15 Apr
2011
15 Apr
'11
5:39 p.m.
On 4/15/11 6:55 PM, Brent Hilpert wrote:
The metrology world typically uses calculable capacitors for C. For
voltage, Josephson junctions are the top dog, driven by a primary
frequency standard. This is a primary standard because the only other
parameters are e and Planck's constant. I believe quantum Hall effect
devices are still used for R standards but I haven't kept up with that.
SET (single-electron tunneling) devices are en vogue as current
standards but I don't know much about those either.
In electronic metrology, "measurements" are carried out by comparison
to these standards or derivatives thereof. Voltage is measured
potentiometrically via either a solid-state null detector or a SQUID
(superconducting quantum interference device). For capacitance, as in
Tony's situation, I'd expect a passive AC bridge would be how it's done,
comparing against (absolute best case) a calculable capacitor. That
gets you down to first principles, but the "measurement" is still a
comparison. As you stated above, measuring the RC time constant with a
precision timer and a precision voltage source (both driven by primary
standards, cesium fountain and Josephson junction array) is still
comparing it, by my interpretation, mathematically against numbers
calculated from those primary standards. I'm not sure Tony's instructor
actually had a point.
-Dave
--
Dave McGuire
Port Charlotte, FL
I think the
wordst was when I was aksed to descrtivbe a method of
measuring something (the capacitance of a capacitor). I did so (an AC
bridge) and was told 'That is a compariston and not a measurement', I am
still waiting for a definition of 'measurement' (or an esample of one)
which
does not involve a comaprison to a standard.
I expect that what your instructor was getting at was that the use of an
AC bridge typically involves comparison with another C, which also needs
to be measured. It becomes a 'turtles all the way down' problem. This
would be in contrast to a solution which examines Cx in terms of the
definition of capacitance (time/charge) and breaks it down to more
fundamental/axiomatic measurement units (i.e.,time,mass), even though
this does involve comparison to some other standard.
"That is a comparison and not a measurement" may not be a full
expression of the distinction, but I think your instructor had a point.
I'm not up to scratch on my measurement theory, what are the fundamental
standards these days?, there's oscillation of the cesium atom for T,
there was the Meter bar in Paris for D, but hasn't that been redefined?,
etc. Or, how many fundamental standards do we need to derive everything
else? 3?.