Answers

2014-04-18T22:02:28+05:30
As across 50 ohms voltage is  18v
then across 25 ohm it is 30-18=12 v
and the current through it is I=V/R=12/25=0.48 Ampere
as potential difference across 50 ohm and volt meter is same
let current through voltmeter be Iv and r be its resistance
r(Iv)=50(0.48-Iv)
r(Iv)=24-50(Iv)..(1)
total combination resistance is 
 \frac{1}{Rc}= \frac{1}{50}+ \frac{1}{r}= \frac{r+50}{50r}
Rc= \frac{50r}{r+50}
as
 \frac{V}{I}=Rc
 \frac{18}{0.48}=Rc=37.5
 \frac{50r}{r+50}=37.5 \\ 50r=37.5r+37.5(50)
12.5r=37.5(50)
r=150
therefore resistance of voltmeter is 150 ohm
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2014-04-19T07:51:00+05:30