The galena detector
	A long time ago, before the invention of the radio, it was a well-known fact that some crystals have a ‘one-way’ electric current flow.  If a thin wire with a sharp point is pressed on a crystal, the current can only flow from the wire into the crystal, and not the other way around.  These crystals, known as galena detectors, were used in old crystal receivers and were the predecessor of the semiconductor diode.
	Close-up of a galena detector, the precursor of the crystal diode.
	The material used for the crystal was germanium, silicon or lead sulphate, on which a metal wire was placed.  This steel or gold wire could be moved by a dial.  This was needed because the radio listener had to search for the best reception on the crystal, the location for the highest sensitivity varied a lot.  Especially the horizontal implementation in a glass tube is well known.
	These galena detectors were used for a years without comprehending its operation.  When In the ’30’s and ’40’s crystals were researched, it was found that the detection was caused by the pollution of the crystal.