The best way to increase current sensitivity of a galvanometer is by
The best way to increase current sensitivity of a galvanometer is by
Options
Correct option: (A) increasing number of turns of the coil.
Current sensitivity is the deflection produced per unit current:
where N = number of turns, A = area, B = field, k = torsional constant.
It increases with N, A, B and with a smaller k. The most practical and effective way is to increase the number of turns N — turns can be added easily without making the instrument bulky (unlike increasing A) and without the practical limits of raising B, giving a directly proportional rise in sensitivity.
Marking Scheme
- 11 mark: option (A) increasing number of turns of the coil.
- 2No step-by-step required (single-correct MCQ); reasoning: current sensitivity = rises with N, the most practical parameter to change.
Hint
Current sensitivity = . It is directly proportional to N, so adding turns is the easiest way to raise it.
Quick Oral Answer
Current sensitivity equals , so it is directly proportional to the number of turns N; increasing N is the simplest and most effective way to raise sensitivity, since enlarging the coil area makes the meter bulky and raising the field or lowering the spring constant have practical limits.
Analysis & Explanation
Why (A) is right: Current sensitivity = . It rises in direct proportion to the number of turns N, and adding turns is the simplest practical change — you cannot keep enlarging the coil area (the meter becomes bulky) or endlessly raise the field strength, and reducing k weakens the spring. Hence increasing N is the standard 'best way'.
Why the distractors are wrong:
- (B) Increasing area A and field B does raise sensitivity, but enlarging the coil makes the galvanometer bulky and there is a practical ceiling to the field strength — so it is not the best/most convenient method.
- (C) Decreasing A and B reduces , so sensitivity falls — the opposite of what is wanted.
- (D) Increasing the torsional constant k puts k in the denominator, so sensitivity decreases; a stiffer spring gives less deflection per unit current.
Common Mistakes
- 1Choosing (D) — thinking a stiffer spring (larger k) helps, when k in the denominator actually reduces sensitivity.
- 2Choosing (C) — decreasing area/field reduces and lowers sensitivity.
- 3Overlooking that although (B) increasing A and B does raise sensitivity, it is impractical (bulky coil, field limits), so (A) is the 'best' answer.
Interesting Facts
Current sensitivity and voltage sensitivity differ: increasing N raises current sensitivity but also raises coil resistance R, so voltage sensitivity may not improve.
Modern digital multimeters can resolve currents down to nanoamperes, far beyond a classic moving-coil galvanometer, yet still rest on the same NAB/k principle.
The torsional constant k of the fine phosphor-bronze suspension can be made extremely small, which is how ultra-sensitive mirror galvanometers detect tiny currents.
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Frequently Asked Questions
What is the formula for current sensitivity?
Current sensitivity is the deflection per unit current, , where N is the number of turns, A the coil area, B the magnetic field and k the torsional constant of the suspension. It increases with N, A and B, and decreases as k increases.
Why is increasing the number of turns the 'best' way?
Sensitivity is directly proportional to N, and adding turns is the easiest practical change — it does not make the meter bulky like enlarging the area, does not hit the physical ceiling of field strength, and does not weaken the restoring spring. So increasing N is the most convenient and effective method.
Does increasing sensitivity have any drawback?
Increasing the number of turns also increases the coil's resistance R. Because voltage sensitivity is , boosting current sensitivity by adding turns may not improve voltage sensitivity, so the two sensitivities must be considered separately.