Question

The voltmeter shown inthe figure reads 18v  across the 50ohms  resistor. Find the resistance the voltmeter.

9c33a2776b55a902324b0ef2b59d819b.pdf

Answer 1

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

Answer 2

I=V/R
So 18/15
1.2 ohms