There is a legend about Christopher Columbus winning a wager against his critics by making an egg stand on its tip. That story is often used to illustrate the fact that it is often simple to understand the discovery after it already happened. That principle is undoubtedly true in case of famous inventor Nikola Tesla who discovered laws of rotating (rotary) magnetic field. His discoveries lead to the invention of the polyphase induction motor. Although Tesla didn’t discover alternating current, he was the first one to harness it and use it to do useful work efficiently.