Re: Can anyone solve this problem?

[brian whatcott <inet@INTELLISYS.NET>]

At 20:28 10/22/00 -0700, you wrote:
> ///
> Here's the problem (what's the lesson?):
>
> In the arrangement shown in the figure, a potential difference V
> is applied, and C1 is adjusted so that the voltmeter between
> points b and d reads zero.  This "balance" occurs when C1 = 4.00
> microF.  If C3 = 9.00 microF and C4 = 12.0 microF, calculate the
> value of C2.
>
>        ______________________
>       |         |            |
>       | V      ===C1  ___   ===C4
>     -----      b|____/ V \___|d
>      ---        |    \___/   |
>       |        ===C2        ===C3
>       |_________|____________|
>
>
> John Mallinckrodt      mailto:ajm@csupomona.edu
> Cal Poly Pomona          http://www.csupomona.edu/~ajm

Taking a strict construction of John's circuit (that it is
a DC source, no switch, only C1 may vary)...

It is only the variability of C1 that raises this circuit
 to the possibility of working as advertized.

 Without variation in C1, there would be two cases:
1) Currentless meter - reads any preexisting PD, not depending
 on bridge values within reason
2) Current driven meter - any pre existing PD falls to zero over
some small number of time constants.

With manual variation of C1:
3) Currentless meter - reads any preexisting PD, but with an
alternating component which depends both on the amplitude of
the C1 variation, and on the bridge value ratios.

4)Current driven meter - any pre existing PD falls to zero,
an alternating component is read as in 3)

Some helpful notes: components can be purchased off the shelf
in 1% values and with low temperature constants - some as
low as +- 3 ppm.

 An engineer who seeks job longevity will avoid components
of this kind wherever possible.
She will also avoid bridges capable of resolving 1 ppm in
component values wherever possible - which is nearly always.

These rascals reached the height of their development
about 30 years ago. These days, microprocessors and low cost
time/frequency sources lead to compact, self calibrating,
direct reading meters for the majority of modern design
 requirements.



brian whatcott <inet@intellisys.net>  Altus OK
                Eureka!