The thing that makes Tesla such a compelling, yet sad case is that he was genuinely brilliant and had carved himself a real place in history with his accomplishments.  His contributions to the field of electrical engineering are on a scale to rival that of Edison and Steinmetz and we enjoy the fruits of his labours every time we flick on a light switch.  However, his lonely work habits, refusal to write things down, and flat-out eccentricities have made him one of those figures that historians cross the street to avoid.

AC Induction Motor

Tesla's first great invention was the AC induction motor.  For those of you who weren't paying attention in science class, an electric motor works by flipping the field of an electromagnet, causing the attraction/repulsion of the magnets to spin the armature around.  The sort of small motors that you see in things like toy cars do this with metal brushes that touch two metal semi-circles.  As the armature spins, the brushes touch one semi-circle, then the other, and each time the electrical current in the armature reverses and therefore its magnet poles do so as well, so the armature spins 'round and 'round like a puppy chasing its own tail.

Tesla was the first inventor to come up with a practical way of using AC power to achieve the same reversal of polarity; only this time it's done with clever wiring.  Unlike DC, where the electrical current always flows in one direction, AC current flows in both directions.  By wiring some the magnetic sections of the motor one way and then their neighbour sections in reverse, the polarity would reverse automatically with the current. 

Of course, the trick was how to supply current to the armature itself without burning out the contacts with the high voltages that AC power required.  Tesla's answer was to use induction.  That is, whenever AC power reverses itself, it causes an electrical field to collapse and re-establish it.  When this happens, power is transmitted.  It's what allows certain makes of cordless toothbrushes to recharge without being plugged into anything.  You just stick them into a coil in a charger base and the induction field transmit the power to the brush's battery.  It's also the reason why you don't stand under an AC power line with a steel fishing rod unless you want a shocking surprise.   Anyway, in Tesla's motor, the induction field set up by the AC current feeds power to the armature without any direct wiring needed.

AC Power

But the really neat thing about electrical motors is that if you get one working you also have a perfectly good electrical generator in one of technology's rare twofers.  An electric motor works by taking electricity in and turning it into motion.  But if you take a motor and spin it, out comes electricity.  All this means that when Tesla perfected his motor, he was well on his way toward building a new generation of AC dynamos that form the basis of our modern electrical grid.  Up to that time, the main source of electricity was DC, which relied on low voltage and heavy amperage.  In practical terms that meant it took a lot of oomph to push electricity through a wire and  one could only transmit power about a mile from the generator.  AC power, on the other hand, used high voltages with low amperage and therefore could be transmitted over great distances. 

When Westinghouse  bought up Tesla's patents it sparked commercial war between Westinghouse and Thomas Edison, who was a great backer of DC.  There followed years of bitter propaganda battles, but in the end the AC system won out and Tesla had his greatest triumph when the Niagara Falls power station was built with machines that bore his design and name.

The Tesla Coil

Curiously, despite his achievements, Tesla never had a very good theoretical grasp of what electricity actually is.  He tended to ignore developments in physics.  In fact, he greeted Einstein's theory of relativity with downright hostility.  For Tesla, electricity wasn't a thing of electrons and energy states, but of fluids, vibrations, and harmonics in a system which he seemed to understand, but which made his explanations the thing of which headaches are made.

Whatever his theory, Tesla still managed to get results.  He was fascinated with high frequency electricity, but mechanical generators could only go so fast before they started to fly apart, so he developed devices that could provide higher and higher frequencies without moving parts.  The most famous of these was the Tesla coil; famed denizen of  science fairs and cheap plasma globes the world over.    This high-voltage transformer is familiar to anyone who has seen an old Frankenstein movie where they were used to generate the electrical arcs that are apparently necessary if you're going to be a dedicated mad scientist.  They also produce an electrical field that light fluorescent tubes and similar devices at a distance; a spectacular parlour trick that led Tesla down more than one rabbit hole.

Radio

Tesla's interest in high frequency electricity had other benefits.  A number of his circuits were basic to radio technology.  Because he didn't understand how electromagnetic radiation worked, Tesla thought that sending messages through the air required  transmitting huge amounts of energy, so he never produced a working system, but his patents did predate those of Marconi by several years and Tesla was awarded precedence by US supreme court in 1943.

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