>Although deka and hecto are SI, they are unnecessary. Stick with units in 1
>000s or 1 000ths to simplify use and understanding, particularly by the
>general public.
If the above refers to general teaching of SI, then I must disagree. The
following are reasons why I think we should not seek to ignore units
between milli and kilo.
1. One of the disadvantages of having a single unit (of length for example)
is that a convenient size for measuring one set of objects is not suitable
for another. The Imperial/Colonial systems get around this by using several
different units (e.g. inch, foot, yard, mile) and using whichever one is
appropriate. The metric system uses prefixes to achieve the same result, but
since the prefixes are not chosen arbitrarily, the multiple value must be
sufficiently small to offer maximum flexibility, while not being so small
that we have a surplus of different prefixes.
Now the scale of convenience is not linear. For example, the difference
between 10 and 100 is more significant than that between 1 000 000 000
and 10 000 000 000 even though a single order of magnitude is involved in
each case. Note that I am talking about human perception here, not just
percentage difference. The difference between 10 and 100 seems more
significant to the average human that 0.0000001 & 0.00000001.
Thus around the base unit a smaller prefix multiple is required than far away
from it. That is why having nothing between kilogram and megagram is not as
big a problem as having nothing between gram and kilogram. Once things get so
big or small, you're talking in scientific notation anyway. So units like deka,
hecto, centi and deci are useful in that they give us the ability to use a size
of measurement that is suitable to what we are measuring.
Unlike the Imperial/Colonial systems, the subunits can be readily converted
by inspection, which is a huge disadvantage of these older systems.
So if you remove some prefixes, you put the metric system at a disadvantage
because of the relative inconvenient size of the resultant units, which is
not in our interests.
2. If any of these rare units are in regular use, then not teaching the full
range will mean someone will eventually stumble over one of the unlearned
ones, e.g. decibels, dekanewtons, hectopascal et, and conclude that the
metric system has exceptions and contradictions of its own.
3. In teaching, it is important to stress the consistent way in which the
metric system is put together. All the different quantities have the same
range of prefixes, and they go up (or down) in 10s as far as the thousand (or
thousandth) and in thousands thereafter. Otherwise you have 'holes', and one
of the beauties of the metric system is lost (i.e. once you learn the prefixes
for meter, you've already learned the prefixes for every other measurement, as
opposed to learning that 12 inches make a foot, 3 feet make a yard etc, which
gives you no inkling of how ounces, pounds & stones are put together).
Keep it simple and consistent.
4. None of the above precludes any particular trade or profession from
establishing a convention that some prefixes are more appropriate than others
*for their particular group*. Thus it is quite reasonable for engineers to
use mm rather than cm, as they
(a) need the precision offered by a subunit that small and
(b) using whole integers is always easier than decimals which in turn
are easier than fractions.
However, this does not mean that cm shouldn't be used outside such a group, or
that cm should be ignored in teaching the metric system. When it comes to
measuring people's height, the cm is much more convenient than the mm, as
height is rarely measured to that exactitude, and you therefore have a
superfluous digit. Remember that people are most comfortable with whole
numbers up to 100, which is why percentages are so easy to visualize.
For the same reason, clothing sizes rarely fit to the exact mm, so cm are
more appropriate here.
[Aside: I note that someone claimed that the Australian clothing industry's
failure to metricate when compared to building was due to the former's use of
cm and the latter's use of mm. This would be a more convincing argument if
they could cite a successful migration of a clothing industry elsewhere using
mm. I believe that cm are quite suitable (no pun intended) for clothing, and
the relative failure is due to other factors such as greater familiarity of the
public with legacy units when dealing with body dimensions (even in AU, some
people still quote height in feet and inches).]
Exactly the same goes for the hectopascal (having the same numerical value
as the older millibar is a definite safety plus for aircraft, and hecto is
just as valid a prefix as kilo).
Similarly whole meters or kilometers are appropriate for other systems (there
is not much point using meters to measure intercity distances for example).
So please, stop treating hecto, deka etc as something to be swept under the
carpet. If they are not a convenient unit for your purpose, fine, don't
use them, but they are just as valid as kilo and milli, and may be more
appropriate for other people.
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Tom Wade, EuroKom | E-Mail: [EMAIL PROTECTED] (all domain mailers).
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