题目:[99年河北省中考题]如图所示电路中,R_1、R_2为两个定值电阻,且 R_1+R_2=8欧,小灯泡上标有“6V、12W”字样.当开关 S_1、S_3闭合,S_2断开时,电流表示数为 I;当开关S_2、S_3闭合,S_1断开时,电流表示数增加0.5安,此时小灯泡正常发光.(设电源电压灯丝电阻均不变) Title: [99 Hebei Provincial Examination Questions] In the circuit shown in the figure, R_1 and R_2 are two fixed-value resistors, and R_1+R_2=8 Euro, and the small bulb is marked with the word “6V, 12W”. When the switches S_1 and S_3 are closed, when S_2 is opened, the current display number is I; when the switches S_2 and S_3 are closed, and S_1 is opened, the current display number is increased by 0.5 A. At this time, the small light bulbs emit light normally. (Set the supply voltage to the filament resistance constant)