┌——□—┬—S2┐
│````````````│`````│
├S1—┬□┘ ````│
│``````│```````````│
│``````S3``````````│
│``````│```````````│
└电源┴————┘
闭合S1和S2两电阻并联;
只闭合S2时一个电阻被连入电路;
只闭合S3时两电阻串连接入电路;
┌——□—┬—S2┐
│````````````│`````│
├S1—┬□┘ ````│
│``````│```````````│
│``````S3``````````│
│``````│```````````│
└电源┴————┘
闭合S1和S2两电阻并联;
只闭合S2时一个电阻被连入电路;
只闭合S3时两电阻串连接入电路;