A Tesla Coil is a special type of transformer invented by Nikola Tesla that is able to generating extremely large voltages using a phenomenon known as electrical resonance.
We learned that from the description of this video.
Apparently, if you modulate the number of sparks created, you can affect the tone of the emissions.
Regardless of how it happens, the result is fascinating. Watch thse coils replicate Lynyrd Skynyrd's Sweet Home Alabama.