Tesla coil
Electrical oscillation and resonant air-core transformer circuits had been explored before Tesla. Resonant circuits using Leyden jars were invented beginning in 1826 by Felix Savary, Joseph Henry, William Thomson, and Oliver Lodge. and Henry Rowland built a resonant transformer in 1889. Elihu Thomson invented the Tesla coil circuit independently at the same time Tesla did. Tesla patented his Tesla coil circuit April 25, 1891. and first publicly demonstrated it May 20, 1891 in his lecture "Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination" before the American Institute of Electrical Engineers at Columbia College, New York. Although Tesla patented many similar circuits during this period, this was the first that contained all the elements of the Tesla coil: high voltage primary transformer, capacitor, spark gap, and air core "oscillation transformer".