André-Marie Ampère made the revolutionary discovery that a wire carrying electric current can attract or repel another wire next to it that’s also carrying electric current. The attraction is magnetic, but no magnets are necessary for the effect to be seen. He went on to formulate Ampere’s Law of electromagnetism and produced the best definition of electric current of his time.
Ampère also proposed the existence of a particle we now recognize as the electron, discovered the chemical element fluorine, and grouped elements by their properties over half a century before Dmitri Mendeleev produced his periodic table.
The SI unit of electric current, the ampere, is named in his honor.