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You mentioned not to connect the two ports together. but that's basically what you're doing since your electromagnet is just a length of wire.
Very cool and simple but I have to say it seems dangerous. It also won't work in many situations. Your device doesn't have a high input impedance like a voltmeter is supposed to. More wraps in the coil will increase the resistance but not to the order of millions of ohms. So, if you test a fresh battery against an old battery you might get good readings. But if you're measuring on a live circuit, this thing is going to suck a bunch of current from the circuit and ruin the readings. In addition, the coil might get extremely hot if a high enough voltage is being measured, so maybe cardboard isn't the safest option. I like the fact that this is technically a voltmeter for probably less than a dollar in parts. But it really could be unsafe, especially for people new to electronics, and you should really think about changing or removing this article in my opinion
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