Metric is inaccurate

[joraven]

I know there are metric lovers out there and I agree that the system is less taxing on the brain as any idiot can count in 10's but I've read that when the French determined the sizes they made a mistake in their calculations and their measurement of distance is out of true by a significant amount (on a planetary scale).

I know that since 1959 America has had to adopt 2 conversion rates (geodetic and standard) to try to convert between the accurate imperial and the inaccurate metric measurement.

Could anyone confirm the basis of the French measurement and whether or not the rumour is true?

[qapmoc]

The meter was intended to equal 10-7 or one ten-millionth of the length of the meridian through Paris from pole to the equator. However, the first prototype was short by 0.2 millimeters because researchers miscalculated the flattening of the earth due to its rotation. Still this length became the standard. (The engraving at the right shows the casting of the platinum-iridium alloy called the "1874 Alloy.") In 1889, a new international prototype was made of an alloy of platinum with 10 percent iridium, to within 0.0001, that was to be measured at the melting point of ice. In 1927, the meter was more precisely defined as the distance, at 0�, between the axes of the two central lines marked on the bar of platinum-iridium kept at the BIPM, and declared Prototype of the meter by the 1st CGPM, this bar being subject to standard atmospheric pressure and supported on two cylinders of at least one centimeter diameter, symmetrically placed in the same horizontal plane at a distance of 571 mm from each other.

The 1889 definition of the meter, based upon the artifact international prototype of platinum-iridium, was replaced by the CGPM in 1960 using a definition based upon a wavelength of krypton-86 radiation. This definition was adopted in order to reduce the uncertainty with which the meter may be realized. In turn, to further reduce the uncertainty, in 1983 the CGPM replaced this latter definition by the following definition:

The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

Note that the effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 m�s-1. The original international prototype of the meter, which was sanctioned by the 1st CGPM in 1889, is still kept at the BIPM under the conditions specified in 1889.

[Loosehead]

Phew, well done qapmoc. It is also true that 1 litre of water = Kg = 10cm cube of water and the meter is 10 times the side of the cube. Not sure how that relates to all the steps described by qapmoc though it can't be a conincidence, any ideas qapmoc?

[Ralph]

Perhaps the kilogram was defined after the metre, since one cubic metre equals one tonne (one thousand kilos) of pure water at 4�C (when it's most dense).

My point is though, that how can a measurement system be inaccurate? It can't be! The only time you get inaccuracies in measurements is when you convert from one measurement system to another. If I invented my own measurement system, it would be perfectly accurate as long as I was consistent.

The rumour isn't true!

[joraven]

The point was that the whole basis of their intended system of measurement was to set a size for their metre on the principle of it being able to divide exactly into the measurement from one point on the planet to another - and they miscalculated so failing to achieve their original goal. They would have been just as accurate had they chosen a size at random.

[brachiopod]

I don�t quite get the point of your question, joraven.

Is it a go at the French, the Metric system or what?

Now, if you think the Metric System is inaccurate, how do you prove it is inaccurate? By measurement against, say, a wooden ruler marked in inches?

As you say, you could have chosen a �size� at random. I could invent a unit of length tomorrow called �the leg� � but unless I can define it absolutely, it is inaccurate as any other measurement.

The French had a metal bar to define the metre, the Imperial (avoir dupois) system had a stick (yes, the original �yardstick�). Even back in the 19th Century (the 1870�s if memory serves), the French realised that heat affected the length of the metal rod (by thermal expansion), and so created a new �metre rod� out of, I think, a platinum alloy that had a lower coefficient of expansion.

The British still had their stick.

Since then, the metre has been re-defined several times according to the accuracies of the prevailing technology � initially in terms of wavelengths of light emitted by specific elements (since this is largely independent of temperature / pressure etc.), now by specific fractions of the speed of light. (the unit of time, the second, by which this can be worked out, is similarly defined absolutely by vibrations of a caesium atom)

Originally, yes, the metre was defined as a fraction of the Earth�s radius, (as was the imperial system, think of the nautical mile), and since the Earth has variations in its circumference, then yes, it would be inaccurate.
However, since it has been re-defined in increasingly precise, physical constant terms, the metre can hardly be described as �inaccurate�.

It�s a bit like me sticking my size 9 Docs next to a 30cm ruler and saying it is wrong because my boot is �a foot, innit?�

[jake-the-peg]

I think the originbal definition of the inch was the "length of 3 barleycorns".

I think therefore if we judge the imperial system by your standard of the accuracy of the original definition metric wins hands down!

[ugly_bob]

Hmmm, so, by definition it is not innacurate, just slightly out on what they intended it to be.

[brachiopod]

No, perfectly accurate for what they intended it to be, but improvements in measurement accuracy meant that its definition had to be re-defined to keep up.

The second used to be defined as one 86,400th of an Earth day - fine when clocks and watchs were rare and inaccurate, but when events needed to be measured in nanoseconds, it's obvious that a better, more absolute method of defining the unit was required. Hence, the second is now defined by an exact number of vibrations of a caesium atom.

Although the original ways / reasons of measuring a particular unit have gone - (the length of an Earth day varies anyway, as does its circumference) the new definitions are more accurate definitions of these units.

[Dave.B]

qapmoc:

I propose that your answer is just a verbatim quote from this site:

http://physics.nist.gov/cuu/Units/meter.html

The note about the "engraving at the right" gave it away.

Sorry to point that out, but, well, you know... credit where it's due and all that.