We’re paying a second visit to the Tesla Coil today, exploring more about how it works. 

Broadly speaking, we can wave our hands at the Tesla Coil and talk about inductance and resonance, but what does that really mean, and how does it lead to those lovely purple sparks?

 Electromagnetic induction is the process by which a voltage is produced across an inductor in a changing magnetic field. In this case, we’re taking advantage of the studies of Maxwell and Faraday that showed the relationship between electricity and magnetism. An electrical current generates its own magnetic field; a changing electrical current thus produces a changing magnetic field, and so a changing electrical current in one conductor can induce a current in a nearby conductor. We can carefully choose these to create higher or lower induced voltages.

Electrical resonance occurs when a circuit is built to have a particular resonant frequency, at which the impedance (the way a circuit element resists an alternating current) of different components cancels out to let the circuit build up higher voltages or currents.

Our Tesla coil, circuit above, uses a 5000 volt transformer to charge a large oil capacitor. When the potential across the capacitor reaches the breakdown potential of the spark gap, breakdown across the gap occurs. The spark gap then becomes a conducting part of the RLC circuit, which resonates at a frequency of about 200 kilohertz. The large coil in the resonant circuit is the primary coil of the final transformer and the long coil of very fine wire is the secondary, producing about 200,000 volts at 200 kilohertz.

You can see what’s happening by examining a simulation of a similar circuit’s behaviour, like https://www.falstad.com/circuit/e-tesla.html . The initial transformer creates a high voltage, which eventually builds up enough to exceed the breakdown voltage of the air and make a spark across the spark gap. This then feeds into resonant circuits which build up very high electrical potential, which can create the discharge we see.

This uses Paul Falstad’s Circuit Simulator Applet, which you can explore further at https://www.falstad.com/circuit/index.html

To learn more about Tesla Coils, check out:

Nikola Tesla’s patent: https://patents.google.com/patent/US1119732

Richie Burnett’s Operation of the Tesla Coil: http://www.richieburnett.co.uk/operation.html

Wikipedia: https://en.wikipedia.org/wiki/Tesla_coil

Kelley and Dunbar, “The Tesla Coil,” American Journal of Physics 20(32). https://doi.org/10.1119/1.1933098