Title: Tesla coil
Content:  A Tesla coil is an electrical resonant transformer circuit designed by inventor Nikola Tesla in 1891. It is used to produce high-voltage, low-current, high frequency alternating-current electricity. Tesla experimented with a number of different configurations consisting of two, or sometimes three, coupled resonant electric circuits. Tesla used these circuits to conduct innovative experiments in electrical lighting, phosphorescence, X-ray generation, high frequency alternating current phenomena, electrotherapy, and the transmission of electrical energy without wires. Tesla coil circuits were used commercially in sparkgap radio transmitters for wireless telegraphy until the 1920s, and in medical equipment such as electrotherapy and violet ray devices. Today, their main usage is for entertainment and educational displays, although small coils are still used as leak detectors for high vacuum systems. Originally, Tesla coils used fixed spark gaps or rotary spark gaps to provide intermittent excitation of the resonant circuit; more recently electronic devices are used to provide the switching action required. With solid-state drivers, a Tesla coil can be used to generate audible tones, to produce musical effects.
Title: Nikola Tesla
Content:  Nikola Tesla (, ; 1856 – 7 January 1943) was a Serbian-American  inventor, electrical engineer, mechanical engineer, and futurist best known for his contributions to the design of the modern alternating current (AC) electricity supply system. Born and raised in the Austrian Empire, Tesla studied engineering and physics in the 1870s without receiving a degree, gaining practical experience in the early 1880s working in telephony and at Continental Edison in the new electric power industry. In 1884 he emigrated to the United States, where he became a naturalized citizen. He worked for a short time at the Edison Machine Works in 
Title: Nikola Tesla
Content:  York City before he struck out on his own. With the help of partners to finance and market his ideas, Tesla set up laboratories and companies in New York to develop a range of electrical and mechanical devices. His alternating current (AC) induction motor and related polyphase AC patents, licensed by Westinghouse Electric in 1888, earned him a considerable amount of money and became the cornerstone of the polyphase system which that company eventually marketed. Attempting to develop inventions he could patent and market, Tesla conducted a range of experiments with mechanical oscillators/generators, electrical discharge tubes, and early X-ray imaging. He 
Title: Nikola Tesla
Content:  From the 1890s through 1906, Tesla spent a great deal of his time and fortune on a series of projects trying to develop the transmission of electrical power without wires. It was an expansion of his idea of using coils to transmit power that he had been demonstrating in wireless lighting. He saw this as not only a way to transmit large amounts of power around the world but also, as he had pointed out in his earlier lectures, a way to transmit worldwide communications. At the time Tesla was formulating his ideas, there was no feasible way to wirelessly transmit communication signals over 
Title: Nikola Tesla
Content:  In the summer of 1889, Tesla traveled to the 1889 Exposition Universelle in Paris and learned of Heinrich Hertz's 1886–1888 experiments that proved the existence of electromagnetic radiation, including radio waves. Tesla found this new discovery "refreshing" and decided to explore it more fully. In repeating, and then expanding on, these experiments, Tesla tried powering a Ruhmkorff coil with a high speed alternator he had been developing as part of an improved arc lighting system but found that the high-frequency current overheated the iron core and melted the insulation between the primary and secondary windings in the coil. To fix this problem Tesla came up with his "oscillating transformer", with an air gap instead of insulating material between the primary and secondary windings and an iron core that could be moved to different positions in or out of the coil. Later called the Tesla coil, it would be used to produce high-voltage, low-current, high frequency alternating-current electricity. He would use this resonant transformer circuit in his later wireless power work.
Title: Nikola Tesla
Content:  he worked on improving and developing new types of electric motors, generators, and other devices. In 1887, Tesla developed an induction motor that ran on alternating current (AC), a power system format that was rapidly expanding in Europe and the United States because of its advantages in long-distance, high-voltage transmission. The motor used polyphase current, which generated a rotating magnetic field to turn the motor (a principle that Tesla claimed to have conceived in 1882). This innovative electric motor, patented in May 1888, was a simple self-starting design that did not need a commutator, thus avoiding sparking and the high maintenance of constantly servicing and replacing 
Title: Resonant inductive coupling
Content:  In 1894 Nikola Tesla used resonant inductive coupling, also known as "electro-dynamic induction" to wirelessly light up phosphorescent and incandescent lamps at the 35 South Fifth Avenue laboratory, and later at the 46 E. Houston Street laboratory in New York City. In 1897 he patented a device called the high-voltage, resonant transformer or "Tesla coil." Transferring electrical energy from the primary coil to the secondary coil by resonant induction, a Tesla coil is capable of producing very high voltages at high frequency. The improved design allowed for the safe production and utilization of high-potential electrical currents, "without serious liability of the destruction of the apparatus itself and danger 
Title: History of the Tesla coil
Content:   In addition to its use in spark-gap radio transmitters and electrotherapy described above, the Tesla coil circuit was also used in the early 20th century in x-ray machines, ozone generators for water purification, and induction heating equipment. However, in the 1920s vacuum tube oscillators replaced it in all these applications. The triode vacuum tube was a much better radio frequency current generator than the noisy, hot, ozone-producing spark, and could produce continuous waves. After this, industrial use of the Tesla coil was mainly limited to a few specialized applications which were suited to its unique characteristics, such as high voltage insulation testing. In 1926, pioneering accelerator physicists Merle Tuve and Gregory Breit built a 5 million volt Tesla coil as a linear particle accelerator. The bipolar coil consisted of a pyrex tube a meter long wound with 8000 turns of fine wire, with round corona caps 
Title: Nikola Tesla
Content:   After 1890, Tesla experimented with transmitting power by inductive and capacitive coupling using high AC voltages generated with his Tesla coil. He attempted to develop a wireless lighting system based on near-field inductive and capacitive coupling and conducted a series of public demonstrations where he lit Geissler tubes and even incandescent light bulbs from across a stage. He spent most of the decade working on variations of this new form of lighting with the help of various investors but none of the ventures succeeded in making a commercial product out of his findings. In 1893 at St. Louis, Missouri, the Franklin Institute in Philadelphia, Pennsylvania and the National Electric Light Association, Tesla told onlookers that he was sure a system like his could eventually conduct "intelligible signals or perhaps even power to any distance without the use of wires" by conducting it through the Earth. Tesla served as a vice-president of the American Institute of Electrical Engineers from 1892 to 1894, the forerunner of the modern-day IEEE (along with the Institute of Radio Engineers).
Title: Wizard: The Life and Times of Nikola Tesla
Content:  Seifer follows the life of Nikola Tesla, the Serbian American Invention|inventor, electrical engineer, mechanical engineer, physicist, and futurist. He covers the high points of the inventor's life through his designs used in the modern alternating current system, experimentation with high frequency current and wireless power transmission, wireless remote control, X-ray imaging, and Tesla's "death ray". Seifer goes on to cover Tesla's downfall, attributing it to Tesla's megalomaniacal, neurotic, self-destructive tendencies, and Tesla's interactions with financier J. Pierpont Morgan. Seifer based his book largely on primary sourced documents including Tesla's writings and patents. Also including papers of Tesla's in the Tesla archives in Yugoslavia as well as manuscripts in the United States. Seifer filed Freedom of Information Act requests to get documents from the FBI and other United States government agencies.

Question: what did nikola tesla invented that is of use today?
Answer: Tesla coil.