What is a Tesla Coil?The Tesla Coil is a machine for generating extreme high voltages. It's sort of like the Van De Graaff generator you might have played with in high school science classes, but much more powerful. When you fire it up, the shiny donut/sphere-shaped part on top is energized with about 500,000 volts of high-frequency current. Huge sparks shoot out from it with a deafening noise and the whole room stinks of ozone.
The original Tesla coil
The Tesla coil was invented more than 100 years ago, as part of mad genius Nikola Tesla's plan to transmit electrical power without wires. There is a great article on Tesla and his inventions here: http://www.unmuseum.org/tesla.htm Basically, he thought that by building a big enough Tesla coil, with a high enough voltage, he could ionize the whole Earth's atmosphere, allowing it to conduct electricity. As he found out, millions of dollars and two nervous breakdowns later, this wasn't going to work. It wasn't a complete waste of time, though. Marconi borrowed heavily from Tesla's work to create his first radio transmitter, which was basically a Tesla coil with a large wire antenna on top instead of the small sphere or toroid that Tesla used. From then on, the evolution of the Tesla coil split along two separate lines.
The Tesla coil as a radio transmitter
In its radio transmitter guise, the apparatus changed dramatically as wavelengths got shorter and vacuum tubes replaced the spark gap. Circuits related to the Tesla coil are still used in radio and television receivers, but they operate at low voltages with not a spark to be seen, and are no longer known by the name "Tesla coil".
The Tesla coil as a power supply
In its high-voltage power supply guise, the Tesla coil drove apparatus such as x-ray machines, electron microscopes, particle accelerators, and all sorts of bizarre electro-medical appliances, up until the 1950s. When the Cockroft-Walton multiplier was invented, it superceded the Tesla coil in most of these applications. And, when penicillin was discovered, doctors found that it worked better than zapping the patient with millions of volts.
Modern developments in Tesla coiling
Today, Tesla coils of this kind are mainly used for entertainment purposes on account of the spectacular lightning-like discharge that they create. Therefore, modern Tesla coils are designed to produce the biggest, most impressive show of sparks possible. Tesla coiling appeals to amateurs and hobbyists because it's all basically 100-year old technology that you can bodge together in your garage. You can build a perfectly good coil entirely from things scavenged out of the trash. At the other extreme, you can spend tens of thousands of dollars. Either way, Tesla coiling is an exciting business. It's all about pushing components to the limit and hoping nothing blows up or catches fire.