Dear Tom, I have interspersed some remarks.
On 22/03/06 1:31 AM, "Tom Wade" <[EMAIL PROTECTED]> wrote: > Greetings, > >> But if we are going to get people to use the SI in their personal lives >> we will have to 'allow' the centimeter for use. One of the frequent >> gripes against the SI is that (to some people) it does not feel "natural" >> because (they say) it does not related to a 'body measurement'. To get >> people to "think metric..." we have to come up with something they can >> readily see for a comparison. > > I am in complete agreement with Linda. The cm is every bit as SI as the > mm, and which you use depends on which is more appropriate to the particular > application. It also depends on your choice of how long you want to take with your metric transition or metric conversion process. My observation is that metrication using millimetres is always much faster than metric conversion using centimetres. > Where you need the precision, then mm is the one to us, which > would cover engineering and scientific applications. Where you don't need > more precision, then cm are just fine (the whole number argument only applies > if you have to use something like 4.5 cm, but then you need greater precision > than 1 cm, so mm would be better). My point is that it is not as simple as this. Your choice should also include an allowance for the additional time, additional aggravation, and additional expense that are needed for a centimetre based conversion. > Ultimately, it is to be hoped that people will be using metric units for their > height, and that is a very good example of cm precision. I doubt if even in > Australia people measure their heights in mm. It is more likely to be done in > cm (note that a height of 1.74 m will probably be spoken as "one seventy four" > which although in meters is really a cm measure). Thank you for this argument in support of not using centimetres. Australians have tried to use centimetres for height for over thirty years now. Our present practice is a muddle of centimetres, feet and inches (sometimes with fractions), and fortunately a growing trend to use metres. The latter is partly driven by a need for people to know their Body Mass Index and this calculation requires height to be in metres. When I worked in training with our state police force, I recommended the use of metres with intervals of 0.05 metres so that height estimates in metres would have either a 5 or a 0 ending. This was consistent with the previous practice of guessing heights 'on the trot' in intervals of two inches. Height also epitomises one of the problems with centimetres -- they always come with conversions. When people choose millimetres, or in this case metres, they simply take a few days to do enough measurements to develop a millimetres mindset and a range of millimetre 'rules of thumb' and that's the end of the metrication process. The use of centimetres, with their attendant conversions, simply lingers on and on. > Getting someone to remember > their height as 1741 mm rather than 174 cm is simply not going to happen. > Other examples of quantities that people might want to measure in which cm > are the more appropriate are: In both of these cases I would recommend that this person's height be rounded to 1.75 metres. As you say, there is no need for any more precision such as 1.741 metres. > How far from the kerb have I parked ? In Australia, this is listed as 250 millimetres but I suppose that people (if they think of this at all) might think of this length as a quarter of a metre. > What size monitor is this ? Again a classic example of the failure of centimetres to be accepted quickly within the Australian (and the UK and USA) community. We're happy that the separation on the microchips is measured in nanometres, we're happy that the circuit boards are laid out in micrometres or millimetres, and we're happy that the case is built using millimetres. All of these metric transitions happened so quickly and easily that we no longer consider them at all -- they have quickly and quietly disappeared from our collective consciousness. But not the one single dimension where we decided to use the centimetre for screen sizes. This is still the subject of much debate, conjecture and discussions such as this one. > How high should I hang this mirror ? As I'm 1.85 metres and it is about 0.1 metres from my eyes to the top of my head, then the minimum height for the top of the mirror is 1.9 metres. If I choose 1.95 metres and if I was having this fitted by a trade person in Australia, they would mark this at 1950 millimetres. > There are quite a lot of things you will come into contact with in everyday > life, whose size you want to estimate, but not to more than a 1 cm precision. That's true but since centimetres seem to always come with conversions to feet and inches, many people continue for years to make these estimates in old pre-metric measures. People will also make estimates and then record them with unnecessary precision, such as converting their height to something like 164.1 centimetres. > I have never bought into the argument that using cm slows down a metric > transition. Let me be absolutely clear on this issue. I have simply observed that metrication using millimetres proceeds much faster that a metric conversion program using centimetres. This is not an argument -- it is an observation. I was working in building when I observed that the building industry in Australia proceeded rapidly using millimetres and completed the process within a year. I was also working in textiles the observed that the textile industry in Australia is still trying to come to terms with centimetres after 35 years experience with them. I did not directly observe that the building industry in Canada. I have simply observed from afar and I