One of the major early works about electricity and magnetism was written by Scottish physicist James Clerk Maxwell (1831–1879), who in 1873 published *A Treatise on Electricity and Magnetism.* It contained his mathematically based theory of the electromagnetic field. These equations, now known as *Maxwell’s equations,* include four partial differential equations that provided a basis for the unification of electric and magnetic fields, the electromagnetic description of light, and, ultimately, Albert Einstein’s theory of relativity. Although most people recognize Isaac Newton’s work on mechanics, few remember Maxwell’s electromagnetic theories (including the idea of the electromagnetic wave) when it comes to classical physics. But his theories eventually led to many things we take for granted today, including radio waves and microwaves.