Inductance Coil & Magnetic-Field Converter

Inductance was discovered in 1830 by Joseph Henry, a young science professor at Albany Academy in upstate New York. While experimenting with multilayer electromagnets wound on iron horseshoes, Henry noticed that opening the switch produced a sharp painful spark across the contacts. He realised, correctly, that the collapsing magnetic field was inducing a high-voltage transient back into the same coil — the first observation of self-induction.

Michael Faraday in London made the same discovery independently in 1831 and, with greater institutional support, published first. Faraday's law of induction — EMF = -dΦ/dt — became the single equation underpinning every transformer, generator, motor, inductor, and ignition coil in industrial civilisation. The unit of inductance was nonetheless named the henry in 1893 to honour Henry's priority of observation.

Inductors became practical in 1837 with Charles Wheatstone's electric telegraph, which used relay coils to repeat weak signals across long copper lines. Each relay was a magnetic switch whose pull-in current was set by its inductance and resistance — the same τ=L/R that the tool above shows live. By 1858 the transatlantic telegraph cable depended on careful inductor design to maintain dot-and-dash legibility across 3000 km of seawater.

The transformer transformed the world. In 1885 William Stanley, working for George Westinghouse in Great Barrington, Massachusetts, installed the first commercial AC distribution system — using transformers wound on iron cores to step voltage up for transmission and back down for lamps. Three years later Nikola Tesla's polyphase induction motor put inductors at the heart of every factory drive. The "war of currents" between Edison's DC and Westinghouse-Tesla's AC ended decisively in 1893 when AC won the Chicago World's Fair contract.

The radio era multiplied inductors into millions. Marconi's 1901 transatlantic transmission used hand-wound spark-gap coils; by the 1920s, RCA receivers each contained a half-dozen tuning inductors. The ferrite core, patented by J. L. Snoek and the Philips lab in 1946, packed huge inductance into small volumes and made portable AM radios possible. The same ferrites today power switching converters and wireless charging pads.

Modern inductors span 14 orders of magnitude. A picohenry-scale PCB trace at the input of a microwave amplifier sets the matching network impedance; a 100 nH chip inductor in a smartphone Wi-Fi front end stays well below its self-resonant frequency; a 10 mH common-mode choke on a power supply blocks line-borne EMI; a 100 H industrial reactor in a substation smooths a 1 GW DC link. The tool above puts all of them on one slider.

The most consumer-visible inductor today is the Qi wireless charging pad. The Wireless Power Consortium standard, adopted in 2010 and now built into nearly every smartphone, pairs two ~10 μH coils across an air gap at 110-205 kHz, transferring up to 15 W via Faraday induction. The same physics scaled to 11 kW now charges electric cars in your garage without a plug — 195 years after Joseph Henry first felt the spark.