OT: Length of the Meter?

Not just any ten-millionth, but a ten-millionth of the quarter-meridian passing through _Paris_. Pure Greenwich envy. ;)

Soon after their Revolution, the French defined the metre that way, and dispatched surveyors to measure it, despite the fact that both ends of that line are at sea. The idea promptly sank as a result (like so many subsequent French initiatives).

More recently, even the SI folks have abandoned the guillotine and tried to reduce the impact of redefinitions on pre-existing equipment and scientific results. So when time and frequency became more accurate than any mechanical measurement, the definition of the metre was switched over to time and frequency. (And by the way it's

Cheers

Phil Hobbs

The one about the equator and pole is the original one and was derived geom etrically. That's right, when they told us people thought the world was fla t they lied. (you have to go pretty far back for that, alot farther than li ke the 1400s, the only people on the Nina, Pinta and Santa Maria who though t they would sail off the edge of the Earth were totally uneducated)

The one about the light is just a contemporary equivalent, measured to the original. It's just an easier way NOW to do it.

Same way with a second. There were latitudes, hours, minutes and then secon ds. Now they say it is so many vibrations of the cesium atom or something l ike that, but that is a contemporary thing. It is just easier NOW.

It's also interesting how they figured out ohms, volts and amperes. Wiki sh ould have a good article on all that, including your original question.

passing through _Paris_. Pure Greenwich envy. ;) "

Are they STILL sticking their middle fingers out at each other ? (you DO know that story right ?)

that line are at sea. The idea promptly sank as a result "

How come that became the number five ? Did they try it five times ? They also can't spell. "cinq" ? Worse than Spanish.

Is that why that neutrino apparently went faster than the speed of light ?

I think they did something stupid with a cable in their test gear. You'd think they would check stuff like that before they went public.

But they didn't specify the time of the day/month/year to make the measurement.

The length of the meter is however big you choose it. It could be some tiny meter with barely a scale, for a tuning indicator in a stereo receiver, or it could be nice and big, the meter in my Senior Voltohmyst VTVM.

As for a metre, everyone knows it's 39.37 inches long.

As for your question, it has to be based on some reality, not a random even number.

Michael

;)

Robespierre & Co took a bit of discouraging, I suppose.

Cheers

Phil Hobbs

Yes, I understand it was switched from the physical length of a metal bar to the accuracy of a cesium clock and speed of light. But I'm still curious how the length of the metal bar was measured to within 1 part in 300,000,000?

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Interferometry of one sort or another, I think. A Michelson interferometer using a He-Ne laser will give you 2m/632.8 nm = 2.8 million cycles per metre. Not too high a SNR required to get to 1 part in 300,000,00 from there, assuming you know where you start from.

Cheers

Phil Hobbs

It probably wasn't, any more than the pole-to-equator distance was measured to PPB accuracy. I suspect they did their best and moved on.

Speaking of measurement standards and such good clocks are needed to make good length standards by measuring how far light travels in a certain amount of time. Accuracy of the timekeeping limits accuracy of length. Resolution is important too, the goal being to get smaller and smaller parts per part. And of course the rate of time passing has to be kept track of since it changes depending on the clock's distance to massive objects, like planets. As we all know the closer to massive objects we are the slower time passes. So those folks living in sky scraper penthouses may have a better view but pay for it in shorter lives compared to those living on the ground floor. At this time (pun intended) we can measure elevation changes by comparing how fast clokcs tick. Or resonate. Or whatever. In fact, GPS isn't very accurate unless we account for the satellite speed (which slows time) and the distance from Earth (which speeds time up). But we can measure smaller elevation differences than the mean distance a satellite is from Earth by measuring the rate of time passing. In fact, we can measure elevation differences of less than 3 feet by observing the difference in time passing. So if you take your atomic clock off the bench and set it on the floor to make room for your death ray prototype, you had better account for the time difference so that your GPS pointing device makes your death ray take out that nerd who is vying for your job and not his babe secretary. Eric

"Michael Black"

.... Phil

Absolutely, we should recognize other folks' hard work.

Of course 200 years ago, the English used to capture more French (and especially Spanish) ships than they built themselves. ;)

Cheers

Phil Hobbs

Yes, they probably ran the test 1000 times and averaged it out.

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I think it was more like 5-10 ppb at the time--ISTR one of the last official measured values of c was 299792456.3 m/s, back in the early 70s (when I was into memorizing stuff like that). ;)

Cheers

Phil Hobbs

The meter itself was two scratches on a platinum-iridium bar. I guess that could be measured pretty well. One micron would be (...calculates furiously...) 1 PPM.

I don't know how the scratches were made. You can make atomically sharp scratches in some materials, but I imagine the parallelism error would be more than a ppb.

There was probably a standard way of comparing metre standards, so if you take the standard metre as the combination of the bar and the method, it might be better than 0.1 ppm.

Cheers

Phil Hobbs

The metre should have been how far light travels in 1 ns. Given how arbitrary the pole-to-equator thing was, they could have made 1 metre = 1 foot, or 1 yard, except that they were French.

Interesting that 1" is precisely 25.4 mm.

The inch was changed to be precisely 25.4mm some time last century previously it was about 25.400051mm in USA and about 25.399956mm in the Brittish commnwealth. and previos to that different.