Dave... I went back and checked in my college Physics textbook, Halliday & Resnick, Vol. II, circa 1960, and you are correct about the Ampere being defined based on the force between two parallel wires. However, H&R does not specify a vacuum nor negligible wire cross section. The latter seems reasonable to minimize the effects of geometry. They also say that at the time, NBS was using a balance beam technique with a moving coil between two fixed coils as the primary measurement standard.
Where Avogadro's number comes in is that 1 coulomb is defined as "the amount of charge that flows through a given cross section of wire in one second IF there is a steady current of one Ampere". In other words, if I moved a coulomb of charge in one second, then the current must have been one Ampere. Kind of a strange way to state it given that one of the equations given for charge is Q= the integral of I*d(t), implying that current and time are the are the measurables. So I think in a way we are both correct: you have the definition of the standard, and I cited an equivalence that is based on the fundamental units of the mks system. In a table in the appendix called "Symbols, Dimensions, and Units for Physical Quantities" there are listed about 60 quantities and their primary units (Length, Mass, Time, and Charge). For example, capacitance has dimensions of T^2 * Q^2 / M^2 * L^2, with the derived unit Farad. Force has dimensions of M*L/T^2, with derived units of newtons. This fits with F=MA, that is, force is derived from mass, length, and time, all of which have fundamental standards. The Kg is a slug of something carefully stashed in a cave in France ( a little license here, please), the meter is a bunch of wavelengths of a Krypton dance, and the second is based on...oh, wait, this is the time-nuts forum. So what is bugging me is that the Newton, a derived unit, is being used to define the Ampere, which appears to be a fundamental part of the definition of the Coulomb, a primary unit. This strikes me as backwards. However, it does make sense that the method used to determine a 'standard' value for the Ampere might not be possible using such a strict dependency on direct ties to primary units. OK, I think I have meandered far enough OT once again as to put this to rest for now. Regards, Tom Holmes, N8ZM Tipp City, OH EM79xx -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Dr. David Kirkby Sent: Tuesday, February 23, 2010 8:12 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Primary Standards... Tom Holmes, N8ZM wrote: > My recollection of the definition of an Ampere is 6.02 x 10^23 electrons per second (Avogadro's Number, I believe) passing a point in a conductor. To this day, I wonder how they managed to count all those electrons. But it does suggest that the silver deposit approach might be a better method of building a standard. Seems, though, like you'd have to make a darned high resolution weight measurement. That certainly was not the definition I learned during my EE degree, and neither is it the one given on Wikipedia - not that I'd call Wikipedia a standard. My recollection is the same as Wikipedia's - though I could not remember the bit about it needing to be a vacuum. But if you stuffed mu-metal between the wires, it would tend to reduce the force, so I can well believe its defined in a vacuum. I think as someone else said, this depends on one's definition of a "standard". There's no one standard definition of a standard (pun intended). Dave _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
