In the circuit shown in figure, a voltmeter reads 30 volts when it is connected across 400 ohm resistance. Calculate what the same voltmeter will read when it is connected across the 300 ohm resistance.

Explanation

Let the resistance of the voltmeter be `R_(V)` and the current flowing through it be `I_(V)`.

Then `I_(V) R_(V) = 30` and `(I - I_(V)) xx 400 = 30`
Also, `300 xx I = 30` or `I = 0.1 A`
Therefore, `0.1 - I_(V) = 3//40`
or `I_(V) = 0.1 -(3)/(40) = (4 - 3)/(40) = (1)/(40) A`
`:. (1)/(40) xx R_(V) = 30` or `R_(V) = 1200 Omega`

For the circuit in Fig. `S6.7`,
`I_(V) xx R_(V) = (I - I_(V)) 300` or `I_(V) xx 1200 = (I - I_(V)) 300`
or `4I_(V) = I - I_(V)` or `I = 5I_(V)` (i)
Also, `300 (I - I_(V)) + 400 I = 60` or `700 I - 300 I_(V) = 60`
`3500 I_(V) - 300 I_(V) = 60` or `I_(V) = (60)/(3200) = (3)/(160) A`
Therefore, reading of voltmeter is `(3)/(160) xx 1200 = 22.5 V`