Which of the following instruments has the highest internal resistance?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Voltmeter
A galvanometer is converted into an ammeter by a low shunt resistance in parallel, and into a voltmeter by a high resistance in series.
\( \displaystyle \text{Ammeter}: R_A = \frac{GS}{G+S} \lt S \text{ (very small)} \)
\( \text{Voltmeter}: R_V = G + R \text{ (very large)} \)
A milliammeter needs a larger shunt than an ammeter, so its resistance lies between those of the ammeter and the galvanometer. The order is ammeter < milliammeter < galvanometer < voltmeter, so the voltmeter has the highest resistance. It must, because it is connected in parallel and should draw almost no current.
NCERT (2026-27):
The Moving Coil Galvanometer – “… For measuring currents, the galvanometer has to be connected in series, and as it has a large resistance, this will change the value of the current in the circuit. To overcome these difficulties, one attaches a small resistance \( r_s \), called shunt resistance, in parallel with the galvanometer coil; so that most of the current passes through the shunt.”
The Moving Coil Galvanometer – “… Usually we like to keep the disturbance due to the measuring device below one per cent. To ensure this, a large resistance \( R \) is connected in series with the galvanometer. … Note that the resistance of the voltmeter is now,
\( \displaystyle R_G + R \simeq R : \mathrm{large} \)”