# Tesla coil: what is it and how does it work?

In 1891, Nikola Tesla invented the Tesla coil, a transformer that produces very high voltage and high frequency current.

## How does the Tesla coil work?

This is an explanation of the original model’s operation. An alternate voltage source (AC mains), whose value is usually between 100V and 127V or between 220V and 240V, depends on where you live. This source is connected to a high voltage transformer and the latter increases the voltage to thousands of volts. The relation below shows that for the voltage on secondary winding ($E_{s}$) be higher than on primary ($E_{p}$), the number of turns on secondary winding ($N_{s}$) must be higher than on primary winding ($N_{p}$).

$\frac{E_{p}}{E_{s}}=\frac{N_{p}}{N_{s}}$

The high voltage on secondary winding charges a capacitor or a capacitor bank. The spark gap consists of two pieces of metal separated by an air space and it’s connected to the capacitor and the second transformer’s primary winding. When the capacitor is charged, a high voltage appears on spark gap, from a certain voltage level, the air is no longer an insulator and becomes a conductor.

When appears a spark between the spark gap’s conductors, a current circulates between the high voltage capacitor and the second transformer’s primary. As a result, the voltage on capacitor decreases and the spark on spark gap disappears. As a result, voltage source charges the capacitor and the cycle continues.

### Resonance and the second transformer

Although the source provides a frequency of 50 or 60Hz, the capacitor and the second transformer’s primary winding, which is not linked to power source, operate in a resonance frequency, since both are energy storage components.

The equation of resonance frequency ($f_{s}$) of an LC circuit. Where $L$ and $C$ are inductance and capacitance, respectively.

$f_{s}=\frac{1}{2\pi\sqrt{LC}}$

On the second transformer, the primary winding has thick wires with few turns, while the secondary winding has thinner wires, but with a much higher number of turns. Therefore, the voltage is much higher. One of the secondary winding’s terminals must be grounded, while the other side has a sphere or a toroid made of conductor material, to avoid charge loss due to the corona effect.

## Tesla coil applications

In the past, Tesla coils’ high voltage was used to generate X-rays, the coil was also used in experiments with illumination, phosphorescence and electrotherapy. Currently, it’s used for education and entertainment. On YouTube, you can find videos where Tesla coils play famous songs.