When the switch is at contact 1, R 1 is connected in series with the light bulb, which increases the circuit resistance, decreases the current and darkens the light bulb;
When the switch is at contact 2, R 1 and R2 are connected in series with the light bulb, so that the resistance of the circuit increases again, the current decreases and the light bulb darkens;
When the switch is at contact 3, the resistance in the circuit is the largest, the current is the smallest and the light bulb is the darkest;
Therefore, when the switch is turned from contact 0 to contact 3, the brightness of the bulb gradually darkens.
(2) filament resistance:
RL = U2P =( 12V)224 w = 6ω，
When the switch is at contact 1, the bulb is connected in series with R 1.
r 1 = 13RL = 13×6ω= 2ω，
Total resistance in the circuit:
r = RL+r 1 = 6ω+2ω= 8ω；
Current in the circuit:
I = UR = 12v8ω= 1.5A，
Actual power of the lamp: PL = I2RL = (1.5a) 2× 6Ω =13.5w. 。
Answer: (1) When the adjusting switch is turned from contact O to contact 3 in turn, the brightness of the light bulb becomes dim;
(2) If the regulating switch is turned to contact 1, the actual power of the bulb is13.5w. 。