DISCLAIMER: This project involved working with mains voltage, and the energies involved could easily be lethal, if not cause serious injury. Please do not try to recreate what you see here.

While I was home for the holidays, I was feeling a little bored. While looking around my workspace for project ideas, I came across a transformer that I removed from an old microwave oven.

Aware of the dangers of doing anything with the high voltage output of such a transformer, I decided the best thing to do would be to carry out a common modification: Rewind the secondary coil to perform as a low voltage, high current supply.
The basic principle of a transformer is such that the output voltage on the secondary coil is proportional to the input voltage on the primary, differing by the ratio of the number of turns between the two windings.
Thanks to the conservation of energy, the ~6 amps flowing through the primary at 120V will be converted to several hundred amps at a low voltage(determined by the turn ratio) available at the secondary. Passing this high current through a few pieces of thin metal allows us to melt the metal together, forming a strong bond.
After performing the modifications to the transformer, I sketched out and built a simple frame out of scrap lumber in order to hold the two electrodes in such a way that two pieces of sheet metal could be placed in between, and some force could be applied.
In the future, I would like to modify it so that both electrodes are on the same side, so that I can weld nickel strips to 18650 lithium cells to create custom battery packs.