Nikola Tesla is not a well known man, but he is the father of our electric age. He was the pioneer of Alternating Current (AC), the type of electricity that comes out of your wall socket. This is different from Direct Current (DC), the type of electricity that comes from a battery. Tesla knew that AC was a much better way to deliver electricity to your house and he helped design the electrical grid we use today.

Though Tesla didn’t like the way the world would look with so many wires running about to deliver power. He started work on a way to deliver power wirelessly so that there would be no need to run so many cables around the planet.
Around 1891, Nikola Tesla invented his "Tesla Coil" with the intention of transmitting electricity through the air. He conducted much research in this area. Indeed, he spent the majority of his career attempting to achieve wireless power. His setup was simple. He purposed using a few coils spread across the globe, to transmit electrical energy through the earth. Wherever power was needed one would only need a receiving coil to convert the power into a useful form. Tesla had successes in this area but his investors refused to support further research.

The wireless power research was not a total loss however. Tesla also used the coils to experiment in radio transmission. Indeed, today at the very heart of every radio on will find a circuit exactly like that used in the Tesla Coil. The basic principles of transmitting information over radio waves have not changed since Tesla's time.
As time progressed, Tesla's claims about the coil became more sensational. He claimed that he could use it to build Death Rays and other wild inventions.
As a high-voltage power supply, 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 When the Cockroft-Walton multiplier was invented, it superseded the Tesla coil in most of these applications.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.

But what exactly is a Tesla Coil?

A Tesla Coil is a machine for generating extreme high voltages. When you fire it up, the shiny donut/sphere-shaped part on top is energized with about 1 000 000 volts of high-frequency current. Huge sparks shoot out from it with a deafening noise.

A Tesla Coil works much in the same way your cell phone charger works, but in reverse. Instead of reducing voltage, it increases it. This is called a transformer. It has two main sets of windings, a primary winding and a secondary winding. The difference in the number of turns in each winding determines if the voltage out will be higher or lower than the voltage in and by how much. A Tesla Coil has very few turns on the primary (input) side, and many turns on the secondary (output) side. This increases the voltage tremendously.

A high voltage power supply charges up a capacitor (kinda like charging up a battery). When the capacitor reaches a high enough voltage, the spark gap fires. The spark gap is like a switch. It goes on when the voltage gets high, and turns off when the voltage gets low. When the spark gap fires, the energy stored up in the capacitor dumps into a 1:X step-up transformer. The primary coil is about 10 turns of heavy wire. The secondary coil is about 1000 turns of thin wire. 10:1000 or 1:100: feed in 10 000 volts get out 1 000 000 volts. ZZZZZZZAAAAAAPPP. It all happens at a rate of over 120 times per second, often generating multiple discharges in many directions.
But a Tesla Coil does much more then increase voltage, it increases frequency. You can see a schematic of a Spark Gap Tesla Coil above. The capacitor and spark gap work together to create a very high frequency pulse to drive the primary coil. Tesla designed it this way because once you create very high voltage electricity at a very high frequency, you can create a field of energy with which to transmit power. This resonant condition is kinda like pushing a kid on a swing. If you give the kid a little kick at the right moment, it will go a little higher.

In addition to seeing uses in industry and science, Tesla Coils can be found in people's homes and on the shelves in department stores: the plasma ball. The Plasma Ball that many people are familiar with is in fact a scaled down version of a Tesla Coil. 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.