Re: Picofarad Capacitor Standard

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

Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 

Oh, Ho.... the standard capacitor thing..
Check out NBS Circular 74... it has how to build standard capacitors...
As a practical matter, if you're working at a known frequency, a long
coaxial line might be the best approach, because, especially in the shorted
form, it has a well defined value, with end effects taken care of.
Cragg
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, December 09, 2003 5:13 PM
Subject: Picofarad Capacitor Standard


 > Original poster: "John H. Couture" <couturejh-at-mgte-dot-com>
 >
 >
 > Paul N, Antonio, All -
 >
 > I have attempted to make a homemade capacitor with a reasonably exact
 > capacitance value at the low picofarad level for toroid research. This
 > capacitor consists of two flat metal plates at a certain distance from one
 > another. The main problem of this type of capacitor at low pf capacities
 > would be the edge effects. I have tested the capacitor and developed three
 > curves as shown below to find where they intersect when varying the
distance
 > between the plates. I believe the point of intersection of the curves is
 > where the edge effects are eliminated? In other words the plates at .25
 > inches giving 40 pf would be the real world capacitance of the two plates
 > with the elimination of edge effects.
 >
 > The theoretical equation for two metal plates at a distance d apart in air
 > is
 >
 >         Picofarads = 8.85 (area sq meters)/(d meters)
 >
 >         1 inch     = 39.37 inches
 >         1 sq meter = 39.37^2 = 1550 sq inches
 >
 >         Picofards = (39.37/1550) sq inches/inches
 >             "     = .225 sq inches/inches
 >             10 pf = .225 sq inches/1
 >         Area sq inches = 10/.225 = 44.4 (6"x7.42")
 >
 >    pf1 - Theoretical capacity    y = 10 X^(-1)
 >    pf2 - Beckman DM25L           y = 21.3 X^(-0.45)
 >    pf3 - Wavetek DM27XT          y = 15.6 X^(-623)
 >
 > The equations above are power regression equations obtained from Excel.
Note
 > that the 3 curves intersect near the d = 0.250 inches point. You can make
a
 > graph that illustrates this condition by entering the values into your
Excel
 > program and using the Chart Wizard.
 >
 >        d inches   pf1    pf2     Pf3
 >         0.125      80     56      55
 >         0.250      40     40      38
 >         0.500      20     28      25
 >         0.750      13.3   23      19
 >         1.000      10     21      15
 >         1.500       6.7   19      12
 >
 > The values for pf2 and pf3 were adjusted for leads, etc. The two meters
are
 > rated +/- 2%+4 for the 2nf range. The results above show that the meters
 > only agree with each other and with the theoretical capacitor at the point
 > of intersection
 > (d = .25"). This is also the point where the capacitor edge effects are
 > eliminated?
 >
 > I would be interested in all comments.
 >
 > John Couture
 >
 > ------------------------------------
 >
 >