Tesla Coils and Induction Coils

A Tesla coil is a resonant transformer that produces very high voltages at high frequencies, invented by Nikola Tesla in the 1890s. Unlike an ordinary transformer, it uses no iron core and relies on resonance between two tuned circuits to step up the voltage. The result is a device capable of generating long, dramatic electrical discharges in air, which made it a popular demonstration of high-voltage and high-frequency phenomena and an early tool in research on wireless power and radio.

A classic Tesla coil has a primary circuit of a few turns connected to a capacitor and a spark gap, and a secondary of many turns wound on a tall, narrow former. The primary capacitor charges from a high-voltage supply until the spark gap breaks down, suddenly connecting the capacitor across the primary coil. This forms an oscillating circuit that rings at high frequency, and because the secondary is tuned to the same frequency, energy transfers resonantly into it, building up an extremely high voltage at the top terminal. The high frequency means the discharges, although at very high voltage, tend to flow over the surface of the body rather than deep through it.

The induction coil, an earlier device sometimes called a spark coil or Ruhmkorff coil, also generates high voltages but works differently. It uses an iron core and an interrupter that repeatedly breaks the primary current, so that the rapidly collapsing magnetic field induces a high voltage in a secondary winding of many turns. Induction coils were used to power early X-ray tubes, radio spark transmitters and ignition systems. Both devices illustrate how high voltages are produced from lower-voltage supplies, but the Tesla coil's resonance and lack of an iron core distinguish it from the iron-cored induction coil.

Frequently asked questions

How does a Tesla coil reach such high voltages?

It uses resonance: the primary and secondary circuits are tuned to the same frequency, so energy transfers efficiently into the many-turn secondary, building up a very high voltage.

How does a Tesla coil differ from a normal transformer?

A Tesla coil is air-cored and resonant, operating at high frequency, whereas a conventional transformer uses an iron core and works at the mains frequency without resonance.

What is an induction coil?

An iron-cored device that uses an interrupter to repeatedly break the primary current, inducing high-voltage pulses in a many-turn secondary, used historically for X-ray tubes and ignition.