(1) When S 1 is closed and S2 is open, it can be known from the problem that I=U/R0, then U=IR0, which is the circuit voltage.

When S 1 and S2 are closed, R0 and Rx are connected in parallel, and the total resistance is: R=R0Rx/(R0+Rx).

Then I=U/R=IR0/[R0Rx/(R0+Rx)]

Solution: Rx=IR0/(I 1-I)

(2) The second set of circuit diagrams can be experimented with reference to the circuit diagram on the first floor, which will not be repeated here.

(3) The circuit diagram of the third group is described as follows:

R0 and Rx are connected in series in the circuit, with a voltmeter connected at both ends of R0 and a switch connected at both ends of Rx. A switch is connected in series near the power supply to control the main circuit.

Experimental operation instructions and calculation of unknown resistance;

First, close the switch and determine that the voltage across R0 is U, that is, the power supply voltage;

Then turn off the switch, measure the voltage at both ends of R0 as U0, calculate the circuit current I0 according to U0, calculate the current Ix=I0 flowing through Rx according to the equality of currents in series circuits, calculate Ux=U-U0 according to U=U0+Ux, and calculate the resistance of unknown resistor according to Rx=Ux/Ix.