Between 1882 and 1888, Tesla went from an obscure telephone engineer in Budapest to the inventor of a motor that would help power the modern world. The key was an idea about magnetism that came to him, he said, during a walk in a city park.

AC and DC

To understand Tesla's achievement, we need two terms. Direct current (DC) flows steadily in one direction, like water through a pipe. Alternating current (AC) reverses direction many times a second, flowing back and forth. Early electrical systems, including Thomas Edison's, used DC. AC had one huge advantage: its voltage could easily be raised or lowered with a transformer. High voltage can be sent over long distances with little loss, then reduced to safer levels for homes. DC systems of the 1880s could not do this efficiently, so power stations had to be within about a mile of customers.

But in the early 1880s AC had a major weakness: there was no practical AC motor. Factories needed motors, and without one, AC was useful mainly for lighting.

The vision in the park

In 1882, Tesla later wrote, he was walking in a park in Budapest with a friend at sunset, reciting lines from Goethe's Faust, when the solution came to him in a flash. He drew it in the sand with a stick. The idea was a rotating magnetic field. If two or more alternating currents, out of step with each other, flowed through coils arranged around a circle, they would create a magnetic field that turned around the circle continuously. A metal rotor placed inside would be dragged around by the field, without any commutator or direct electrical connection to the rotor. This was the principle of the induction motor.

Tesla's account of the park was written decades later and may be polished. But his notebooks and patents confirm that he worked out the principle in these years.

Paris and Edison's company

In 1882 Tesla moved to Paris to work for the Continental Edison Company, installing and repairing DC lighting systems. In 1883, in Strasbourg, he built his first working model of an AC induction motor in his spare time. He could not find investors in Europe. His manager encouraged him to go to America and meet Edison himself.

Arriving in New York

Tesla arrived in New York in June 1884, aged twenty-eight, with little money, according to his own account only a few cents, some poems and calculations. He went to work at Edison's machine works, improving DC dynamos. Edison was already famous for the phonograph and the practical incandescent light bulb, and was building DC power stations in Manhattan.

The two men had very different styles. Edison worked by tireless trial and error, testing thousands of materials; Tesla favored theory and calculation. Tesla later told a famous story: that Edison or his manager promised him $50,000 if he could redesign the company's dynamos, and when he succeeded, Edison laughed and said, “Tesla, you don't understand our American humor.” The story comes only from Tesla, told years later, and historians cannot confirm it. Either way, Tesla left Edison's company in early 1885, after about six months.

Hard times

Tesla's next venture was a company to develop arc lighting in Rahway, New Jersey. He designed a system, but the investors took control of the company and pushed him out, leaving him with worthless stock. In the winter of 1886 to 1887, by his own account, he worked as a ditch digger to survive, a bitter experience for a trained engineer.

Patents and a famous lecture

In 1887 his luck changed. A superintendent at Western Union, Alfred Brown, and a lawyer, Charles Peck, backed a new company, the Tesla Electric Company, to develop his AC ideas. Tesla quickly built working motors and filed a series of patents on his polyphase AC system: generators, transformers and induction motors that worked together. The first were granted in 1888.

In May 1888, Tesla presented his system in a lecture to the American Institute of Electrical Engineers. It made his reputation overnight. At almost the same time, the Italian physicist Galileo Ferraris published his own independent work on rotating magnetic fields. Simultaneous discovery is common in science; Tesla's advantage was a complete, patented, practical system.

Worked example: why AC mattered

Imagine a growing town in 1888 that wants electric light and power for factories. A waterfall ten miles away could drive generators. Compare the two systems.

• DC. The power cannot be sent ten miles without huge losses unless the copper wires are impossibly thick. The town must build small coal-burning power stations in every neighborhood instead, and cannot use the waterfall.
• AC without a motor. Transformers let the town send high voltage cheaply from the waterfall, and lights work well. But factories cannot run their machines.
• AC with Tesla's motor. High voltage carries power from the waterfall, transformers reduce it for homes, and induction motors run factory machines. One system serves both light and power, from a distant source.

That last combination is essentially how the world's electric grids still work. The induction motor, rugged and simple, with no sparking commutator, became one of the most widely used machines on Earth, in factories, pumps, fans and appliances. Tesla's idea from the Graz lecture hall had become real.