Cool, thanks! Makes one wonder when the accuracy measuring time interval or frequency was finally better than the accuracy measuring distance. John WA4WDL -------------------------------------------------- From: "Jim Lux" <jimlux@earthlink.net> Sent: Monday, June 24, 2013 9:02 PM To: <time-nuts@febo.com> Subject: Re: [time-nuts] Measuring speed of light or reproducing a metre > 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. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >