The Franklin Institute Logo Case Files logo  Guglielmo Marconi:
Application of Radio Waves to Communication, 1918  Marconi
Magnetic Detector  Marconi's magnetic detector, nicknamed the
"Maggie," consists of a continuous iron band moving through a
glass tube to which permanent magnets are attached. The tube is
spirally wound with insulated copper wire; the wire terminals of
this primary coil are connected between the antenna and a
grounded connection. A secondary coil of insulated wire is wound
on a spool which surrounds the primary; its wires are connected
to headphones.  In the detector's operation, each section of the
iron band is magnetized as it passes the permanent magnets and
moves on. When each radio signal reaches the primary coil, the
magnetism in the passing iron band changes. The changing
magnetism induces a current in the secondary coil, resulting in
a clicking sound in the earphones.  Memorandum (290K) Franklin
Medal Certificate text.