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A voltmeter has a sensitivity of 1000 Ω/V reads 200 V on its 300 V scale. When connected across an unknown resistor in series with a millimeter. When the milliammeter reads 10 mA. The actual resistance of the unknown resistor will be?(a) 20 kΩ(b) 18.57 kΩ(c) 21.43 kΩ(d) 22.76 kΩI have been asked this question in an international level competition.My question is based upon Advanced Problems on Error Analysis in Electrical Instruments topic in chapter Measurement of Resistance of Electrical Measurements

Answer» CORRECT option is (c) 21.43 kΩ

The best explanation: Resistance of voltmeter,

RV = 1000 × 300 = 300 kΩ

The Voltmeter is in parallel with an unknown resistor,

RX = \(\frac{R_T R_V}{R_T – R_V} = \frac{20 × 300}{280}\) = 21.43 kΩ.


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