(1)当电流0.4 A时，电压8 V，$R=\frac{8\ \text{V}}{0.4\ \text{A}}=20\ \Omega；$
(2)电源电压$U=IR_{0}+8\ \text{V}=0.4\ \text{A}\times R_{0}+8\ \text{V}，$当电流1.2 A时，电压0 V，$U=1.2\ \text{A}\times R_{0}，$解得$R_{0}=10\ \Omega，$$U=12\ \text{V}；$
(3)中点时，$R'=10\ \Omega，$总电流$I=\frac{12\ \text{V}}{10\ \Omega+10\ \Omega}=0.6\ \text{A}，$电压表示数$U'=0.6\ \text{A}\times10\ \Omega=6\ \text{V}。$
