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GATE 2019 EE – Question 42

Electrical and Electronic Measurements · Measurement of voltage, current, power, energy and power factor · 2 marks · Multiple choice

A moving coil instrument having a resistance of $10\ \Omega$, gives a full-scale deflection when the current is 10 mA. What should be the value of the series resistance, so that it can be used as a voltmeter for measuring potential difference up to 100 V?

  1. $9\ \Omega$
  2. $99\ \Omega$
  3. $990\ \Omega$
  4. $9990\ \Omega$

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Correct answer: (D) $9990\ \Omega$

Explanation

For 100 V at 10 mA the total resistance must be $\frac{100}{0.01}=10{,}000\ \Omega$. Subtracting the coil's $10\ \Omega$ gives a series resistance of $9990\ \Omega$.