English: Photo of a carbon detector or "microphone detector" from 1909, a primitive coherer radio wave detector used in the first radio receivers. It was used from about 1900 to 1910 to receive Morse code radiotelegraphy transmissions from the first spark radio transmitters . The microphone detector was invented by David Edward Hughes, who stumbled on radio wave communication in 1879 while he was experimenting with carbon microphones. It consists of a steel needle resting across two sharp edged carbon blocks. One carbon was connected to the aerial and the other was connected to ground, so the radio waves from the antenna were applied across the needle contact. With no radio signal the device had a large resistance (megohms) but the radio wave altered the steel-carbon contact causing its resistance to drop. An earphone in series with a battery was connected across the carbons, so when the needle conducted electricity it made a hissing noise in the earphone. Greenleaf Whittier Pickard first observed radio wave rectification in 1902 with a similar detector. A semiconducting layer of corrosion on the needle which caused the device to act like a diode may have been responsible for the rectification. This was the first semiconductor diode. Information from Douglas, Alan (April 1981). "The Crystal Detector". IEEE Spectrum 18 (4): 64–69. DOI:10.1109/MSPEC.1981.6369482. ISSN 0018-9235.