Evans wrote:Chris Hall wrote:No measuring system is more 'rational' than any other. They are all artificial constructs - metric more perhaps so as it is based on such standards (when it comes to the metre) as the distance travelled by light in a specific fraction (1/299 792 458) of a second.
Well, originally the meter was defined as "1/10,000,000th the distance from the Earth's equator to the North pole", which in no way counters your point about choices, but
is more rational. Based on that statement, anybody can re-derive the approximate length of a meter from calculations and measurements of the world we live in.
That said, I get annoyed when a ruler isn't marked in inches down both edges because I have to pay attention to which edge I'm using: the inch edge, or the meaningless edge.
Yes, the metre was originally defined as 1/10,000,000th the distance from the Earth's equator to the North Pole, the quadrant of the earth's circumference, and surveying that took 6 years. Curiously enough, they undertook that survey using a standardized non-metric measurement called the
toise. A
toise is like the fathom, about 6' long. The
toise was divided into 6
pieds, or "feet" and 72
pouces (inches). The
pouce was divided into 12
lignes (lines). So, 864
lignes = 1
toise. Later investigation determined that, at the time of the survey, 1
toise equaled 1,949.03632 mm. Yes, a metre was determined by surveying using imperial French measures.
Previous survey work lead to the provisional determination that 1/10,000,000 of the Earth's equator to the North Pole was 443.44
lignes - this became the official standard for the metre. If a
toise is 1,949.03632 mm, then 1/864th of that, 1
ligne is 2.255829mm. So, multiply that by 443.44 and you get 1000.32484460mm.
The eventual official survey result lead to the provisional 443.44
lignes being changed to 443.296
lignes, which equals 1000.0000052207...mm. A lot closer to the modern definition but still not quite correct. This 443.296
lignes distance however became the reference standard. Later it was determined that the survey value was slightly too short and the metre was redefined, but not officially, as 443.31 lignes: later work increased the value to 443.39 lignes. Even though the scientific standard was revised twice, the initial standard of length of the metre stayed as it was. How rational is that?
Anyway, two issues here, to address your point Evan more succinctly (it is my hope at least...):
- numbers like 1,000,000, 10,000,000, 1/10,000,000 etc. are not intuitively graspable by humans. Yes, many can intellect such numbers, and we sure have counting systems to go there, however, biologically speaking we are set up to deal with much smaller numbers in terms of cognition. In our normal life experiences, our brains have become capable of representing small numbers, but helpless at accurately reflecting very large ones. So, making a metre 1/10,000,000 of a distance we cannot apprehend in the first place is not terribly helpful.
-the intellection that setting the length of the metre as 1/10,000,000 of the earth's quadrant -the meridional definition - might seem rational and reasonable, something that could be agreed upon by all, however a problem crops up in that the earth is not a perfect spheroid, but an oblate spheroid. Along which longitude one measures is going to affect the result, plus doing those surveys in each country is way impractical, for a number of reasons .
Also, defining a metre (or any other length standard for that matter) by a physical object, like a platinum bar, is going to lead to problems with wear and tear on that bar, and leads to difficulties in spreading the standard without actually moving the bars from place to place, etc.
The contention that "anybody can re-derive the approximate length of a meter from calculations and measurements of the world we live in" is incorrect I think, if you base it on the 1/10,000,000 of the earth's quadrant idea.
Others realized, and early on, the shortcomings inherent in the definition too. In the 1960's the metre was redefined as:
"The metre is the length equal to 1 650 763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton 86 atom". This measurement was undertaken in a perfect vacuum.
Do you intuitively understand how far that is? I don't personally. It's perfectly rational to make it equal something like that, and it is at least a method which is readily internationally reproducible in a laboratory.
Later on, it was discovered that the krypton wavelength line was found to be asymmetrical, so different wavelengths could be found for the laser light depending on which point on the krypton line was taken for reference.
Therefore that length standard for the metre was abandoned in favor of the current one,
"The metre is the length of the path travelled by light in vacuum during a time interval of 1 ⁄ 299,792,458 of a second"
I put it to you that humans cannot intuitively grasp either the speed of light or 1/299,792,458 of a second. It is very specific and exact yet seems a highly artificial construct. They could specify
any length measurement you like, inches, shaku, whatever, by some amount of distance that light travels in a vacuum - it would be equally hard to grasp for most folks.
And I say that in the world of woodworking, we are dealing with intuitively-graspable numbers nearly all the time. Whatever system you use, it matters only that you are conversant with it and are not confused by it. I tend to prefer inch scale myself but am frequently converting measures here and there to metric for certain machine settings. I like the high factorability of the inch as well.