IM 2769-10E
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12
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6. PRINCIPLE OF OPERATION
6.1 Basic Principles
Fig. 6.1 shows the theoretical diagram of the double bridge circuit.
When the resistances N and n are adjusted so that the current flowing in
the galvanometer G is reduced to zero, the unknown resistance X is expressed
by equation (6.1) as follows:
X = × S + ×
–
(6.1)
N
M
N
M
n
m
m ×
m + n +
Where is the lead resistance from the terminal Ps of S to the terminal Px of X.
The 2769 double bridge is designed so that the fixed resistors and
slide resistors have the following relationship:
(6.2)
N = n and M = m
Therefore,
From equation (6.1),
X = × S
– = 0
N
M
n
m
N
M
In other words, the low resistance X can be accurately measured without influenced by
the lead resistance.
E
G
N
n
m
M
Px
C
2
Ps
X
S
Fig. 6.1 Theoretical Circuit Diagram
6.2 Error
6.2.1 Error due to Current Lead
Practically,
N
M
n
m
–
is not perfectly zero, but is approximately ±(10
-3
to 10
-4
).
For example, m + n ,
0.99 ,
m
= 10 m
Ω
m + n
Then, the second term of equation 6.1 is ±1 × 10
-5
Ω.
If S = 0.1 Ω, the resistance value is X = 10
-2
Ω ± 1 × 10
-5
Ω according to equation (6.1).
This means that the error is ± 1% of the dial reading.
