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Examoogle • User • info@examoogle.com • EE-2024-8821
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To avoid the effect of stray magnetic field in A.C. bridges we can use
Magnetic screening
Wagner earthing device
Wave filters
Any of the above
Magnetic screening
Quick Summary: 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.
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.
e=−NdtdΦ — Induced EMF due to stray flux
S=μAl — Reluctance of the magnetic path
When a bridge circuit is subjected to an external time-varying magnetic field, induced voltages (e=−NdtdΦ) 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.
Stray magnetic fields cause inductive interference in A.C. bridge circuits.
Magnetic screens utilize materials with high relative permeability (μr).
Screening is essential at low frequencies where electromagnetic interference is dominant.
Electrostatic shielding is distinct and uses conductive enclosures to address capacitive stray effects.
High effectiveness against low-frequency magnetic interference
Does not require physical disconnection of the circuit
Improves accuracy of high-sensitivity impedance measurements
Adds bulk and weight to the measurement setup
Expensive materials like Mu-metal are required for best performance
High-precision inductance bridges (e.g., Maxwell bridge)
Laboratory-grade impedance analyzers
Sensitive electronic instrumentation
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.
A is correct — Magnetic screening is the standard technique to mitigate errors induced by stray magnetic flux in sensitive A.C. bridge components.
Always differentiate between magnetic screening (for flux interference) and Wagner earthing (for stray capacitance/grounding issues) in A.C. bridge questions.