Magnetic detector

Lore & Background

The magnetic detector was developed by Marconi during his transatlantic radio communication experiments in December 1902, when he found the coherer too unreliable and insensitive for detecting weak long-distance signals. Marconi devised a configuration with a moving iron band driven by a clockwork motor passing by stationary magnets and coils, providing continuous sensitivity. Ernest Rutherford claimed he had also invented this configuration. The detector was the official detector used by the Marconi Company from 1902 through 1912, when the company began converting to the Fleming valve and Audion-type vacuum tubes, and it remained in use through 1918. The device worked by hysteresis of the magnetization in the iron wires. Permanent magnets magnetized the iron band along its axis, and the radio signal from the antenna passed through an excitation coil, canceling the hysteresis and causing a change in magnetic flux that induced a current pulse in a pickup coil connected to a telephone receiver. The radio signal from spark gap transmitters consisted of pulses of radio waves repeating at an audio rate, producing a musical tone or buzz in the earphone. The detector produced electronic noise heard as a "hissing" or "roaring" sound in the background, due to the Barkhausen effect. The operator had to keep the mainspring wound up; if forgotten, the band would stop and the detector would cease working. The detector could only be used with earphones, not with recording instruments like the siphon paper tape recorder.

Reader's Guide

The magnetic detector was significant as a more sensitive and reliable alternative to the coherer, which was prone to false triggering and insensitivity. Its robustness made it especially valuable on ships, where vibration would disrupt other detectors. The detector's use on the RMS Titanic during its sinking in 1912 highlights its historical importance in maritime wireless communication. However, it was not as sensitive as the Fleming valve, which began to replace it around 1912. The detector required careful design: the permanent magnet field strength had to match the radio frequency excitation coil's field, the tuner impedance had to be low, and the telephone earphone impedance had to match the pickup coil. Its legacy lies in bridging the gap between the unreliable coherer and the later vacuum tube detectors, serving as the primary detector for Marconi stations during a critical period of early radio development.

Did You Know?

• The magnetic detector was sometimes called the 'Maggie'.
• It was used on the RMS Titanic to summon help during its sinking on 15 April 1912.
• The iron band consisted of 70 strands of number 40 gage silk-covered iron wire.