By 1840, ensconced in a laboratory he’d set up in the family home, Joule was building batteries, electromagnets, and motors, and investigating their behavior. One of his earliest observations was his most significant. He noticed that as an electric current runs through a wire, the wire becomes warmer. Electricity, in other words, can produce heat as well as do work by turning a motor. (From now on I shall use the word work to mean what Carnot called motive power
