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What is a Tesla Coil?
In the 1880's and 1890's, Nikola Tesla had invented many alternating current (AC) devices. One of the most spectacular was his high frequency high potential transformers that are most commonly called 'Tesla Coils'. There are actually a few different kinds of Tesla Coils; I will be discussing what is known as a Classic Tesla Coil. A Tesla Coil is a resonant transformer, meaning it has a tuned primary and tuned secondary resonant circuits. The Primary circuit is a tank circuit consisting of a capacitor, inductor, and spark gap. The secondary circuit is a series resonant circuit consisting of a coil, a 'terminal', and an earth ground connection. By 'terminal' I mean an object, usually a sphere or toroid. This terminal has a surface capacitance with respect to ground and acts like a capacitor. The primary and secondary circuits must be tuned to the same frequency, or in other words the resonant frequency of the primary circuit must be the same as the resonant frequency of the secondary circuits.
How does a Tesla Coil work?
Again I must stress that I am discussing a Classic Tesla Coil. In a Classic Tesla Coil a high voltage transformer is used to power the primary circuit. Typical values would usually fall between 7,000V and 20,000V. This transformer charges a capacitor. When the voltage across the capacitor reaches a determined amount it will spark across the spark gap and discharge through the primary coil. This causes a resonant situation to occur between the primary coil and capacitor. The magnetic field formed in the primary coil is magneticaly coupled to the secondary coil. This induces current to flow in the secondary coil that corresponds to the magnetic field in the primary coil, so the resonant frequencies of the primary and secondaru circuits must be matched. The oscillations in the secondary coil then continue to rise in amplitude until the voltage is high enough to 'break' out of the terminal. When these currents 'break' out from the terminal, the air is temporarily ionized and conducts electrons. The secondary coil is connected to a low resistance ground path. This is to provide the source for the electons to come from. Ground connections are usually long copper rods burried several feet in the ground that are connected to the base of the secondary coil using large copper wire or flashing. Many people don't realize that the electricity you see shooting from the top of a Tesla Coil does not come from the outlet, but comes from the ground.