Wheatstone bridge
Study the principle of the Wheatstone bridge for precise resistance measurement and its balanced condition.
What is Wheatstone bridge?
An electrical circuit used for measuring an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes an unknown component.
Key formula / rule: Balance Condition of Wheatstone Bridge
Key points
- Understand the construction and working principle of a Wheatstone bridge.
- Derive the condition for the balance of the Wheatstone bridge.
- Calculate an unknown resistance using the balanced Wheatstone bridge formula.
- Recognize the applications of the Wheatstone bridge.
Common exam trap
Incorrectly applying the balance condition (e.g., P/R = Q/S).
Definitions
- Term
Wheatstone Bridge
- Meaning
An electrical circuit used for measuring an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes an unknown component.
- Term
Galvanometer
- Meaning
An instrument for detecting and measuring small electric currents.
- Term
Null Detection
- Meaning
The process of detecting a condition where no current flows through a measuring instrument (like a galvanometer), indicating a balanced state in a circuit.
Learning objectives
Understand the construction and working principle of a Wheatstone bridge.
Derive the condition for the balance of the Wheatstone bridge.
Calculate an unknown resistance using the balanced Wheatstone bridge formula.
Recognize the applications of the Wheatstone bridge.
Formulae
- Name
Balance Condition of Wheatstone Bridge
- Note
Where P, Q, R are known resistances and S is the unknown resistance. This holds true when the galvanometer shows zero deflection.
- Expression
\frac{P}{Q} = \frac{R}{S}
- Name
Calculation of Unknown Resistance
- Note
Rearranged from the balance condition to directly find the unknown resistance S.
- Expression
S = R \× \frac{Q}{P}
Prerequisites
Ohm's Law
Kirchhoff's Laws (especially Kirchhoff's Voltage Law and Current Law)
Series and Parallel combinations of resistors
Concept of potential difference and current
Common mistakes
Incorrectly applying the balance condition (e.g., P/R = Q/S).
Assuming the bridge is balanced without checking galvanometer deflection.
Not considering the internal resistance of the galvanometer or battery.
Using it for very high or very low resistances without modifications.
Keywords
Wheatstone Bridge
Resistance Measurement
Balance Condition
Galvanometer
Null Detection
Kirchhoff's Laws
Electrical Circuits
Practice preview
In a Wheatstone bridge, if the galvanometer is replaced by a voltmeter, what will be the reading when the bridge is balanced?…
easy
A Wheatstone bridge is balanced. If the galvanometer resistance is 50 Ω and its sensitivity is such that a current of 1 mA causes full-scale deflection. If the galvanometer shows half-scale deflection, what is the curren…
medium
In a Wheatstone bridge, the resistances in the four arms are P, Q, R, and S. The bridge is balanced. If the battery voltage is doubled, what happens to the current through the galvanometer?…
medium