Ed Huntress wrote:

Ha! Well, I wouldn't go that far. I'd say that they have had mixed success
in "cleaning up" the metric system itself. There are units that are in
common use (kgm, for kilogram [mass]; calorie; liter; micron), for the
same reason many of our inch/Imperial units are in common use: they relate
to experience of the senses, or they apply a single dimension to a
commonly used unit when the SI demands multiple-dimension units.

Ah, you are confusing things!
SI-units are the smallest possible set of units to do all the calculations.
Metric units are based on the SI-units. They do have a few "shortcuts" like
the liter or bar (= kPa). F.e., the liter is clearly derived from SI (1
dm^3). But nothing like kgm, as kg is already mass.
The micron is not a new unit, it is just an lazy abbrev. of micrometer (µm).

Some old non-SI-non-metric units seem to be more persistent. Like calorie or
HP. Calorie only for nutrition, HP almost only for cars.

And coming back to SI and liter. As soon as you do make math with it, the
liter gets unhandy. So you'll convert it as soon as you write down its
value.
Example, that also shows the advantage of the factors-of-ten-thing:
Volume = 4.2 l = 4.2 dm^3 = 4.2 * 10^-3 m^3

And an example (verbose) to do math with the units:
If I want to know the height of water poured onto 1m^2 (with the values from
above)

height = Volume / area = 4.2 * 10^-3 m^3 / 1 m^2
Units: m^3 / m^2 = m^3 * m^-2 = m
Result: 4.2 * 10^-3 m = 4.2 mm

That's elegant! No pocket calculator involved.


Nick
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