Re: Picofarad Capacitor Standard

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br> 

Tesla list wrote:
 >
 > Original poster: "John H. Couture" <couturejh-at-mgte-dot-com>
 >
 > Antonio -
 >
 > Thank you for your reply to my post. My test capacitor consisted of two
 > rectangular plates separated by 0.25"
 > 6"x7.42" = 44.5 sq inches. C=40 pf
 > This compares with your two disks separated by 0.25"
 > R = 9.56 cm = 44.5 sq inches. C=40pf

I don't have how to simulate rectangular capacitors, but I would
expect similar results. The capacitance would tend to the ideal
value when the plates are at small distance, but would always
be somewhat greater than it.

 > You mentioned that the capacitance (test caps) will always be greater than
 > the ideal value. Note that the tests I made indicated that the test cap
 > capacitance became less not greater when the plates were closer than 0.25".
 > This is the intersection point of the test curves. I wonder if this has any
 > significance because it occurs after the intersection of the curves. It
 > might relate to edge effects?

Maybe you overcompensated for the meter lead capacitances? It's tricky
to measure small capacitances. If you, for example, measured the
capacitance between the plates at a large distance, assumed that this
was zero capacitance, and then discounted this value from the other
measurements, you discounted the large distance capacitance between
the plates, that is not zero, but one half of the free-space capacitance
of the plates (added to the lead capacitance in a different geometry).
I would do the compensation by first measuring the capacitance between
the test leads only, at the position that they would assume when
connected to the plates, and discount this value from the measurements.
Even so, the simple addition of the plates would change the correct
value to be discounted a bit.

 > Your Inca program is very good. I use the program often to check the outputs
 > from my own program.

Thank you. Suggestions on how to improve it are welcome.

Antonio Carlos M. de Queiroz