如图所示的电路中,电阻$R_{1}$的阻值为$20\Omega$,电源电压不变,$S_{1}$、$S_{2}$断开,$S_{3}$闭合时,电流表的示数为$0.6A$；$S_{1}$断开,$S_{2}$、$S_{3}$闭合时,电流表的示数为$0.9A$.求:
$(1)$电源电压及$R_{2}$的阻值.
$(2)S_{1}$闭合,$S_{2}$、$S_{3}$断开时,电阻$R_{1}$两端的电压.
$(3)S_{1}$、$S_{2}$闭合,$S_{3}$断开时,电流表的示数.