Nikola Tesla filed his first United States patent in 1886 and his last in 1928. The 112 patents he received during that span cover an extraordinary range of electrical inventions: motors, generators, lighting systems, power transmission, radio, remote control, and energy conversion. All of these patents have long since expired. The legal protection they provided ended decades ago.
But the technology they describe has not expired. Many of Tesla’s inventions are so fundamental to modern electrical engineering that they remain in daily use, not as historical curiosities but as the working principles behind devices that billions of people rely on. The alternating current motor that powers your refrigerator, the transformer that steps voltage up and down in the power grid, and the resonant circuits in your radio all operate on principles that Tesla patented more than a century ago.
The AC Induction Motor
Tesla’s most consequential invention is the alternating current induction motor, patented in 1888 (US Patent 381,968). The induction motor converts alternating current into rotary mechanical motion using a rotating magnetic field, a concept that Tesla developed and that remains the foundation of industrial motor design today.
The principle is elegant. Alternating current flowing through stationary coils (the stator) creates a magnetic field that rotates in space. A conductive rotor placed within this field experiences induced currents that create their own magnetic field, and the interaction between the two fields causes the rotor to spin. No physical contact between the stator and rotor is needed. There are no brushes, commutators, or sparking contacts to wear out.
This simplicity makes the induction motor extraordinarily reliable. Modern induction motors, which operate on exactly the same principle that Tesla patented, have operational lifetimes measured in decades. They power industrial machinery, HVAC systems, water pumps, elevators, conveyor belts, and electric vehicles. An estimated 45 percent of all electricity generated worldwide is consumed by electric motors, and the majority of those motors are induction motors based on Tesla’s design.
The Tesla Coil
The Tesla coil, patented in 1891 (US Patent 454,622), is a resonant transformer circuit that produces high-voltage, low-current, high-frequency alternating current. Tesla designed it as a tool for investigating the properties of high-frequency electricity and for experiments in wireless energy transmission.
While the Tesla coil is best known today for its spectacular electrical discharges (the dramatic arcs of artificial lightning that appear in science museums and films), its underlying principle of resonant energy transfer has found practical applications across modern technology.
Wireless charging for smartphones and electric toothbrushes uses resonant inductive coupling, the same principle that Tesla demonstrated with his coils in the 1890s. RFID tags (used in contactless payment cards, building access systems, and inventory tracking) transfer energy wirelessly between a reader and a tag using resonant coupling. Medical devices, including certain types of implanted pacemakers, use wireless energy transfer to recharge batteries without surgical intervention.
Tesla’s vision of transmitting power over long distances without wires has not been realized at the scale he imagined. But the short-range wireless power transfer that his coil principle enables is now a routine feature of consumer electronics.
The Polyphase AC Power System
Tesla did not merely invent the AC motor. He designed a complete system for generating, transmitting, and utilizing alternating current. His patents covered polyphase generators (which produce multiple synchronized AC waveforms), polyphase transmission systems, and polyphase motors designed to work together as an integrated system.
The polyphase AC system that Tesla developed for George Westinghouse in the late 1880s became the foundation of the modern electrical power grid. The system installed at the Niagara Falls hydroelectric plant in 1895, which used Tesla’s polyphase generators and transmission system, was the first large-scale demonstration that AC power could be generated at one location and transmitted over long distances to consumers. The success of the Niagara installation settled the “War of Currents” between AC (championed by Tesla and Westinghouse) and DC (championed by Edison) decisively in favor of AC.
Every modern power grid in the world uses three-phase alternating current, the system that Tesla patented. The voltages, frequencies, and specific equipment have evolved, but the fundamental architecture remains his.
Radio and Remote Control
Tesla’s contributions to radio are the subject of one of the longest-running priority disputes in the history of technology. Guglielmo Marconi is conventionally credited with inventing radio, but Tesla demonstrated radio-frequency transmission and reception before Marconi’s earliest experiments. In 1943, the United States Supreme Court ruled that Tesla’s radio patents had priority over Marconi’s, a ruling that came too late to benefit Tesla (who had died earlier that year) but that confirmed his contribution.
Tesla also demonstrated one of the earliest examples of remote control. At an 1898 exhibition at Madison Square Garden, he controlled a small boat wirelessly using radio signals. The audience was astonished; some suspected a trick, and one spectator reportedly asked if the boat was controlled by a trained monkey hidden inside. Tesla’s patent for the “teleautomaton” (US Patent 613,809) described the principles of remote radio control that would later be used in unmanned vehicles, drones, and robotic systems.
Lighting and High-Frequency Applications
Tesla held several patents for electric lighting systems, including designs for fluorescent and gas discharge lamps that anticipated commercial fluorescent lighting by decades. His experiments with high-frequency currents, described in his famous 1892 London lecture, demonstrated that electrical energy at sufficiently high frequencies could light lamps wirelessly and pass through the human body without causing harm.
Modern applications of high-frequency electricity include microwave ovens (which use high-frequency electromagnetic energy to heat food), induction cooktops (which use high-frequency magnetic fields to heat cookware directly), and plasma lighting (which uses high-frequency excitation of gas to produce light). While Tesla did not invent any of these specific devices, his foundational work on high-frequency alternating currents established the scientific and engineering basis on which they were later developed.
Tesla’s Publications
Tesla communicated his ideas not only through patents but through lectures and publications that described his experiments in vivid, accessible detail. His 1892 lecture to the Institution of Electrical Engineers in London, in which he demonstrated the behavior of high-frequency, high-voltage alternating currents, is one of the most celebrated presentations in the history of electrical engineering.
Kronecker Wallis’s edition of Tesla’s Experiments with Alternate Currents presents this lecture in full, documenting the experiments that first demonstrated the practical potential of high-frequency electricity. The book captures Tesla at the peak of his powers: a brilliant experimentalist showing an astonished audience what the future of electricity would look like.
The individual patent illustrations that document Tesla’s most iconic inventions are available as posters: the Tesla Coil, the AC Generator, the Electric Light, and the Electromagnetic Motor. Each reproduces the original patent drawings that Tesla submitted to the United States Patent Office, the technical blueprints of inventions that still power the modern world.
Why Tesla’s Patents Endure
Most patents become obsolete within years of being filed. The technology they describe is superseded by newer, better approaches. Tesla’s patents are unusual in that the fundamental principles they describe have not been superseded. The induction motor has been refined and optimized over the past century, but its operating principle is unchanged. The polyphase AC system has been scaled up enormously, but its architecture is the same. Resonant energy transfer has found new applications, but the physics is what Tesla described.
This endurance reflects something about the nature of Tesla’s inventions. He was not optimizing existing technologies. He was discovering fundamental principles of electrical engineering and embodying them in practical devices. The principles he discovered are not the kind that become obsolete. They are as permanent as the laws of electromagnetism themselves. A century from now, electric motors will still use rotating magnetic fields, power grids will still use polyphase alternating current, and wireless energy transfer will still use resonant coupling. Tesla’s patents did not merely describe products. They described physics.
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