- an air-core transformer used to produce high voltages of high-frequency alternating currents.
- a step-up transformer with an air core, used for producing high voltages at high frequencies. The secondary circuit is tuned to resonate with the primary winding
- An electrical device that generates extremely high voltages, usually for the purpose of creating dramatic electric arcs and lightning effects or for producing x-rays. Tesla coils use step-up transformers to boost the voltage of a power supply and build up large charges in a capacitor. A spark gap periodically shorts out the capacitor, releasing its charge in huge current flows, that generate extremely high voltages (up to ten million volts) through an open-air transformer.