Tesla coil is a kind of "step-up transformer" which works by resonance principle (note: electromagnetic induction principle is different from ordinary transformer, so don't confuse it).

Definition of resonance:

There is a concept in physics called * * * vibration: when the frequency of the driving force is equal to the natural frequency of the system, the amplitude of the forced vibration of the system is the largest, which is called * * * vibration. Resonance in the circuit actually means the same thing: when the excitation frequency of the circuit is equal to the natural frequency of the circuit, the amplitude of the electromagnetic oscillation of the circuit will also reach the peak. In fact, * * * vibration and resonance express the same phenomenon. This essentially the same phenomenon has different names in different fields.

A simple LC resonant circuit is used in Tesla coil.

Definition and related characteristics of LC resonance;

A circuit consisting of an inductor L and a capacitor C can resonate at one or several frequencies, which is collectively called a resonant circuit.

According to the connection mode of components, resonant circuits can be divided into series resonance (also known as voltage resonance) and parallel resonance (also known as current resonance) (see Baidu Encyclopedia for a more detailed definition).

The characteristics of the resonant circuit at resonance are as follows

1. The current increases sharply.

2. High voltage appears across L and C. ..

This phenomenon of high voltage is very useful in radio and electronic engineering.

Tesla coils use the high voltage generated by resonance to make lightning.