David Dameron wrote: > Hi all, > I just realized that a meter is defined by the speed of light., see > http://physics.nist.gov/cuu/Units/meter.html > It is only to 9 significant digits, so if the speed of light (in some > controlled environment) is measured more precisely than this, the meter and > all other derived length units will change? > > (I was taught that 1 meter was 39.37 inches, to define the inch > , but now I see more of 1 inch = 2.54 cm, as someone just referred to.) > > I find the standard for the Ampere, mentioned in the nist pages above more > difficult, as 2 infinite wires to measure the force between cannot be > found! Was the coulomb the standard before? Does anyone have other web > pages to recommend? > (Am still learning about the 1948 changes to electrical units, > international and absolute volts, etc. Before finding this list, did not > think much about the differences, about 500 ppm., with a 3 1/2 digit dvm.) > > David D. > David
The 1m = 39.37 inch definition applies to the US survey inch, a unit only used in surveys. Since around 1958 or 1959 the US customary inch has been identical to the international inch: 1 international inch = 25.4mm. The 2ppm difference is significant in geodetic survaeys. In practice realising the ampere used to mean building a current balance. The abstract definition employing infinite wires can easily be used together with a little calculus to calculate the force between non infinite wires of wound into ci=oils and other shapes. Before the advent of the current balance the unit of charge was defined electrochemically in terms of the weight of a standard metal (platinum??)electroplated from solution on to the cathode of a electroplating cell. The unit of current being defined by extension from the unit of charge. Note the SI units as we know them today were not then in use. Bruce _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
