Arago Disc Apparatus

9 Jan 2026

The Arago disc apparatus demonstrates that a rotating conductive disc, such as copper, can induce torque on a freely suspended magnet, causing it to spin in the same direction. 

It demonstrates the principle of electromagnetic induction as well as the existence of eddy currents and magnetic damping. The operating principle is used today in devices known as eddy current brakes, a type of brake used to slow or stop trains by generating eddy currents and dissipating kinetic energy as heat. The unit pictured is recorded in the National Inventory of Historic Scientific Instruments by Charles Mollan, entry 1942 UCP 169. The device comprises mahogany and brass, with large wheel and handle turn spindle to rotate the copper disc as well as glass disc and needle. The device was fabricated circa mid-19th Century.