To avoid the effect of stray magnetic field in A.C. bridges we can use

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Examoogle · Accepted Answer

Magnetic screening

Explanation: Magnetic screening is used in A.C. bridges to protect components from external electromagnetic interference and stray magnetic fields. By enclosing sensitive bridge components within a high-permeability magnetic shield, the flux linkage from external sources is redirected around the component, preventing induced EMF errors in measurement.

Working Principle: When a bridge circuit is subjected to an external time-varying magnetic field, induced voltages ($e = -N \frac{d\Phi}{dt}$) occur in the inductive components of the bridge, causing inaccurate null detection. A magnetic screen made of high-permeability material (like Mu-metal) provides a low-reluctance path for external flux, diverting it away from the sensitive bridge elements. This significantly reduces the stray magnetic flux density ($\,B\,$) inside the shielded volume, thereby minimizing magnetic interference.

Key Formulas: $e = -N \frac{d\Phi}{dt}$ — Induced EMF due to stray flux; $S = \frac{l}{\mu A}$ — Reluctance of the magnetic path

Key Points: Stray magnetic fields cause inductive interference in A.C. bridge circuits.; Magnetic screens utilize materials with high relative permeability ($\,\mu_r\,$).; Screening is essential at low frequencies where electromagnetic interference is dominant.; Electrostatic shielding is distinct and uses conductive enclosures to address capacitive stray effects.

Advantages: High effectiveness against low-frequency magnetic interference; Does not require physical disconnection of the circuit; Improves accuracy of high-sensitivity impedance measurements

Disadvantages: Adds bulk and weight to the measurement setup; Expensive materials like Mu-metal are required for best performance

Applications: High-precision inductance bridges (e.g., Maxwell bridge); Laboratory-grade impedance analyzers; Sensitive electronic instrumentation

Additional Info: Option B (Wagner earthing device) is primarily used to eliminate the effect of stray capacitance to earth, not magnetic interference.. Option C (Wave filters) are used to eliminate harmonic content from the supply source rather than external field interference.

Answer: A is correct — Magnetic screening is the standard technique to mitigate errors induced by stray magnetic flux in sensitive A.C. bridge components.

Exam Tip: Always differentiate between magnetic screening (for flux interference) and Wagner earthing (for stray capacitance/grounding issues) in A.C. bridge questions.

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