Many people should already be familiar with the concept of a transformer, which could be a step-down transformer, that has a certain number of turns – anywhere from 200 to maybe 400 – as its primary winding, and which has a secondary winding with fewer turns.
The output voltage follows as the AC input voltage, times the ratio of secondary turns to primary turns – or, divided by the ratio of the number of primary turns to the secondary turns. The output current gets stepped up, by whatever factor the output voltage was stepped down.
What some people may not have the imagination to realize, is that it is fully valid for the secondary winding, to consist of only 1 turn, or of 2 turns. If that is the case, then the output voltage will also be ~1/200 the input voltage, and the output current will be ~200 times the input current, as long as a well-designed transformer, with a suitable core material, preserves the energy well.
If that amount of current flows through a solid bar or chunk of metal, even a solid chunk of metal will get hot. And this is the basis of a soldering gun. Its secondary winding is just a solid bar of metal, that loops through the same transformer core, which the primary winding looped through.
There are some caveats to this design. One is, that if the secondary winding really does just consist of a solid bar of metal, we will have problems with eddy currents, stealing away some of our energy. After all, the ferritic core material itself is commonly laminated, to avoid the possibility of eddy currents there; the same consideration could be given to this secondary loop…