Electromagnetic induction

Lore & Background

He connected one wire to a galvanometer and the other to a battery, observing a transient current—which he called a 'wave of electricity'—when the battery was connected and disconnected. This induction was due to the change in magnetic flux. Within two months, Faraday found other manifestations, including transient currents from sliding a bar magnet in and out of a coil, and a steady DC current from rotating a copper disk near a bar magnet (Faraday's disk). Faraday explained induction using his concept of lines of force, but scientists at the time widely rejected his theoretical ideas because they were not formulated mathematically. An exception was James Clerk Maxwell, who used Faraday's ideas as the basis of his quantitative electromagnetic theory. In Maxwell's model, the time-varying aspect is expressed as a differential equation, which Oliver Heaviside referred to as Faraday's law, though it differs slightly from Faraday's original formulation and does not describe motional emf. Heaviside's version is the form recognized today as the Maxwell–Faraday equation. The law states that an induced current will flow in the direction that opposes the change which produced it, reflected by the negative sign in Faraday's law.

Reader's Guide

Electromagnetic induction is a cornerstone of classical electromagnetism and modern technology. James Clerk Maxwell later mathematically formalized this as Faraday's law of induction, which he generalized into the Maxwell–Faraday equation, one of the four Maxwell equations that unify electricity, magnetism, and optics. The principle has found widespread applications, including electrical components such as inductors and transformers, and devices such as electric motors and generators. Faraday's law describes two distinct phenomena: motional emf (from a moving wire in a magnetic field) and transformer emf (from a changing magnetic field). Albert Einstein noted that both situations correspond to relative motion between a conductor and a magnet, and the outcome is unaffected by which one moves—a key insight that contributed to his development of special relativity. The law remains fundamental in physics and engineering, enabling the generation and transformation of electrical energy.

Did You Know?

• Michael Faraday's first experimental demonstration of induction used two wires wrapped around opposite sides of an iron ring, similar to a modern toroidal transformer.
• Faraday observed a transient current, which he called a 'wave of electricity', when connecting and disconnecting a battery to one wire.
• The Maxwell–Faraday equation is one of the four Maxwell equations in the theory of electromagnetism.