Magnetic detector

The magnetic detector—affectionately called "Maggie" by wireless operators—emerged because the coherer was killing radio's commercial promise. By 1900, Guglielmo Marconi had demonstrated transatlantic wireless, but the coherer detector remained maddeningly unreliable: metal filings that clumped together when radio waves arrived, then had to be physically tapped to reset for the next signal. The coherer limited transmission to 8-12 words per minute when telegraph operators could send 50.

The adjacent possible had been prepared by Ernest Rutherford, who in 1895 discovered that alternating magnetic fields could diminish the magnetization of iron—and used this principle to detect wireless signals at Canterbury College in New Zealand. Rutherford briefly held the world distance record for radio communication at 1.5 miles. But he moved to Cambridge to study atomic physics, leaving the magnetic detection principle unexploited.

Marconi patented his refined version in 1902. His design used 70 strands of silk-covered iron wire formed into an endless band, driven by clockwork past two permanent magnets arranged to magnetize the band in opposite directions. Radio signals cancelled the iron's hysteresis—the lag in magnetic reversal—causing a sudden magnetic flux change that induced current pulses in a pickup coil connected to headphones. Unlike the coherer, Maggie required no reset mechanism and produced audio directly.

During the SS Philadelphia voyage of February 1902, Marconi achieved audio reception at 2,100 miles versus 1,550 miles for coherer-tape systems. The magnetic detector became standard equipment on maritime vessels from 1903 to 1912. On January 23, 1909, Jack Binns used one aboard the SS Republic after a collision—his distress signals led to the rescue of 1,650 of 1,656 lives, "monopolizing newspaper front pages everywhere." When RMS Titanic sank in 1912, its Marconi room contained a magnetic detector alongside newer crystal receivers. Over 700 survivors owed their rescue to wireless calls enabled by the rugged detector that operators trusted in North Atlantic swells.