[time-nuts] Measuring speed of light or reproducing a metre

Jim Lux jimlux at earthlink.net
Mon Jun 24 21:02:04 EDT 2013


On 6/24/13 4:16 PM, jmfranke wrote:
> The tuning fork was used with a clock. The clock was checked against
> astronomical measurements.
>
> http://www.schoolphysics.co.uk/age16-19/Wave%20properties/Wave%20properties/text/Speed_of%20light/index.html
>
>
> http://www.nhn.ou.edu/~johnson/Education/Juniorlab/C_Speed/2007-PhysToday-RefFrame-Michelson.pdf
>
>
> http://www.loc.gov/item/magbellbib002940 synchronizing two forks, letter
> to Bell.
>
> http://www.otherhand.org/home-page/physics/historical-speed-of-light-measurements-in-southern-california/the-mount-wilson-station-1922-1928/
>
excellent.. and I found on one of those pages the link to the US 
Geodetic Survey information
http://www.otherhand.org/wp-content/uploads/2012/03/A-Geodetic-Measurement-Of-Unusually-High-Accuracy.pdf

The "Pasadena" baseline was almost as long as the 22 mile measurement, 
and stretched from Pasadena to the east (San Dimas, etc.)

I like the comment that direct measurement of the baseline to 1 part in 
500,000 wasn't considered particularly challenging ("routine"), but 
transferring that measurement to the "MICHELSON" "ANTONIO" path was 
challenging.

Sure.. a few inches in 20 miles isn't particularly challenging...
They measured it with 4 different tapes and came up only 18mm difference 
among the measurements. That's some careful chaining. They were using 50 
meter invar tapes: that means they had to put that tape out, pull it 
straight to the rated tension, etc. about 700 times along the path.

A great picture of the tape going through a house along the baseline, in 
one window and out another.
Ultimately, they measured the baseline (down on the flats) to 1 part in 
11.6 million, and they estimate the probable error of the 
MICHELSON-ANTONIO line was 1 part in 5 million.



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