In the early days of radio, there were no such thing as amplifiers. Consequently, a weak radio signal could not be heard at a great distance. Inventors developed something called a coherer, to remedy this-it was a glass tube filled with metal filings. It acted as a sort of relay-if it detected a radio signal, the metal grains would align, and allow the passage of a local current. In effect, it was a primitive form of amplifier.
How did these things work? Has the principle behind them ver been used since?
Wikipedia is your friend.
Ralph: You will find most if not all that you want to knowat: <http://en.wikipedia.org/wiki/Coherer>
Simplified answer: Metal filings conduct poorly. When in the presence of an electromagnetic signal, they clump together and conduct better. So if you are passing current through a coherer, the current flow will tend to mimic the radio signal. There are any number of reasons why such a device is sub-optimal - this is left as an exercise for the student.
I wasn’t real happy with the Wikipedia offering, so I put together my own coherer page on Citizendium.org sometime back. Also, there is a nice do-it-yourself page at Simple Homemade Coherer.
Awesome demo of a working coherer here.
BTW, “coherer” is an excellent Scrabble word.