >> I might be mistaken with my views of proper metric usage yet I fell the need >> to share the embarrassment of asking if there is a place held in the proper >> Si metric system that includes Decimeter. > > Tim, > > Yes, SI does include the decimetre. > > The SI metric system has a basic unit for every type of measurement and a > series of prefixes used to form larger and smaller units. The officially > recognized and sanctioned prefixes DO include "deci-" which may be appended to > any basic unit to form a unit which is one tenth as big as the basic unit. > Thus, for example, it can be used with "metre" to make "decimetre" where: > > one decimetre = one tenth of a metre, or 1 dm = 0.1 m > > Of course these relations can be reversed. So, we can write: > > one metre = ten decimetres, or 1 m = 10 dm > > It is often easier to discuss these things in this second form since it does > not involve fractions. > > However, most of the prefixes in the SI metric system are used to make new > units that are 1000 times as big as the next smaller unit, not just 10 times. > To illustrate some of the more common prefixes, and using "metre" as the base > unit for example, we have: > 1 gigametre = 1000 megametres > 1 megametre = 1000 kilometres > 1 kilometre = 1000 metres > 1 metre = 1000 millimetres > 1 millimetre = 1000 micrometres > 1 micrometre = 1000 nanometres > etc. > > In the earliest forms of the metric system, there were also prefixes for every > step-of-ten. There were only six at that time. Using "metre" as the base > again: > 1 kilometre = 10 hectometres > 1 hectometre = 10 decametres > 1 decametre = 10 metres > 1 metre = 10 decimetres > 1 decimetre = 10 centimetres > 1 centimetre= 10 millimetres Question, Bill. What about the myriametre (mym)? Was myria (symbol my) not one of the original prefixes standing for 10 000 metres? > However, having a new prefix for EVERY factor of ten soon proved to be too > cumbersome. As the metric system was used for larger and larger things > (distances to stars) and smaller and smaller things (masses of protons and > electrons) there would need to be too many prefixes for convenience. At some > point the decision was made that, when adopting additional new prefixes, only > powers of 1000 would be used. > > Since that time, one finds that in most areas, ONLY the powers of 1000 are > used. However, hecto-, deca-, deci- and centi- ARE STILL A PART of SI and > there are a number of areas in which some of them are in quite common use. The hectare is in fairly common use, but I'm stretching to find many others. I recall that the area of some leather skins were described in square decimetres but few other examples come to mind. Some people think in decimetres when they want to calculate the volume of something about the size of a bucket because the numbers are fairly small and the resulting cubic decimetres can then be simply called litres since 1 cubic decimetre is exactly equal to 1 litre. Bucket calculation Width of bucket at top = 250 millimetres (based on the width of my hand = 100 mm) = 2.5 decimetres Width of bucket at bottom = 200 millimetres = 2 decimetres Height of bucket = 250 millimetres = 2.5 decimetres Calculated volume of bucket = 9.9 cubic decimetres = 9.9 litres (say 10 litres). A similar trick can be used if you wanted to find the volume of (say) a tin can. In this case you might choose centimetres for calculating in the full knowledge that your result in cubic centimetres is exactly equivalent to the volume in millilitres. Tin can calculation Width of can at top and bottom = 100 millimetres (based on the width of my hand = 100 mm) = 10 centimetres Height of can = 120 millimetres = 12 centimetres Calculated volume of can = 942.5 cubic centimetres = 942.5 millilitres (say 940 millilitres). > The prefix "centi-" is in very prevalently use as the "centimetre", of course, > although there are efforts in some parts of the world (Australia, for > example). A better example of the attempt to eliminate it (the centimetre) and use millimetres instead is from South Africa where the government made a conscious decision to 'prefer' millimetres in all industries except the textile industry, and their metric conversion was completed within the ten years they had set for the task in all industries except the textile industry. They also encouraged the teaching of millimetres in schools as the preferred small metric unit for measuring. Here is an extract from the July/August 1978 edition of the South African Metrication News. > 1. One of the most important aims of the SI is the simplification and > rationalisation of units, both for measurement and for use in calculations. > The number of multiples and sub-multiples is accordingly restricted by giving > preference to the use of prefixes that represent steps of 1 000 (x 103) > > 2. If this preferred range of prefixes is combined with the division of > numerals into groups of three then this makes for extreme ease of conversion > from, say, mm to m by simply moving the decimal indicator to the next > available space viz: 1 725 352 mm = 1 725,352 m = 1,725 352 km > > 3. If the centimetre is interposed between the millimetre and the metre it has > several disadvantages: > >> (i) It destroys the simplicity of the system, >> >> (ii) It makes it impossible to use the 'groups of three' method to change >> from centimetres, say, as the gap is in the wrong place viz: 76 322 cm = >> 763,22 m But there will be a tendency - using the 'gap' theory - to make the >> answer 76,322 m (a tenfold error). >> >> (iii) It is universal practice in technical drawings to use millimetres only. >> If this practice is employed all that has to be done is to write 'all >> dimensions in millimetres' at the top of the drawing and then