Tony Duell ard at p850ug1.demon.co.uk
Thu Oct 16 14:34:56 CDT 2008

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> > I rememebr having a long arguement with a physics teacher who
> > insisted a bridge circuit couldn't be used to measure capacitance,
> > but only to compare capacitors.  I am strill trying to find a
> > definition of 'measurement' which does not involve comparison to a
> > standard.
>
> Yes, but that standard does not have to be another of the type of thing
> being measured.

No, on the other hand, you can measure by comparing to a standard of the
same time. A lot of common measuring instruments, starting with the
ruler, do exactly that.

Incidnetally, an AC-excited bridge circuit can meausure capacitance in
terms of a standard capacitor, but you can also balance R:C against L:R
(taking the obvious meaning). This is more commonly used to measure
inductance (in that it's easier to get a stable stnadard capacitor than a
stable standard inductor), but there's no reason in principle why you
can't measure capacitance in terms of inductance and the ratio of 2
resistors.

> In the case of capacitance, I think it's fair to draw a distinctino
> between measuring capacitance in the form of "this capacitance is 12.7
> times that one" and measuring capacitance in the form of, say, "it
> takes 137ms for a 10mA current to charge this capacitance up to 50mV"
> or "with a 50kohm resistor this capacitance exhibits an R-C time
> constant of 27ms".

I would agree...

However, M.G.Scroggie's 'Radio Laboratory Handbook' makes the probably
valid point that in general it is more accurate to measure something
against a standard of the same time (there are fewer 'conversions' to
different physical quantities, each of which may not be exact). Said book
(of course) treats assorted bridge circuits as methods of measuring
'circuit constants'.

As does the _excellent_ 'Measurements in Radio Engineering' by Professor
Terman (I am sure everyone here knows who he is. Now, I am sure there are
people who know more about electrical measurements than Professor Terman,
but I don't think such people are exactly common, and I am darn sure said
physics teacher wasn't one of them.

-tony

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