A sensitive galvanometer can be converted into a voltmeter suitable for measuring potential difference by connecting which type of resistance with it?

Introduction / Context:
A galvanometer is a delicate instrument that detects and measures small currents. With suitable modifications, it can be used either as an ammeter or as a voltmeter. The question asks how to convert a galvanometer into a voltmeter, which measures potential difference across a circuit element. Knowing which resistance arrangement achieves this conversion is an important application of basic circuit principles and is frequently asked in electricity and electronics questions.

Given Data / Assumptions:

• We start with a sensitive galvanometer that can measure small currents.
• The goal is to modify it so that it measures voltage (potential difference).
• We may connect either high or low resistance, in series or parallel, with the galvanometer.
• We assume the galvanometer has a known internal resistance and full scale deflection current.

Concept / Approach:
A voltmeter is designed to measure potential difference across a component without drawing significant current from the circuit. To achieve this, it must have very high resistance so that only a small current flows through it even at the maximum voltage it is supposed to measure. To convert a galvanometer into a voltmeter, a suitable high resistance, called a multiplier, is connected in series with the galvanometer. The series combination then has a large total resistance. When a voltage is applied across this combination, the current is limited, and the galvanometer deflects proportionally to the applied voltage. Connecting high resistance in parallel would lower the total resistance, which is undesirable for a voltmeter.

Step-by-Step Solution:
Step 1: Recall that a good voltmeter should have very high resistance to minimise current drawn from the circuit.Step 2: The galvanometer by itself has relatively low resistance and is designed for small currents.Step 3: To increase the overall resistance, we connect a high resistance in series with the galvanometer, creating a large combined resistance.Step 4: When a voltage is applied across this series combination, the current is limited and proportional to the voltage due to Ohm's law, allowing calibration in volts.Step 5: Therefore, the correct modification is to connect a high resistance in series with the galvanometer.

Verification / Alternative check:
Compare this with the method of converting a galvanometer into an ammeter. For an ammeter, we want very low resistance so that it does not change the current in the circuit. A low resistance shunt is connected in parallel with the galvanometer in that case. This contrast helps you remember that voltmeters require high resistance in series, while ammeters require low resistance in parallel. Textbooks on basic electronics and circuit theory consistently explain voltmeter conversion using a high series resistance, confirming the correctness of this arrangement.

Why Other Options Are Wrong:
High resistance in parallel, option A, would create a low equivalent resistance path and is used for ammeter shunts, not for voltmeters. Low resistance in parallel, option B, again lowers the overall resistance and is associated with ammeter conversion, not voltmeter conversion. Low resistance in series, option D, would not sufficiently increase the total resistance and would allow too much current to flow, disturbing the circuit and possibly damaging the galvanometer. Only option C, high resistance connected in series with the galvanometer, satisfies the requirement for a voltmeter.

Common Pitfalls:
Students often mix up the modifications needed for ammeters and voltmeters, especially under exam pressure. A useful memory aid is to associate voltmeters with "very high resistance" and the word series, while associating ammeters with "very low resistance shunt" and the word parallel. Also, always think about the effect on the circuit: voltmeters must not significantly change the circuit conditions, so they must draw minimal current, which requires high resistance. Keeping this physical reasoning in mind helps you choose the correct configuration even if you momentarily forget the standard rule.

Final Answer:
High resistance connected in series with the galvanometer