Physics · Moving Charges And Magnetism · NEET
No. Current sensitivity SI = NBA/C is deflection per unit current (div/A). Voltage sensitivity SV = NBA/CR is deflection per unit voltage (div/V). They differ by one factor of R, the galvanometer resistance: SV = SI/R. Here N = turns, B = radial field, A = coil area, C = torsion constant of the spring. A galvanometer with high current sensitivity can still have low voltage sensitivity if its resistance R is large.
When you double N (turns), current sensitivity SI = NBA/C doubles, because SI is proportional to N. But the coil is now twice as long a wire, so its resistance R also roughly doubles. Voltage sensitivity SV = NBA/CR has N on top and R on the bottom. If both double, they cancel: SV = (2N)BA / (C x 2R) = NBA/CR = unchanged. So doubling turns raises current sensitivity but leaves voltage sensitivity nearly the same.
Use the master relation SV = SI / R, so R = SI / SV. Watch the units. If SI = 5 div/mA and SV = 20 div/V, convert 5 div/mA = 5 div / (10^-3 A) = 5000 div/A. Then R = 5000 / 20 = 250 ohm. This is the exact NEET 2018 question below. Never divide the raw numbers 5/20 without fixing the mA to A conversion.
A galvanometer becomes an ammeter using a small shunt, and a voltmeter using a large series resistance. For a sensitive voltmeter you want high voltage sensitivity (large deflection for a tiny voltage). For detecting small currents you want high current sensitivity. Because SV = SI/R, low galvanometer resistance helps voltage sensitivity.
Current sensitivity of a moving coil galvanometer is 5 div/mA and its voltage sensitivity (angular deflection per unit voltage applied) is 20 div/V. The resistance of the galvanometer is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Current sensitivity SI = NBA/C, the deflection per unit current, where N = number of turns, B = radial magnetic field, A = coil area, C = torsion constant of the suspension. Its unit is div/A (or rad/A).
Voltage sensitivity SV = NBA/CR = SI/R, the deflection per unit voltage, where R is the galvanometer resistance. Its unit is div/V.
SV = SI / R. Voltage sensitivity equals current sensitivity divided by the galvanometer resistance. This single relation lets you find R = SI/SV, which is exactly what NEET 2018 tested.
No. Increasing turns raises current sensitivity, but it also raises resistance in proportion, so voltage sensitivity stays almost unchanged. This is the classic NCERT observation and a favourite NEET trap.
