The circuit is shown in Fig. 1.18.
The wattage of a lamp is given by :
W = I2 R = V2/R
∴ 100 = 2302/R
∴ R = 529 Ω
Resistance of each lamp under stated condition is = 6 × 529 = 3174 Ω
Equivalent resistance of these four lamps connected in parallel = 3174/4 = 793.5 Ω
This resistance is connected in series with the voltmeter of 1500 Ω resistance.
∴total circuit resistance = 1500 + 793.5 = 2293.5 Ω
∴ circuit current = 230/2293.5 A
According to Ohm’s law, voltage drop across the voltmeter = 1500 × 230/2293.5 = 150 V (approx)