Here is a picture of my initial Tesla coil in operation. It produces 60,000 volts of electricity at a frequency of 130,000 cycles per second. The air at the top of the coil glows blue because the voltage is so high there that the air ionizes and gives off light. 

The glowing glass tubes at the bottom of the picture change low frequency alternating current (60 cycles per second) to high frequency alternating current (130,000 cycles per second, or hertz). 

The tall coil has 2,200 turns wound on it, while the wide, short coil at the bottom of the picture surrounding it contains 20 turns of wire. The two coils form a resonant transformer and the energy fed to the short coil by the glass tubes is transferred to the tall coil, causing the voltage to be increased from about 2,000 volts to 60,000 volts. 
The voltage is so high, that light bulbs and fluorescent tubes light up some distance away from the Tesla coil, without anything even connected to them! 
Nikola Tesla's dream was to send electricity to homes without wires, by having huge Tesla coils send electric power through the air across the countryside. In this way electric cars, fuel-less electric aircraft, etc. can receive power without carrying heavy, limited batteries.
Tesla managed to light up one hundred 100-watt light bulbs, 25 miles away from his coil. 
Adding a toroid terminal to the top of the coil to increase the surface area and hence allow the voltage to build up before charges can start leaking off into the surrounding air. This is superior to a spherical terminal because the electric field does not extend downwards into the coil windings themselves. Adding the terminal increases the capacitance of the coil, and so also requires re-tuning the primary to a lower operating frequency (100,000 hertz).
The phenomenal increase in voltage that results!!! Voltages are estimated to be several millions. All this from a 275 watt power supply connected to the 120 volt 60 cycle line using a spark gap instead of vacuum tubes (much more efficient because PEAK power is several tens of kilowatts) you can imagine there is no limit to how large "artificial lightning" can be made using a Tesla transformer and a spark gap.

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