In the early 20th century, Tesla coils provided high frequency and high voltage electricity for radio transmission, X-ray machines, electro-therapy and early particle accelerators. Other electrical generation methods have since eclipsed the Tesla coil, so in modern times they have little practical value except for high voltage demonstrations.
Nikola Tesla, who lived from 1856 to 1943, developed the Tesla coil in 1891 as part of his work with alternating currents. Unlike transformers, which have a primary and a secondary winding around an iron core, Tesla coils use induction from a smaller primary coil to boost voltage in the secondary coil, using air to insulate the secondary coil. This allows much higher voltages than a conventional transformer. Tesla coils can boost input voltage from 110 volts AC to between 100,000 and 1,000,000 volts AC.
Tesla’s initial experiments and demonstrations suggested many uses for his coil. He used it to create some early X-rays, and because Tesla coils produce high frequency energy, they transmit radio waves as well. This property also allows wireless AC power transmission. Tesla, along with others, worked on larger and larger coils hoping they could solve the numerous problems with such transmission. Some experimenters used Tesla coils in early particle accelerators, but they abandoned them in favor of voltage multipliers and electrostatic solutions.
By the end of the 1920s, the Tesla coil had fallen out of favor as a high voltage generator and radio wave transmitter. It remained a laboratory demonstration tool until hobbyists revived interest in it in the late 1950s. Today numerous sites on the Internet describe how to build and operate Tesla coils.