例给你一个电流表、一个电压不变的电源、一个定值电阻R(阻值已知)、两个开关、导线若干.请你利用上述器材测出一个未知电阻Rx的阻值. 要求:(1)说明实验原理,画出测量时所用的电路图;(2)指出需要测量哪些物理量;(3)写出被测电阻Rx的最后表达式(用测出的物理量表示). Example gives you an ammeter, a constant voltage source, a fixed value resistor R (known resistance), two switches, and several wires. Please use the above equipment to measure the resistance of an unknown resistor Rx. Requirements: ( 1) Explain the principle of the experiment, draw the circuit diagram used for measurement, (2) indicate which physical quantities need to be measured, and (3) write the last expression of the measured resistance Rx (represented by the measured physical quantity).