CHAPTER 7
DC BRIDGES
7.1 Aim of the Experiment
The purpose of this experiment, to examine measurement principles of resistance with
Wheatstone (Kelvin) bridges.
7.2 Theoretical Information of the Experiment
Two terminal resistance can be measured by Wheatstone (Kelvin) bridge. Lower
measurement bound of this bridge is 1 Ω because of the resistance of the bridge’s connection
points is about 1 Ω. Upper measurement bound of the bridge is 1 MΩ for range of using
galvanometer. Additionally, this bound is grown to around 10 GΩ by using galvanometer
which have high impedance and high sensibility.
Measurement Methodology of DC Bridge
Wheatsone (Kelvin)
Murray & Varley (Telli)
Thomson
Wheatstone-meter
Murray and Varley bridges that run like Wheatstone bridge, are used for measurement of
cable and cable connection. The difference Murray and Varley bridges than Wheatstone
bridge is to be able to calculate the distance from potential point of failure to bridge point of
connection.
Thomson (or Kelvin) bridge is used for measurement of resistance which have four terminal
and small values from 1µΩ to 10Ω. Additionally, DC-Comparator Proportion bridges is
similarly used for the same purpose but these bridges are more sensitive than Thomson
bridges.
� � =� �
(7.1)
7.3 Experiment
7.3.1
R1
10 K pot
R4
22 K
A
V=6V
R3
2.2 K
R2
470
Figure 7.1

Set the circuit at Figure 7.1.

Apply 6V DC to input voltage.

Set the R1 resistor to 1K and its multiples and read the ammeter.

Fill Table 7.1.

Set the ammeter to zero (0) and find the R balance.

Find the real value from Equation 7.1.

Find the percent (%) error using real value and measured value, then save Table 7.2.
Rpot
1k
2k
3k
4k
5k
6k
I
Table 7.1
R balance
% Error
Table 7.2
7k
8k
9k
10 k
7.3.2
R1
4.7 K
R4
A
V
R2
1 K pot
R3
Figure 7.2

Set the circuit at Figure 7.2.

Apply 5V DC to the input voltage.

Find the R2 balance resistor at R3=2.2K and R4=22K, then save this value to
Table 7.3.

Read the ammeter and save Table 7.3.

Find percent (%) error and save Table 7.3.

Apply 10V DC to the input voltage while R3 and R4 is equal.

Read the R2 balance resistor and the current passing from the ammeter and save
Table 7.3.

Calculate the R2 balance resistor and find percent (%) error, then save Table 7.3.

Change R3=220 and R4=2.2K while input voltage is constant.

Read the R2 resistor and the current passing from the ammeter, then save Table
7.3.

Calculate R2 resistor and find percent (%) error, then save Table 7.3.
Case I
Case II
I (mA)
Measured
Resistance(Ω)
Calculated
Resistance(Ω)
Percent
Error (%)
Table 7.3
7.4 Materials

DC power supply

Various resistances (220, 470, 2.2 K, 4.7 K, 22 K)

10 K potentiometer

Ampermeter

Connecting cables
7.5 Research Questions About the Experiment
1.
Explain how the circuit at Figure 7.1 works.
2.
Where DC bridges are used? Why?
Case III
CHAPTER 8
AC BRIDGES
8.1 Aim of the Experiment
The purpose of this experiment, to examine measurement principles of inductance (L) and
capacitance (C) with impedance bridges.
8.2 Theoretical Information of the Experiment
Impedance bridge constitute the basic of AC bridge circuits. There are a lot of types of
bridges used for measurement L, C and mutual induction coefficient. Some of most used AC
bridges circuits are Symmetrical Inductance, Anderson, Hay, Oven, Maxvell-Vien bridges.
Additionally, Campbell, Felici Balance, Hartshorn and connected to ground–Wagner
Resonance bridges are used for measurement of mutual induction coefficient.
The basic measurement circuits used for measurement of capacitance are serial-RC, Wien
de SAUTY, transformer and Schering bridges.
8.3 Application of the Experiment
8.3.1 Measurement of Capacitance with De Sauty Bridge
Figure 8.1

Set the circuit at Figure 8.1

Cn = 470 nF

Set the signal generator to 100 Hz square-wave.

Connect an unknown capacitance to Cx.

Set the ammeter to balance with R1 and R2 potentiometers.

Calculate the Cx from Cx= Cn (R2/R1).

Save the results to Table 8.1.
R1
R2
Table 8.1
8.3.2 Measurement of Impedance with De Sauty Bridge
Figure 8.2

Set the circuit at Figure 8.2

Connect an unknown inductance to Lx.
CX

Set the signal generator to 100 Hz square-wave.

Set the ammeter to balance with.

Measure values of R1 and R2.

Calculate the Lx from Lx= Ln (R1/R2)

Save the results to Table 8.2.
R1
R2
LX
Table 8.2
8.4 Materials

Signal Generator

2 number potentiometer (1 K, 10 K)

2 number capacitor

2 number inductance

Ammeter

Connecting cables
8.5 Research Questions About the Experiment
1
Why we use signal generator instead of DC power supply in AC bridges?
2
Where the AC bridges are used? Why?