Invented by Nikola Tesla, Tesla coils are electrical resonant transformer circuit that produce high voltage, low current, high frequency alternating current (AC) electricity. Tesla coils can produce higher voltages than electrostatic machines and other artificial sources of high voltage discharge. Tesla used his namesake devices to conduct groundbreaking experiments in electrical lighting, x-ray generation, electrotherapy, high frequency alternating current phenomena, phosphorescence, and wireless electrical energy transmission.
A Tesla coil transformer operates very differently than “traditional” iron core electrical transformers. Conventional transformers use tightly coupled windings, and their voltage gain is determined by the ratio of the numbers of turns in these windings. This design is highly effective at normal voltages, but running these transformers at high voltage can cause the insulation between the windings to break down, leading to damage.
Tesla coils, however, utilize loosely coupled windings with large air gaps between them. Whereas traditional transformers may have over 97% of their fields coupled, Tesla coils’ design allows for only 10-20% sharing of the magnetic fields between the primary and secondary windings. Tesla coils transfer energy via their loose couplings from the primary oscillating resonant circuit to the secondary over numerous radio-frequency cycles.
As energy is transferred from the primary circuit to the secondary, the secondary’s output voltage increases until all available primary energy has been transferred (less losses). Even with sometimes-considerable spark gap losses, a properly designed and constructed Tesla coil can successfully transfer over 85% of the stored energy from the primary capacitor to the secondary circuit. Tesla coils can achieve significantly greater voltage than conventional transformers, owing to the wide separation—and therefore effective insulation—of the long, single layer solenoid of the secondary winding.
The loose coupling of circuits in Tesla coils leads to voltage gain that is proportional to the square root of the ratio of secondary and primary inductances. As the secondary and primary windings are wound to be resonant at the same frequency, voltage gain is also proportional to the square root of the ratio of the primary capacitor to the stray capacitance of the secondary.
Tesla coils were commonly used commercially in spark gap radio transmitters for wireless telegraphy until the 1920s. They were also used in medical equipment of the time, including electrotherapy and violet ray devices. Today, small Tesla coils can be used as leak detectors in high vacuum systems, though modern devices are mainly used for entertainment and educational purposes.