在 Hacker News Daily 上看到「“You don't have a voltmeter. Do you have a potato?” (diy.stackexchange.com)」這則討論，原討論在「Is white wire with grey stripes positive or negative wire?」這邊，有人問了一個問題，說手上有一個交流轉直流的變壓器，想要重新接線到下面這種端子，但不確定變壓器輸出的兩條線裡面，正極與負極分別是哪條：
下面的答案很厲害啊，開頭先說如果你沒有伏特計 (像是三用電錶)，那手上有沒有馬鈴薯：
You don't have a voltmeter. Do you have a potato?
把馬鈴薯切半，把 DC 端的兩條線插到同一顆馬鈴薯上面，大約相距 2cm，插電不久後就可以看到有一端會變綠，那端就是正極，把本來插到馬鈴薯上的端子剪掉後再接線就可以了：
With the power adapter unplugged from your electrical outlet, cut the wires, strip a little insulation from the ends, twist the strands of each wire into a point. Do not allow the bare wires to touch each other from this point on.
Cut the potato in half. Take one half, and poke both wires into the cut face of the potato about 2 cm apart. Plug in the power adapter to the wall outlet. In a short time, the potato around one of the wires will turn green. That is the positive wire.
Unplug the adapter and clip off the ends of the wires so you have clean wire for soldering your new plug.
不確定產生的是不是銅綠，但能給出這種解法的確很有娛樂性 XDDD