[例题]如图,用一个电阻很大的伏特表测定滑动变阻器 R_2两端的电压,已知 R_1=30欧姆,当滑片 P 在变阻器最右端时,伏特表的读数为4.5伏特,滑片 P 滑到变阻器中点时,伏特表的读数为4.5伏特,滑片 P 滑到变阻器中点时,伏特表的读数为3伏特。求:(1)电源的电压和变阻器的总电阻。(2)P 在两次位置时,安培表读数的差。 [Example] As shown in the figure, the voltage across the sliding rheostat R_2 is measured with a large resistance voltmeter. R_1 = 30 ohms is known. When the slide P is at the rightmost end of the varistor, the voltmeter reading is 4.5 volts. Slide P When sliding to the midpoint of the rheostat, the voltmeter reads 4.5 volts, and when the slide P slides to the midpoint of the rheostat, the voltmeter reads 3 volts. Find: (1) The voltage of the power supply and the total resistance of the varistor. (2) P The difference in readings of the ammeter at two positions.