Two resistors of 100Ω each are connected in series with a 9 V batte1y. A voltmeter of 400Ω resistance is connected to measure the voltage drop across one of the resistors. The voltmeter reading is ____ V.

We need to find the voltmeter reading when a 400 ohm voltmeter is connected across one of two 100 ohm resistors in series with a 9V battery.

Two resistors: $$R_1 = R_2 = 100 \, \Omega$$ in series

Battery voltage: $$V = 9$$ V

Voltmeter resistance: $$R_V = 400 \, \Omega$$

The voltmeter (400 ohm) is connected in parallel with one 100 ohm resistor:

$$R_{parallel} = \frac{R_1 \times R_V}{R_1 + R_V} = \frac{100 \times 400}{100 + 400} = \frac{40000}{500} = 80 \, \Omega$$

$$R_{total} = R_{parallel} + R_2 = 80 + 100 = 180 \, \Omega$$

$$I = \frac{V}{R_{total}} = \frac{9}{180} = 0.05 \text{ A}$$

$$V_{reading} = I \times R_{parallel} = 0.05 \times 80 = 4 \text{ V}$$

Therefore, the voltmeter reading is Option 1: 4 V.