A, the voltmeter measures the power supply voltage. When the slider moves, the power supply voltage remains unchanged and the voltage representation number remains unchanged, so A is correct;

B, the light bulb is connected at both ends of the power supply, the voltage at both ends of the light bulb remains unchanged, the actual power of the light bulb remains unchanged, and the brightness of the light bulb remains unchanged, so B is correct;

C. When the slider slides from the A end to the B end, the resistance of the sliding rheostat connected to the circuit increases, and the resistance of the branch where the sliding rheostat is located increases, and the power supply voltage remains unchanged. According to ohm's law, the branch current where the sliding rheostat is located becomes smaller, the main current becomes smaller, and the indication of ammeter A 1 becomes smaller, so C is correct;

D, the bulb is connected at both ends of the power supply, the voltage at both ends of the bulb is constant, the resistance of the bulb is constant, the current flowing through the bulb is constant, and the number displayed by ammeter A2 is the same, so D is wrong;

So choose ABC ..