The copper wire has a significant amount of AC electricity running through it, causing it to act like a really strong electromagnet. In the metal slug, eddy currents form due to the magnetic field the copper wire is causing while the copper wire has high frequency AC flowing through it. The metal slug's electric resistance causes a portion of the electric energy to turn into heat, but the heat builds up until the metal slug becomes white hot and melts.
![](https://i.ytimg.com/vi/3iBztmCuwgk/mqdefault.jpg)