(20 12? Dongying) As shown in the figure, the power supply voltage remains unchanged, and the switch S 1 remains closed. When the switch S2 changes from the off state to the closed state, it indicates

(20 12? Dongying) As shown in the figure, the power supply voltage remains unchanged, and the switch S 1 remains closed. When the switch S2 changes from the off state to the closed state, it indicates that When the switch is off, the equivalent circuit diagram is shown in figure 1. When the switch is closed, the equivalent circuit diagram is shown in Figure 2.

The voltmeter V 1 always measures the power supply voltage before and after the switch S2 is closed.

∴ The indication of voltmeter V 1 remains unchanged;

The voltmeter V2 measures the voltage across the series circuit R 1 before the switch S2 is closed, and measures the power supply voltage after the switch S2 is closed.

∴ According to the fact that the total voltage of the series circuit is equal to the sum of the divided voltages, the index of V2 increases after the switch is closed;

The current A2 before the switch S2 is closed measures the current of the series circuit and is short-circuited after it is closed.

∴ ammeter A2 indicates a drop;

The greater the resistance, the greater the resistance than any part.

∴ According to I=UR, we can see that the series circuit current is measured before the switch S2 is turned off, and the simple circuit current of R 1 is measured after the switch S2 is turned off, and the indicated value of A 1 increases.

So choose B.