What is "coherers"

Usage examples of "coherers".

It's the lingo, the ab­breviations, the barnyard to attic chitchat of amateurs with keys, with Marconi coherers or Fessenden barreters-and you can listen for a violin solo pretty soon now.

They used spark-gap transmitters and coherers to pick up the signals, and with enough work, I thought I could get one going.