Question

A D.C supply of 120 V is connected to a large resistance X. A voltmeter of resistance 10 kW placed in series in circuit reads 4V. This is an unusual use of voltmeter for measuring very high resistance. The value of X is –

• A. 390 kW
• B. 290 kW
• C. 190 kW
• D. 300 kW

Answer 1

Answer: (B)

290 kW

Answer 2

i = $\frac{120}{X + 10K\Omega}$

Reading of voltmeter = $\frac{120}{X + 10K\Omega}$ × 10

$\frac{1200}{X + 10K\Omega}$ = 4

X = 290 KW