The constantly fluctuating electric current flowing in from the power grid is wound through a series of turns around an iron ring to generate a magnetic field. Iron is magnetically permeable, so the magnetic field is almost entirely contained in the iron. The ring guides the magnetic field in green below around and through the center of the opposite coil of wire.
The ratio of coils on one side to the other determines the change in voltage. To go from V household wall voltage to, say, 20 V for use in a laptop power adapter, the output side of the coil will have 6 times fewer turns to cut the voltage to one sixth its original level. Tesla coils do the same thing, but with a much more dramatic change in voltage. First, they employ a pre-made high voltage iron core transformer to go from V wall current to roughly 10, V.
The wire with 10, volts is wrapped into one very large primary coil with only a handful of turns. The secondary coil contains thousands of turns of thin wire. This steps up the voltage to between , and one million volts. This potential is so strong that the iron core of a normal transformer cannot contain it.
Instead, there is only air between the coils, which can be seen in a Tesla coil below:. The large primary coil with few turns is on the bottom. The secondary coil with thousands of turns is the cylinder standing up vertically, separated from the lower coil by air.
The Tesla coil requires one more thing: a capacitor to store charge and fire it all in one huge spark. The circuit of the coil contains a capacitor and a small hole called a spark gap. When the coil is turned on, electricity flows through the circuit and fills the capacitor with electrons, like a battery.
This charge creates its own electric potential in the circuit, which tries to bridge across the spark gap. Tesla used his brainchild to research such diverse areas as lighting, X-rays and electric power transmission. Customized Tesla coils are now frequently used to ignite powerful mercury and sodium streetlamps. Although they have now been largely replaced by more modern circuitry, Tesla coils frequently show up in popular culture, most commonly in the form of high-tech guns in video games, blasting bolts of lightning at adversaries.
On the big screen, a Tesla coil was used to produce lighting effects for the film "Star Trek: The Motion Picture. Last modified on 10 December A regular power source fed through a transformer can produce a current with the necessary power at least thousands of volts. The power source is hooked up to the primary coil. The primary coil's capacitor acts like a sponge and soaks up the charge. The primary coil itself must be able to withstand the massive charge and huge surges of current, so the coil is usually made out of copper, a good conductor of electricity.
Eventually, the capacitor builds up so much charge that it breaks down the air resistance in the spark gap. Then, similar to squeezing out a soaked sponge, the current flows out of the capacitor down the primary coil and creates a magnetic field. The massive amount of energy makes the magnetic field collapse quickly, and generates an electric current in the secondary coil.
The voltage zipping through the air between the two coils creates sparks in the spark gap. The energy sloshes back and forth between the two coils several hundred times per second, and builds up in the secondary coil and capacitor.
Eventually, the charge in the secondary capacitor gets so high that it breaks free in a spectacular burst of electric current. The resulting high-frequency voltage can illuminate fluorescent bulbs several feet away with no electrical wire connection. In a perfectly designed Tesla coil, when the secondary coil reaches its maximum charge, the whole process should start over again and the device should become self-sustaining.
In practice, however, this does not happen.
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