Picofarad Capacitor Standard

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

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

------------------------------------