[time-nuts] Primary Standards...

[Ian Sheffield]

In my checkered past, I once legally owned, for about two minutes between 
signatures, the British Standard Pound weight. It was a bar of platinum.
I confess I did wonder how easy it would be to sell a pound of platinum, as 
it would have been a bit difficult to cut it up...


----- Original Message ----- 
From: "Tom Holmes, N8ZM" <tholmes at woh.rr.com>
To: "'Discussion of precise time and frequency measurement'" 
<time-nuts at febo.com>
Sent: Wednesday, February 24, 2010 3:00 PM
Subject: Re: [time-nuts] Primary Standards...


> 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: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] 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
>
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