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I was interested in building a simple electromagnet for the kids - and many tutorials online suggest just looping a wire around a nail as pictured below. But how is this safe ? Isn't this essentially an electrical short directly across the battery ? (You wouldn't take a wire and directly short it across a battery.) Why would coiling wire around a nail make this any different ?

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  • $\begingroup$ you need wire covered with insulation. $\endgroup$
    – Alex Trounev
    Commented Dec 4, 2018 at 1:13
  • $\begingroup$ Okay, but even with insulation isn't it damaging on the battery ? The reason I ask is that I actually want to use a lab power supply as the power source - but don't fancy shorting across the terminals (and damaging my power supply). Should I add a ceramic resistor ? Or is there some property of twisting the wire like this which creates extra resistance whilst under load ? $\endgroup$
    – Patrick Rynhart
    Commented Dec 4, 2018 at 1:23
  • $\begingroup$ @PatrickRynhart : As I noted in my answer, shorting an ordinary battery, such as AA, is probably OK, but don't do that with your power supply - the voltage there can be higher and you don't know its internal resistance. $\endgroup$
    – akhmeteli
    Commented Dec 4, 2018 at 1:28
  • $\begingroup$ Thanks @akhmeteli. If I limited the output voltage of my power supply to around 9V or so, and added a 1-2 Ohm resistor in series would this be safe enough ? (I did a quick google search to find that the internal resistance of a 9V battery is approximately this value.) $\endgroup$
    – Patrick Rynhart
    Commented Dec 4, 2018 at 1:34
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    $\begingroup$ @PatrickRynhart the wire has resistance which can be measured. Voltage is known; Ohm’s law is also known. Calculate the current. $\endgroup$
    – Alex Trounev
    Commented Dec 4, 2018 at 1:38

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If you short a battery, probably nothing spectacular will happen (see short circuit an alkaline battery ), although you should beware of fire hazard. The battery has internal resistance that limits the current.

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