Haefely 2830, User Manual, Page: 21 / 124

Theory
13
The permittivity of free space (vacuum permittivity) also called electric constant

0
is the ratio D/E
in free space. It also appears in the Coulomb force constant 1/4
πε
o
.
D= electric displacement field
E= electric field

0
= vacuum permittivity
C
0
= speed of light

0
= vacuum permeability
The constants c
0
and

0
also called “physical constants” are defined SI units to have exact
numerical values.
s
m
c
8
10 99792458 . 2
0
 =
Am
Vs
7
0
10 4
−  =
 
m
pF
m
F
s
m
Am
Vs
c
... 85 . 8 10 .... 8541878 . 8
10 99792458 . 2 10 4
1 1
12
2
8 7
2
0
0
0
= =






 







= = −
−




C
oil
= Measured capacity of the test cell with liquid insulating dielectric
C
air
= Measured capacity of the test cell with air as dielectric (Tettex Oil Test Cell 2903 approx. 60
pF

20%)

0
= Vacuum permittivity

r oil
= Relative permittivity of the liquid insulating

r air
= Relative permittivity of air (

1)
A= Surface of test cell (Tettex Test Cell 2903 approx. 0.0134 m
2
)
l= Distance between surface ( Tettex Oil Test Cell 2903 approx. 0.002 m)