An ammeter is convertible to a voltmeter by

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Option C: Putting a large resistance in series with the actual measuring part of the instrument

Explanation & Concepts:
An ammeter is converted into a voltmeter by connecting a high-value resistance, known as a multiplier, in series with the instrument. This configuration increases the overall resistance of the circuit to ensure that only a small, proportional current flows through the meter movement, allowing it to measure higher voltage levels without damage.

Working Principle: The moving coil instrument measures current $I_m$. To measure a voltage $V$ higher than the basic range $V_m = I_m \cdot R_m$, a multiplier resistance $R_{se}$ is placed in series. By Ohm's Law, $V = I_m(R_m + R_{se})$. Thus, the multiplier is calculated as $R_{se} = R_m(\frac{V}{V_m} - 1)$, which limits current to the full-scale deflection value while dropping the excess voltage across $R_{se}$.

Key Formulas: $R_{se} = R_m(m - 1)$ — where $m = \frac{V}{V_m}$ is the multiplying factor; $V = I_m(R_m + R_{se})$ — total voltage across the voltmeter

Key Points: A voltmeter must have very high internal resistance to minimize loading effects on the circuit being measured.; The multiplier resistance $R_{se}$ is typically made of Manganin or Constantan due to their low temperature coefficient of resistance.; The multiplier allows the instrument to measure voltages significantly higher than the voltage drop across the meter's moving coil.

Advantages: Extends the utility of a basic PMMC movement for diverse voltage ranges.; High series resistance ensures minimal power consumption from the source.

Disadvantages: The addition of high resistance introduces measurement error if the source impedance is high.; Accuracy is dependent on the stability and precision of the multiplier resistor.

Applications: Multimeter construction.; Calibration of voltage measuring instruments.; Industrial instrumentation panels.

Additional Info: Option B is incorrect because a parallel resistance (shunt) is used to extend the current range of an ammeter.. Option D is incorrect because connecting a low-resistance ammeter in parallel acts as a short circuit, potentially damaging the device.

Answer: C is correct — An ammeter is converted to a voltmeter by adding a high-resistance multiplier in series to limit current flow.

Exam Tip: Always remember: Ammeter = Shunt (low resistance in parallel); Voltmeter = Multiplier (high resistance in series).

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Option A: Changing the scale

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Option B: Putting a large resistance in parallel with the actual measuring part of the instrument

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Option D: Simply installing the instrument in parallel with the circuit

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