In 1876, a German scientist, Ferdinand Braun found that with a point contact the resistance of the junction in the galena depended on which way round the battery was connected. We call this property rectification and the device he created is called a diode. Galena is a semiconductor which found use in early wireless communication systems. For example, it was used as the crystal in crystal radio sets, in which it was used as a point-contact diode to detect the radio signals. The other use of this crystal was discovered in 1907 by the British experimenter H. J. Round of Marconi Labs - the first LED (Light Emitting Diode) in 1927. While probing he noticed that one of the crystals threw off a bit of light. The peculiar thing is that it generates light when power is run through it both forward and reverse biased. This phenomenon is called electroluminescence. recorded during residency at Skolska 28, Prague CZ 06-09 2014
