How is it that I can measure voltage across an unplugged wall wart?

I have a 9V wall wart that's been unplugged for 2 days.  I hooked it up to my volt meter with intentions of checking the polarity as I plug it in.  Without plugging in the wall wart I attached the meter leads to the exposed wire of the wall wart.  It immediately showed a voltage of -10 point something.  I was glad to find the wires were backwards which explains my fried regulator; however how is it that I was able to pick up a voltage, even after discharging it the voltage slowly rose again, unplugged!  Does this have anything to do with earths magnetic effects on the inductor?

NachoMahma6 years ago
. The wall wart wires are an antenna and you are picking up stray voltage. If you put a load on the line, the voltage will disappear.
lemonie6 years ago

They have a smoothing capacitor in 'em which will hold a voltage, see also Nacho.