请帮我讲解这道题---**Extracted Content:** **Question Stem:** 6、如图 9 所示电路，电源电压不变，闭合开关 S，移动滑动变阻器的滑片 P 至某一位置时，电流表的示数为 I₁；移动滑片 P 至另一位置时，电流表的示数为 I₂，电压表示数为 1V。若 I₁∶I₂=4∶3，前后两次滑动变阻器的阻值之比为 2∶3，则电源电压为______V。 **Handwritten Notes:** 2.4 2M-2 = Z **Chart/Diagram Description:** * **Type:** Circuit Diagram. * **Main Elements:** * A power source (represented by the standard symbol, with positive terminal on top). * A switch (S) in series with the circuit. * An ammeter (A) in series, connected to the negative terminal of the power source and the switch. * A fixed resistor labeled "R" in series with the ammeter and switch. * A sliding rheostat in series with the fixed resistor. * A voltmeter (V) connected in parallel across the fixed resistor R. * The sliding contact of the rheostat is labeled "P" and indicated by an arrow pointing downwards onto the resistive element. The circuit is completed by connecting the other end of the rheostat back to the positive terminal of the power source. * **Connections:** The components are connected in a single loop (series circuit) except for the voltmeter which is connected in parallel across the resistor R. The sliding rheostat is used as a variable resistor where the resistance in the circuit is determined by the position of the slider P between one end terminal and the slider terminal. * **Labels:** Components are labeled V, R, P, A, S. The diagram is labeled "图 9" (Figure 9).