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Comment by chriswarbo | Hacker News Reader

The best thing about SI is that it only has one unit per dimension, and no conversion factors between units (it is "coherent"). Unfortunately, this simplicity and elegance usually goes unappreciated; while people focus on arbitrary powers of ten (e.g. that 100 hundredths is one)

http://www.chriswarbo.net/projects/units/metric_red_herring....

PS: The SI unit of time is the "second"; which is actually a sexagesimal fraction (a "pars minuta secunda", AKA a second small part; which is a sixtieth of a "pars minuta", a sixtieth of an hour)

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Just appending 0s to change scale is true in any base. If you'd grown up in base 12 you'd write that 100 one-hundred-forty-fourths is one and that there's nothing particularly simple about A As (84 in your natural numerals), where A=9+1.

That is somewhat idealistic view on SI, and even more so of the metric system. While SI is nominally coherent, it is in practice accomplished by having very awkward values for defining constants, and that factor is excluded from the coherence consideration. E.g. to convert from seconds to meters you need to use conversion factor c, which is very much not 1.

I would also be very careful and not conflate metric and SI systems. There are all sorts whacky metric systems (although most of them historical) that are not nearly as clean as SI. And even SI historically has been messier than its today.

c is used in the definition of the meter, but you don’t remotely need to know that in day-to-day life. As long as the seven base units (length, time, mass, temperature, electric current, luminosity) end up with reasonable values the system is practically coherent.

(are the values reasonable? well, human-scale capacitors and resistors seem pretty heavily skewed away from the center of the scale…)

> e.g. to convert from seconds to meters you need to use conversion factor c, which is very much not 1.

SI cannot "convert from seconds to meters", since it treats length and duration as separate dimensions. Any multiplier which turns a duration into a length must have dimension of length/duration (and hence SI units of meters/seconds); something with those units is a speed, not a conversion factor (which is dimensionless). That's not specific to any particular speed, whether c or otherwise.

My example was weak, but the greater point is that SIs coherence is oversold because it still requires many unnecessary (non-one) constant factors in equations; SI merely brushes the issue under the rug by labeling those as "fundamental" as if they are not almost completely arbitrary.

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