Capacitors Marked in CMs

[Nick]

Just thinking out loud here - my numbers may well be rubbish...

...My friend even found a text book (Morecroft “Princuples of Radio
Communication”. ) and said "I think you will find a CM = capacitance of two 1 sq.centimetre plates one CM apart."

The book says to convert to pF divide by 0.9 (or roughly add on 10%).

Two 1cm2 plates 1cm apart in air (or vacuum) is computed as K * ε0 * A / D where:

• A is the surface area of the plates in sq metres
  
• D is the distance the plates are apart in metres
  
• ε0 is the permittivity of free space, a constant = 8.854e-12 Farads/metre
  
• K is the dielectric constant (aka static relative permittivity, εr) of the insulator, 1 for a vacuum and very nearly that for air.
  

Plugging in the numbers for 2 x 1cm2 plates 1 cm apart in air, gives 8.85e-14 F, i.e. approx 0.09pF...

EDIT: Just found the following: http://www.radiomuseum.org/forum/old_cap...kings.html

cm was used until the 40ies of the last century: 1cm = 0.0885pF*4π ≈ 1.1pF

So, yes, it is indeed a pair 1cm2 plates 1cm apart, but why the "4π"? Four pi? I'm always for pie, especially tarte normande...

Thanks Heavens for SI units - at least they are consistent and make sense...