INDEX
• INTRODUCTION
• AIM
• MATERIAL REQUIRED
• THEORY
• PRINCIPLE OF WHEATSTONE BRIDGE
• PROCEDURE
• DERIVATION
• APPLICATIONS
• LIMITATIONS
• CONCLUSION
• RESULT
• REFERENCES
INTRODUCTION
A Wheatstone Bridge is an electrical circuit used to measure an
unknown electrical resistance by balancing two legs of a bridge
circuit, one leg of which includes the unknown resistance.
Its primary operation in a circuit is its property to provide extremely
accurate measurement. This
characteristics of a Wheatstone Bridge is
often exploited to calculate resistances of
various electronic devices with
minimal uncertainities . It can
also measure inductance, capacitance,
and frequency with the proper
combination and arrangement of inductances and capacitances
in its arms. At the balance condition the
deflection at the centre is null if and only if the product of the
diagnol resistances are found to be equal, that is :
R2/R1 = R4/R3
AIM
The basic aim of the allotted project is to understand and acknowledge the
significance of the ‘Wheatstone Bridge’ in the field of electricity and the
advancements it unfolded which resulted in direct or indirect progress in field of
science and engineering.
Moreover ,it aims at understanding the working of the Wheatstone Bridge and
the mechanism it follows; the project also emphasises on the significance of
Kirchoff’s Laws as well as Ohm’s law in the Wheatstone Bridge and its derivation;
In the end, project discusses about the unignorable important applications of
wheatstone bridge and its limitations as well as the
result and conclusion .
MATERIAL REQUIRED
1. Bulbs 2. Connecting Wire
3. Tape 4. Cardboard
THEORY
Construction Of Wheatstone Bridge :
A Wheatstone bridge circuit consists of four arms of which two arms
consist of known resistances while the other two consist of an
unknown resistance and a variable
resistance or rheostat.
The circuit also consists of a galvanometer
which is connected at
the centre of the bridge.
The overall construction of a simple Wheatstone bridge can
be illustrated by the figure :
Where :
R1, R3 → Known Resistances
Rx → Unknown Resistances
R2 → Rheostat\Variable Resistance
G→ Galvanometer
E→ Electromotive Force Source
Principle Of Wheatstone Bridge
The Wheatstone bridge works upon the Principle Of Null Deflection
that is the ratio of their resistances is equal and no current flows
through the central circuit.
Under normal conditions ,the bridge is in an unbalanced condition
where current flows through the galvanometer connected at the
centre of the bridge.
The bridge is said to be balanced when no current flows though the
galvanometer and it shows zero or null deflection.
PROCEDURE
1. The bulbs R1 and R2 are connected in series using the connecting
wire . The bulbs R3 and R4 are also connected in series.
2. At point A and B the bulbs are connected in parallel using
connecting wires.
3. When the circuit is ready switch it on. We observe that all the four
bulbs glow except the middle one.
i.e the ratio of
R1/R2 = R3/R4
which is the condition for wheatstone bridge.
4. At the balanced condition there exists no potential difference at
the points A and B. Thus there flows no current through it and hence
the bulb does not glow
5. When the bulb R1 is removed the bulb in the centre glows which
means the bridge is unbalanced
i.e the ratio of
R1/R2 ≠ R3/R4
which does not satisfy the wheatstone bridge condition.
DERIVATION
Consider the circuit shown in the figure, which is called wheatstone bridge. Th
bridge has four resistors R1, R2, R3 and R4. Between the two points B and D a
galvanometer is connected .
The cell has no internal resistance hence no current will be flowing across it.
Kirchoffs junction rule applied to the junctions D and B gives us the relation
I1=I3
I4=I2
We apply Kirchoffs rule to the closed loop ABDA
-I1R1 + 0 + I2R2 = 0
I1I2 = R1R2
𝑰𝟏 𝑹𝟐
= …..(1)
𝑰𝟐 𝑹𝟏
Closed loop CBDC
-I4R4 + 0 + I3R3 = 0
I3I4 = R3R4
𝑰𝟒 𝑹𝟒
= …..(2)
𝑰𝟑 𝑹𝟑
From equation 1 and 2
𝑹𝟐 𝑹𝟒
=
𝑹𝟏 𝑹𝟑
APPLICATIONS
The Wheatstone bridge is used for the precise measurement of low resistance.
• Thermometers also use wheatstone bridge for measuring the temperature
which needs to be accurate.
• Quantities such as impedance, inductance, and capacitance can be
measured using variations on the Wheatstone bridge with much
accuracy.
• Photo resistive device uses this circuit.
• Wheatstone bridge can also be used for measuring strain and
pressure.
LIMITATIONS
•Readings mat be inaccurate when the circuit is unbalanced.
• It is only accurate for measurements of low resistance. If the unknown
resistance has a huge value, then the galvanometer becomes difficult to balance.
• When the resistance draws a huge current from the circuit, it displays heating
effect. This leads to an inaccurate reading.
• The Wheatstone bridge is a delicate device. In an off-balance situation,
measurements may not even be accurate.
• Wheatstone bridges are commonly used to measure the resistance of a few
ohms to those few kilo-ohms.
CONCLUSION
Thus, Wheatstone Bridge is a device used to measure the unknown resistance of
a resistor when the other resistance is known.
The device has four arms as four resistors and the current is balanced as a ratio
of lengths and resistances.
For balanced condition the ratio of
R1/R2 = R3/R4
For unbalanced condition the ratio of
R1/R2 ≠ R3/R4
RESULT
The four bulbs glow when the ratio of
R1/R2 = R3/R4
i.e. no current flows through the centre bulb hence it does not glow.
The centre bulb glows when the ratio of
R1/R2 ≠ R3/R4
i.e. the current flows through the centre bulb
REFERENCE
www.byjus.com
www.wikipedia.com
www.britannaci.com
www.greeksofgeeks.org
