a self-educated French physicists and mathematician as well as a key figure in the development of the fields of electricity and magnetism. Ampère is known for providing independent verification of Avogadro's Law. However, his most important work began in the early 1820s, when he attempted to give a combined theory of electricity and magnetism after hearing about experimental results by the Danish physicist Hans Christian Ørsted. Ampère postulated that an electric current is capable of exciting a magnetic field and demonstrated that the direction of the magnetic field is determined by the direction of the current. He developed a quantitative relationship for the strength of a magnetic field in relation to an electric current (known as Ampère's theorem) and theorized how iron becomes magnetized. Ampère also devised a rule governing the mutual interaction of current-carrying wires (known as Ampère's law) and defined the unit of measurement for current flow, now known as the ampère. Ampère's theory became fundamental for 19th century developments in electricity and magnetism.