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Theory
17
The cell constant can also by calculated according the following formula:
air
r
C
l
A
=
 
 
0
12
0
10 113 . 0
1
−
 =

l
A
l
A
l
A
C
K
r r
r
air

= 
 
=
 
= =


 

 

0
0
0
0
0
1
 
l
A
m K with
r
=   
1

5.6 Test Instruments
There are three basic kinds of capacitance, tan

and Power Factor test instruments in use.
Although the high accuracy Schering Bridge must be balanced manually and the balance observed
on a null indicator, it has been widely sold and used for decades up until this day. The capacitance
and dissipation factor can be calculated by reading the position of the balance elements.
The automatically balanced C, tan

measuring instrument performs measurement by the
differential transformer method. The automatic balancing makes operation very easy.
The double vector-meter method is essentially an improvement of the differential transformer
method.
All three methods are in current use for accurate and repeatable measurements of C tan

on
various test objects. The differences basically lie in the resolution and accuracy. Different
instruments are generally developed specially for field or laboratory measurement.
Field instruments are specially constructed for rugged field requirements and are equipped with a
mobile high voltage source. In addition, such instruments provide noise suppression for onsite
use.
Laboratory instruments have been constructed for indoor use where high accuracy specifications
are required. These test systems are built in a modular construction for higher Test Levels. The
systems may be used for daily routine testing, for high precision long duration tests or for
acceptance tests.