Maxwell's Theory of Electromagnetism
Theoretical physics had made significant progress befor6 Einstein. In particular, James Clerk Maxwell (1831-79) had unified magnetism with electricity to give electromagnetism.
By means of four equations, Maxwell explained all the different manifestations of electricity and magnetism - from the emission of light and electric currents to the earth's magnetic field. Maxwell's equations linked the electric and magnetic fields to each other and showed how each of their various manifestations arose as special cases of a general theory.