When the maximum voltage of the power supply is U= 12V, it is obtained by I=u/R, and the currents passing through 10ω and 15ωR0 are 1.2A and 0.8A respectively. Because the maximum allowable current of the sliding rheostat is 2A, the resistance value of ∴15Ω is not optional, and the resistance value of10Ω should be selected.

When the current through the sliding rheostat is 2A and the power supply voltage is U= 12V, the resistance of R0 is the largest, and R0=U/(I-I sliding) =12V/(3A-2A) =12Ω.

When the power supply voltage U=6 and the sliding rheostat is the maximum, the resistance of R0 is the minimum, and the current through the sliding rheostat I-Slip = U/R-Slip = 6V/10ω = 0.6A; The current I0 through I0 = I-I slip = 3a-0.6a = 2.4a；; r0 = U/Io = 6V/2.4A = 2.5ω