During his transatlantic experiments in 1902 Marconi found the coherer to be too unreliable for detecting the very weak signals inherent in long distance transmissions. It was this need that drove Marconi to develop the "Maggie" or magnetic detector.

Maggie proved to be both more reliable and more sensitive than its predecessor, ensuring its commercial success as the standard detector on shipboard receivers from 1902 until about 1914, when it was replaced by the more sensitive crystal and vacuum tube detectors.

The magnetic detector is really a tribute to Marconi's ability as a practical engineer - it would be 20 years before scientists were able to adequately describe how it worked!

Refer to the diagram for a simple (i.e. limited) explanation: A band of Iron wires move continuously around two ebonite pulleys. The wire passes through the center of two concentric coils of wire. The first coil (the primary) is connected to the Antenna and ground. The telephone earpiece is connected to the second coil (secondary). As the band passes under the magnets, the iron atoms are aligned, producing a weak magnetic field. If there is no RF present, the field is unchanged and no change in current is produced in the secondary winding. However, if a RF pulse is present at the antenna, it is passed to the primary coil, producing a magnetic field, which changes the magnetic field on the wire. This change is detected by the secondary coil and is heard as a "click" in the receiver¹.