[time-nuts] Primary Standards...

Tom Holmes, N8ZM tholmes at woh.rr.com
Wed Feb 24 15:00:46 UTC 2010


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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