noted that they are still struggling with this some 30 years after they began their metric conversion using centimetres. I also noted the experience at the Kodak photographic company. Their film division used millimetres for their metrication in about 1916. Within a few years we were all familiar with 8 mm, 16, mm and 35 mm films. At the same time the photographic paper division chose to use centimetres and we are all still converting to ten by eights and six by fours in 2006 -- 90 years later! > It is just that the areas that used the mm (because they needed > the precision) went better because the benefits from whole-number usage were > a significant advantage over fractions of an inch. That's only one of the advantages of using millimetres. There are many others. To my mind, probably the most significant is that millimetres have always seemed to produce direct metrication without going along the metric conversion pathway. > The clothing industry > adopted cm because they don't need the level of precision of mm, and thus the > benefit of metrication wasn't a huge advantage over inches. You're probably right that this is the reason why the textile industry chose centimetres. But centimetres were also chosen without knowing beforehand how slow the metric conversion process is when you choose centimetres. I've said this before, but it bears repeating, that centimetres do not provide textile workers with any clear advantages. Textile cutters still have to cut to 1/2 centimetres and even 1/4 centimetre accuracy for some jobs using expensive fabrics. So there is no removal of fractions. Even worse, the cutter now has to think about decimal fractions as well as vulgar fractions and mixed numbers. > I doubt if > metrication would be any faster if they chose to label clothes in mm sizes > (in fact, it would have made metric quantities far more unwieldy, and thus > slowed down metrication "damn, there are only 846 mm waist sizes - where are > the 849 mm ?"). Textile workers aren't so stupid than to realise that these sorts of precision are unnecessary. Tradition again is that the industry has always worked to 2 inch intervals. For example men's suits went from (say) Size 34, to Size 36, 38, 40, and so on. In a millimetre design these would end in either 50 or 100 millimetres. This is not a trifling issue in a trade where costs can be cut by precise cutting to save enough, often expensive, fabric to cut another piece. I have reported here previously that in the shoe trade, cutting of expensive kangaroo leathers to make running shoes and cross-trainers is made to the nearest tenth of a millimetre to help achieve these savings. Measuring to millimetre precision is this environment is not inappropriate. > People's perception of numbers is not linear. For example, 10 000 km and > 100 000 km are an order of magnitude apart. So are 10 cm and 10 mm, but > people are more sensitive to changes with small numbers, so the same prefix > is fine for the first two, but it's handy to have a choice for the latter. > This is why there is greater granularity (every ten) in prefixes around the > base unit than further away (every thousand). Nice thought, but I think that simple observation doesn't bear this out. Deci, deca, and hecto have been almost universally avoided and the centimetre is restricted to only a few occupations (about 10 per cent of occupations in Australia). > There is a danger of overenthusiastic simplification here. Remember this is > a system to be used by ordinary people. Pick the prefix that you are most > comfortable with, but when educating, you must include everything between > mega and micro at the very least. It depends on what you are educating for. If you are educating builders then this is the complete set required to build a house in Australia: 1000 millimetres = 1 metre 1000 metres = 1 kilometre 1000 millilitre = 1 litre 1000 litres = 1 cubic metre 1000 grams = 1 kilogram 1000 kilograms = 1 tonne 1 metre x 1 metre = 1 square metre 1 metre x 1 metre x 1 metre = 1 cubic metre And I have yet to meet anyone working in the building industry who cannot learn these in more that a few minutes. Australia textile workers have these to learn: 12 inches = 1 foot 3 feet = 1 yard 10 millimetres = 1 centimetre 100 centimetres = 1 metre 1000 metres = 1 kilometre 1 metre = 3 feet and (a bit over) 3 inches 1000 millilitre = 1 litre 1000 litres = 1 cubic metre 1000 grams = 1 kilogram 1000 kilograms = 1 tonne 1 metre x 1 metre = 1 square metre 1 metre x 1 metre x 1 metre = 1 cubic metre Etc. > The centimeter and the liter are your friends. The centimetre is the sort of friend that you will need to get to know very well -- if you choose centimetres for your metric conversion program they will be with you for a very long time. On the other hand, if you choose millimetres your metrication program will be smooth, rapid, and inexpensive and when you are successful your measurements will have become so automatic that you will not even think about them at all. > Tom Wade | EMail: tee dot wade at eurokom dot ie Tom, I don't think that I have given justice to the issues you have raised here with my few simple notes. I will consider this further with a view to collating all of the arguments both for and against millimetres and centimetres that people have been kind enough to share with me over many years. Cheers, Pat Naughtin PO Box 305, Belmont, Geelong, Australia Phone 61 3 5241 2008 Pat Naughtin is the editor of the free online monthly newsletter, 'Metrication matters'. You can subscribe by going to http://www.metricationmatters.com/newsletter Pat is the editor of the 'Numbers and measurement' chapter of the Australian Government Publishing Service 'Style manual for writers, editors and printers'. 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