leave all >> symbols off each dimension - a tremendous saving of time and ensuring that >> errors in transcription are avoided. If centimetres are permitted as well as >> millimetres then it would be necessary to use symbols again after every >> dimension and the risk of error in transcription is very great indeed. > > 4. It should be noted that the objection to centimetre is confined to its use > as a linear measure. When raised to the second and third powers, as in areas > and volumes respectively, it is necessary to employ square centimetres and > cubic centimetres to render the steps between successive multiples of area and > volume, practical ones. > > 5. In South Africa the centimetre is used in the clothing and textile > industries and therefore also for related dimensions of the human body. It > should, preferably, not be introduced elsewhere. > > 6. If the centimetre were given equal status with the millimetre we would have > the situation where some people would specify the dimensions of, say, a piece > of paper in centimetres and others would specify its dimensions in > millimetres. This would be very confusing and would defeat one of the main > objects of introducing a universal language of measurement. > > 7. There is no doubt that the preference that many people have for the > centimetre is merely another throw-back to the Imperial system - it is the > sub-multiple of the metre that is most closely related to the inch - and such > people naturally tend to use it in all applications where the inch was > previously used. This 'cross-section' of people is, however a diminishing one. > > 8. The SI, if it is to retain its simplicity and its coherency, must employ as > few sub-multiples and multiples as possible. Nobody wishes to interpose > centigram between milligram and gram, for example, nor centinewton between > millinewton and newton, nor centivolt between millivolt and volt, etc., and > basically there is no reason why the measurement of length should require > special treatment when it has been proven to be unnecessary for mass, force, > electrical potential, etc. > > 9. One of the objections raised against the millimetre is that it is said to > be difficult to visualize a dimension such as 250 mm. Yet nobody has any > difficulty in thinking of 250 mL as a quarter of a litre and 250 g as a > quarter of a kilogram. Perhaps the answer lies in the fact that we > automatically relate 250 mL to the litre, the 250 g to the kilogram, yet when > it comes 250 mm, we try to visualize 250 tiny divisions on a ruler instead of > relating it to the metre. > > It is suggested that once one cultivates this habit of relating millimetre > dimensions to the SI unit, the metre, much of the 'antipathy' towards the > millimetre will disappear. Australia is an example of a mixed approach to metrication we have had some large successful metrication programs and some others that are still trying to cope with slow, painful and expensive metric transitions. Metrication was rapid in those areas where the millimetre was used but this was only in about 84 % of trades, crafts and professions. In the other 16 %, the centimetre is the prevalent even dominant unit of length and these are the trades, crafts, and professions experiencing the slow transitions. They are also having a marked effect on finishing the metrication process in Australia. The centimetre groups include these occupations: Apparel cutter, baker, canvas goods maker, chef, cook, dressmaker, general clothing tradesperson, nurseryperson, pastry cook, picture framer, sail maker, and tailor, and as these people proceed through their 100 year, or more, metric transitions they constantly remind the rest of us how quickly we changed. Sadly, two of the major groups who are trying to use centimetres for their metric transitions are medicine (doctors and nurses but not dentists and dental technicians) and teachers, especially primary school teachers. In a new detailed curriculum that was promulgated in the last two years the word millimetre was not mentioned. The 84 % of trades that I spoke of earlier who use millimetres as their dominant measuring unit have to train their new recruits 'on the job' as schools in Australia do not train Australians to use the same measuring units that are used in Australian industry. > In Europe it is not uncommon to find wine and beer served by the centilitre or > the decilitre. > > The land area unit, "hectare", while not itself an SI unit, is defined as 1 > square hectometre and the liquid volume unit, "litre" (also not an SI unit > itself) is defined as 1 cubic decimetre. There are other examples. > > However, generally, there is a tendency to gradually eliminate the use of the > old power-of-ten prefixes in favor of using just power-of-1000 prefixes. Many > people would agree, at least, not to introduce the power-of-ten prefixes into > situations where they are not already traditionally used. Others would like > to see them phased out even in those traditional uses. While some people > vehemently protest any effort to eliminate those power-of-ten prefixes, others > are ardently in favor of doing just that. Time will tell. > > I hope this answers your question and gives you some context for understanding > how the decimetre is (or is not) used. > > > > Bill Hooper > 1810 mm tall > Fernandina Beach, Florida, USA Thanks, again, Bill for your comments. Cheers, Pat Naughtin PO Box 305 Belmont 3216 Geelong, Australia 61 3 5241 2008 Pat Naughtin is manager of http://www.metricationmatters.com an internet website that primarily focuses on the many issues, methods and processes that individuals, groups, companies, and nations use when upgrading to the metric system. You can contact Pat Naughtin at [EMAIL PROTECTED]
