rolled caps

From:  richard hull [SMTP:rhull-at-richmond.infi-dot-net]
Sent:  Monday, April 20, 1998 6:54 PM
To:  Tesla List
Subject:  Re: rolled caps

At 05:25 PM 4/19/98 -0500, you wrote:
>From:  Kevin Wahila [SMTP:kevinw-at-stny.lrun-dot-com]
>Sent:  Sunday, April 19, 1998 11:43 AM
>To:  tesla-at-pupman-dot-com
>Subject:  rolled caps
>a while ago i began planning and construction of my first rolled caps.
>since then my coiling was put on hold for some time.  now, as i go back and
>look through my mail and information to refresh my memory, i have found a
>slight problem dealing with the equation to calculate capacitance for a
>rolled cap.  i have two different mails from the list with two different
>            0.224 x K x A
>       C  = ------------- x (N - 1)
>                  D
>C = Capacitance (in microfarads)
>K = Dielectric constant
>D = Distance between plates in inches
>A = Area of plate in inches
>N = Number of plates
>                          2.248965 * 10^-7 * K * A
>C (microfarads) = ------------------------------------ * (N-1)
>                                         d
>!!!Again, you need to multiply this result by 2 for a rolled up cap.
>This is the formula I generally use when making homemade caps.
>ok, so which one is the right one?

Both are correct!!!!

The top one is the one I use, but it is in uufd (pf) not ufd, thus, the
10E-7 in the bottom equation to handle the conversion to ufd and move the
decimal point to 2.2 in the constant.  Naturally, these are for flat plate
caps. 2X for rolled jobs as you note.