Moving Coil Galvanometer – Complete Notes, Revision, Important Questions & Downloads
Moving Coil Galvanometer is centred on Galvanometer Working Principle and Sensitivity: the radial magnetic field gives maximum torque on the coil, the torsion fibre supplies restoring torque, and the final balance produces a linear current-deflection relation. NEET tests this topic through the working equation i = K alpha, through current sensitivity Si = NBA/C, and through voltage sensitivity Sv = NBA/RC. A standard exam move is to ask which quantity must increase or decrease to make the instrument more sensitive, so the student must read the formula rather than memorise only the device name.
NEET Weightage & Exam Pattern
Magnetic Effect of Current| NEET Year | Questions from this Topic | Bar | Marks |
|---|---|---|---|
| 2024 | 1 | 4 | |
| 2023 | 1 | 4 | |
| 2022 | 1 | 4 | |
| 2021 | 2 | 8 | |
| 2020 | 1 | 4 | |
| Topic Weightage | 6 | 24 |
The radial magnetic field is not decorative design language: it keeps sin theta equal to 1 and makes deflection proportional to current.
Current sensitivity and voltage sensitivity are linked but not identical, because voltage sensitivity also depends on circuit resistance.
Preparation Strategy
Start from Torque Balance Remember that magnetic torque NBiA is balanced by restoring torque C alpha. The linear relation between current and deflection follows from this equilibrium, not from a memorised device fact.
Keep the Role of Radial Field Clear The soft iron core and cylindrical pole pieces make the field radial so the deflecting torque stays maximum. This is why the scale becomes linear.
Separate Current Sensitivity from Voltage Sensitivity Current sensitivity rises with N, B, and A and falls with C, while voltage sensitivity also contains the resistance R in the denominator. Students often improve one mentally and forget the other.
Practise Which Quantity to Change Most NEET questions are framed as design choices: increase turns, decrease torsional constant, or change resistance. Read the correct formula and decide from the denominator and numerator rather than guessing by intuition.
Download Topic Notes
PDF · Cheat Sheet · MCQ Set · PYQTopic Coverage
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Quick Revision
Concept → Trap → Example1) Galvanometer Working Principle and Sensitivity
Instrument CoreIn a moving coil galvanometer, magnetic torque NBiA balances restoring torque C alpha, giving i = K alpha where K = C/NBA. Current sensitivity is Si = alpha/i = NBA/C, while voltage sensitivity is Sv = alpha/V = NBA/RC.
- The radial magnetic field keeps the plane of the coil parallel to the field, so the deflecting torque remains maximum for all small deflections.
- Increasing N, B, or A increases current sensitivity, while increasing torsional constant C reduces it.
- Trap: voltage sensitivity does not increase just because current sensitivity increases; the resistance term R must also be checked.
US Curriculum Gaps
Note for NRI/OCI students studying abroad.Instrument Design Details Matter
Some courses describe galvanometers as black-box current detectors, while NEET expects the student to know why the field is radial and how that produces a linear scale.
- soft iron core strengthens and shapes the field
- radial field keeps the torque condition simple
Sensitivity Questions Are Parameter Questions
The exam usually asks how sensitivity changes when N, B, A, C, or R changes, so the student must read the formulas structurally rather than reciting definitions only.
- current sensitivity depends on NBA/C
- voltage sensitivity depends on NBA/RC
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Frequently Asked Questions
Notes · Downloads · Revision · Important QuestionsWhat does a moving coil galvanometer measure?
Why is the magnetic field radial inside the galvanometer?
What is the galvanometer constant?
What increases current sensitivity?
Why is voltage sensitivity not the same as current sensitivity?
Why is deflection proportional to current?
What is the common design trap in sensitivity questions?
What is the most common exam mistake in this topic?
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