The Wheatstone Bridge is one of the most important topics in Electrical Engineering for competitive exams like SSC JE, RRB JE, GATE, UPPCL JE, PSU, and AE exams. It is mainly used for the accurate measurement of unknown resistance by balancing two legs of a bridge circuit.
The Wheatstone bridge is widely asked in JE/AE exams because it is a fundamental measuring instrument topic and forms the base for many sensors and transducer circuits.
What is Wheatstone Bridge?
A Wheatstone bridge is an electrical bridge circuit used to determine an unknown resistance by comparing it with known resistances. The bridge works on the principle of null deflection, which means no current flows through the galvanometer when the bridge is balanced.
Definition of Wheatstone Bridge
Wheatstone Bridge Definition: Wheatstone bridge is an electrical circuit used for precise measurement of unknown resistance by balancing two arms of a bridge network.
Wheatstone Bridge Circuit Diagram
Construction of Wheatstone Bridge
The Wheatstone bridge consists of:
- Four resistors arranged in bridge form
- One DC voltage source
- One galvanometer
- Connecting wires
The four resistances are:
- P → First known resistance
- Q → Second known resistance
- R → Variable known resistance
- S → Unknown resistance
Working Principle of Wheatstone Bridge
The Wheatstone bridge works on the principle that:
When the ratio of resistances in one branch equals the ratio in another branch, the bridge becomes balanced and no current flows through the galvanometer.
At balance condition:
Therefore,
Using this equation, the unknown resistance can be calculated easily.
Balance Condition of Wheatstone Bridge
The bridge is balanced when:
- No current flows through the galvanometer
- Galvanometer shows zero deflection
- Potential difference across galvanometer becomes zero
Balance Equation
or
Derivation of Wheatstone Bridge Formula
Consider the Wheatstone bridge circuit with resistances P, Q, R, and S.
Let:
- I₁ = Current through branch P and Q
- I₂ = Current through branch R and S
At balanced condition, no current flows through galvanometer.
Therefore potential drops are:
I₁P = I₂R
Similarly,
I₁Q = I₂S
Dividing both equations:
Hence,
Experiment on Wheatstone Bridge
Aim
To determine the unknown resistance using Wheatstone bridge.
Apparatus Required
- Wheatstone bridge setup
- Galvanometer
- Battery
- Known resistors
- Unknown resistor
- Connecting wires
- Plug key
Circuit Diagram for Experiment
Procedure
- Connect the circuit properly as per the diagram.
- Insert known resistances in the bridge arms.
- Adjust the variable resistance.
- Observe galvanometer deflection.
- Adjust resistance until galvanometer shows zero deflection.
- Note the resistance values.
- Calculate unknown resistance using bridge balance equation.
Observation Table
| P (Ω) | Q (Ω) | R (Ω) | Unknown Resistance S (Ω) |
|---|---|---|---|
| 10 | 20 | 15 | 30 |
Calculation
S = (20 × 15)/10
S = 30 Ω
Result
The unknown resistance is found successfully using Wheatstone bridge.
Advantages of Wheatstone Bridge
- Very accurate method of resistance measurement
- Highly sensitive
- Simple construction
- Reliable results
- Used for low resistance measurements
- Error can be minimized easily
Disadvantages of Wheatstone Bridge
- Not suitable for very high resistances
- Balancing takes time
- Errors occur due to heating effects
- Not suitable for rapidly changing resistances
Applications of Wheatstone Bridge
- Measurement of unknown resistance
- Used in strain gauges
- Used in temperature sensors
- Used in transducer circuits
- Used in instrumentation systems
- Calibration of measuring instruments
Conditions for Maximum Sensitivity of Wheatstone Bridge
- All resistances should be of same order
- Galvanometer should be highly sensitive
- Battery voltage should be appropriate
- Connections should be tight and proper
Difference Between Wheatstone Bridge and Meter Bridge
| Wheatstone Bridge | Meter Bridge |
|---|---|
| Uses resistors for ratio arms | Uses uniform wire |
| Highly accurate | Moderate accuracy |
| Complex setup | Simple setup |
| Used in laboratories | Used for educational purposes |
Wheatstone Bridge MCQs for JE AE Exams
Q1. Wheatstone bridge is used to measure?
A) Voltage
B) Current
C) Resistance
D) Power
Answer: C) Resistance
Q2. Wheatstone bridge works on which principle?
A) Electromagnetic induction
B) Null deflection
C) Heating effect
D) Electrostatic effect
Answer: B) Null deflection
Q3. At balance condition, galvanometer current is?
A) Maximum
B) Minimum
C) Zero
D) Infinite
Answer: C) Zero
Q4. Balance equation of Wheatstone bridge is?
A) P + Q = R + S
B) P/Q = R/S
C) P × Q = R × S
D) P − Q = R − S
Answer: B) P/Q = R/S
Q5. Which instrument is used to detect balance condition?
A) Ammeter
B) Voltmeter
C) Wattmeter
D) Galvanometer
Answer: D) Galvanometer
Important Points for SSC JE and RRB JE Exams
- Wheatstone bridge is based on null deflection principle.
- At balance condition galvanometer current becomes zero.
- Balance equation: P/Q = R/S
- Used for accurate resistance measurement.
- Very important topic for electrical measurements.
FAQs on Wheatstone Bridge
What is Wheatstone bridge used for?
Wheatstone bridge is used for accurate measurement of unknown electrical resistance.
What is the principle of Wheatstone bridge?
It works on the principle of null deflection where no current flows through galvanometer at balance condition.
What happens when Wheatstone bridge is balanced?
When the bridge is balanced, galvanometer shows zero deflection and no current flows through it.
What is the formula of Wheatstone bridge?
The balance formula is:
P/Q = R/S
Which instrument is used in Wheatstone bridge?
A galvanometer is used to detect balance condition in Wheatstone bridge.
Why Wheatstone bridge is more accurate?
Because it uses null deflection method which reduces measurement errors significantly.
Can Wheatstone bridge measure high resistance?
No, Wheatstone bridge is generally suitable for low and medium resistance measurement.
What are the applications of Wheatstone bridge?
It is used in strain gauges, sensors, transducers, and resistance measurement circuits.
Conclusion
The Wheatstone bridge is an important electrical measuring instrument used for precise resistance measurement. It works on the null deflection principle and provides highly accurate results. The topic is very important for SSC JE, RRB JE, GATE, and other AE/JE electrical exams because theoretical as well as numerical questions are frequently asked from it.