Question

To measure potential difference from a galvanometer, we connect in it:

• A. a high resistance in series
• B. a low resistance in parallel
• C. a high resistance in parallel
• D. a low resistance in series

Hint:

Voltmeter \(⇒\) {V}ery large internal resistance (add a big series resistor).
Ammeter \(⇒\) {A}lmost zero internal resistance (add a small parallel shunt).

Answer 1

The Correct Option is A

Step 1 (What a galvanometer does). A moving–coil galvanometer is a current detector with internal resistance \(R_g\) and full–scale current \(I_g\). By itself it measures small currents, not voltage.
Step 2 (Convert to voltmeter). To measure a potential difference \(V\) without disturbing the circuit, the instrument must draw very little current. We therefore add a large series (multiplier) resistance \(R_s\): \(I=\dfrac{V}{R_s+R_g}\).
Choosing \(R_s\) so that \(I\le I_g\) at full–scale makes a voltmeter whose internal resistance \(R_V=R_s+R_g\) is very large, hence negligible loading of the circuit.
Step 3 (Why others are wrong). A low parallel resistance (shunt) is for ammeters, not voltmeters; a high parallel resistance diverts current unpredictably; a low series resistance draws too much current and loads the circuit.
\(⇒\) We must connect a high resistance in series.