Question

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 error due to the loading effect of the voltmeter is ________

(a) 3.33%

(b) 6.67%

(c) 13.34%

(d) 13.67%

This question was addressed to me in an online interview.

My doubt stems from Advanced Problems on Error Analysis in Electrical Instruments topic in section Measurement of Resistance of Electrical Measurements

Answer 1

Right choice is (b) 6.67%

The explanation is: RT = \( \frac{V_T}{I_T}\)

VT = 200 V, IT = 10 A

So, RT = 20 kΩ

Resistance of voltmeter,

RV = 1000 × 300 = 300 kΩ

Voltmeter is in parallel with unknown resistor,

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

Percentage error = \(\frac{Actual-Apparent}{Actual}\) × 100

= \(\frac{21.43-20}{21.43}\) × 100 = 6.67%.