Wheatstone Bridge

Physics Investigatory Project

1. CERTIFICATE




This is to certify that NISHCHAY
SAXENA, a student of class XII-A has
successfully completed the project under
the guidance of Mr.Mukesh Shrimali
(Subject Teacher) During the academic
year 2019-20 in partial fulfillment of
Physics practical examination conducted
by CBSE.





Signature of external examiner Signature of chemistry
teacher


Signature of principal

2. ACKNOWLEDGEMENT
In the accomplishment of this project
successfully, many people have best owned
upon me their blessings and the heart pledged
support, this time I am utilizing to thank all the
people who have been concerned with project.
Primarily I would thank god for being able to
complete this project with success. Then I would
like to thank my principal Mrs.Shalu Babbar
and physics teacher
Mr.Mukesh Shrimali, whose valuable
guidance has been the ones that helped me
patch this project and make it full proof success
his suggestions and his instructions has served
as the major contributor towards the completion
of the project.
Then I would like to thank my
parents and friends who have
helped me with their valuable
suggestions and guidance has
been helpful in various phases
of the completion of the project.

3. AIM




To study the construction
and application of
Wheatstone Bridge

4. CONTENTS

1. Introduction
2. Components of Wheatstone bridge
3. Circuit Construction
4. Working principle of Wheatstone
bridge
5. Example Circuit
6. Applications of Wheatstone bridge
7. Limitations of Wheatstone bridge
8. Limitations of Wheatstone bridge
9. Summary
10. Bibliography
11. Precautions

5. INTRODUCTION

Samuel Hunter Christie invented the
Wheatstone bridge in the year 1833,
which became popular with the works of
Sir Charles Wheatstone in 1843.
An electrical circuit that is set up to
measure the unknown value of a resistor
and creates a balance between the two
legs of the bridge circuit is called a
Wheatstone Bridge. As shown in the
figure below, three resistances are known
(one is variable/adjustable) and the
fourth one has to be found out.
Compared to the other measuring
instruments such as voltage divider, the
concept of Wheatstone bridge is widely
used because of the accuracy in its
measurement of resistance.

6. Components of
Wheatstone Bridge

 A resistor with an unknown resistance
value.
 Two resistors (with known resistance
value)
 Variable Resistor (a device like
Rheostat or Preset could work)
 Voltage/DC source
 Galvanometer (or any device which
indicates the voltage difference or the
flow of current)
 Connecting Wires
 Circuit Construction
 Construction of Wheatstone Bridge

7. CIRCUIRT
CONSTRUCTION










A Wheatstone bridge is a bridge-type
structure having four resistors, three of
known and one of unknown value.

Here R1, R2, and R3 have known values
among which R2 is adjustable and finally
Rx is the value to be measured. Along
with these resistances, a galvanometer
(Vg) is there between B & D, and a DC
supply between A & C.

8. WORKING PRINCIPLE
OF WHEATSTONE
BRIDGE

Now according to the Wheatstone
bridge principle if the ratio of the two
resistances (R1/R2) on one edge is
equal to the ratio of the two resistances
(R3/Rx) on another edge then there will
be no flow of current between the
midpoints of the two edges of
resistance. This condition of the bridge
is known as the Balanced Bridge
Condition.

In the Balanced Bridge condition, the
current through the galvanometer is
zero and also the voltage difference
between the points B & D becomes
zero, i.e., at both points voltage level
would be the same.

9. Writing equations for the balanced
bridge condition would look like:

R1/R2 = R3/Rx (or) R1 * Rx =R2 * R3

Thus, Rx = R3 * (R2/R1)

This detection of zero current in
galvanometer is of high precision, thus
depending on the level of precision of
known values, the unknown resistance
can be found with the highest rate of
accuracy and precision.

In the Wheatstone bridge experiment,
one resistor should always be variable
in order to obtain a balanced condition.
The Circuit performs at its best when
the regulated voltage source is used,
instead of the current with repelling
characteristics.

10. EXAMPLE CIRCUIT

Let us consider the below circuit where
the bridge is in an unbalanced condition
and we need to calculate the voltage
difference between Q1 and Q2, i.e., Volt
and hence the value of R4 needed to
make the bridge balanced.

11. Example of Wheatstone Bridge
As per the voltage division law,

Vq1 = (R3/(R3+R1)) * Vs ,where Vs
=100volts (voltage source)

Putting values of R3 = 40 ohms, R1=50
ohms, and Vs= 100 volts, we get

Vq1 = 44.4 volts

Similarly, Vq2 = (R4/(R4+R2)) * Vs

putting the values, R4 =50 ohms, R2
=100 ohms, and Vs =100 volts, we get

Vq2 =33.3 volts

Thus, Volt can be found as,

Volt = Vq1 – Vq2

So, Volt = 44.4 – 33.3 = 11.1 volts

12. Now to make the bridge balanced, we can
find a suitable value for R4 as done
below:

R4 = R2 * (R3/R1)

putting the values of R1, R2, and R3, we
have

R4 = 100 * (40/50)

= 80 volts

Therefore, R4 = 80 volts is the value of
resistor which should be used to make
the bridge in a balanced condition.

13. APPLICATIONS OF
WHEATSTONE BRIDGE


 Used in Light detecting devices.
 For measuring the changes in the
pressure.
 For measuring the changes in the
strain of the circuit.
 Used for the Sensing of mechanical
and electrical quantities.
 Also, photo resistive devices use this
circuit.
 Thermometers also use Wheatstone
bridges for the temperature
measurements which need to be
accurate.
 Values like capacitance, inductance,
impedance, etc. can be measured
with some variations in the
Wheatstone bridge circuit.

14. LIMITATIONS OF
WHEATSTONE BRIDGE

Along with all these advantages, there
are a few limitations of the Wheatstone
bridge as well, such as:

 Readings may be inaccurate under
unbalanced conditions.
 The range of measured resistance
varies from a few ohms to mega
ohms.
 Susceptibility for high dc current is
not there.

15. SUMMARY


 Created in 1833, popularized in 1840s
 Wheatstone bridges are one of the
best methods of measuring resistance
due to the basic mathematical ratio
involved.
 Accurate standards with sensitive
enough voltmeter, measurements of
resistance within .05% can be
reached.
 Many calibration laboratories still use
this method today.
 The Wheatstone Bridge are
replaceable; however, for its
simplicity and versatility the circuit is
an indispensible piece of technology

16. BIBLIOGRAPHY



WEBSITES-
www.google.com
www.wikipedia.com
www.scribd.com

Books-
NCERT CLASS
12
th
Physics
PRACTICAL
Physics
CLASS 12
th

17. PRECAUTIONS




 Clean the connecting wires properly.
 Hold the jockey perpendicular to the
wire.
 All connections should be neat and
tight.