the circuit resembles a radio transmitter. Unlike the previous circuits which generate a pulsed output, they generate a continuous sine wave output. Power vacuum tubes are often used as active devices instead of transistors because they are more robust and tolerant of overloads. In general, continuous excitation produces lower output voltages from a given input power than pulsed excitation. Number of coils Tesla circuits can also be classified by how many resonant coils (inductors) they contain: Two coil or double-resonant circuits: Virtually all present Tesla coils use the two coil resonant transformer, consisting of a primary winding to which current pulses are applied, and a secondary winding that produces the high voltage, invented by Tesla in 1891. The term "Tesla coil" normally refers to these circuits. Three coil, triple-resonant, or magnifier circuits: These are circuits with three coils, based on Tesla's "magnifying transmitter" circuit which he began experimenting with sometime before 1898 and installed in his Colorado Springs lab 1899–1900, and patented in 1902. They consist of a two coil air-core step-up transformer similar to the Tesla transformer, with the secondary connected to a third coil not magnetically coupled to the others, called the "extra" or "resonator" coil, which is series-fed and resonates with its own capacitance. The output is taken from the free end of this coil. The presence of three energy-storing tank circuits gives this circuit more complicated resonant behavior. It is the subject of research, but has been used in few practical applications. History 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". Modern-day Tesla coils Modern high-voltage enthusiasts usually build Tesla coils similar to some of Tesla's "later" 2-coil