This field of research is richer with branches than the final results might have suggested.
These magnetic detectors, like the coherer, worked only for detecting the presence
of damped-wave Morse-code transmissions
(where the transmitter was turned on briefly for sending a dot
and a bit longer for sending a dash),
and were made obsolete by continuous-wave transmission for AM radio.
See also in The book of science:
Readings on wikipedia: