Radio detector
The coherer, consisting of a tube containing 2 electrodes a small distance apart with metal filings in between, was used in the first radio receivers at the beginning of the 20th century. It was a key technology, the first device used to detect radio signals in practical wireless telegraphy, and became the basis for radio reception around 1900. It was in widespread use for about 10 years. The hot wire barretter, a highly sensitive thermoresistor developed to permit the reception of amplitude modulated signals, was developed to permit reception of amplitude modulated signals, something that the coherer could not do. It was invented in 1902, and was superseded by the electrolytic detector. A cats whisker, a thin wire that lightly touches a crystal of semiconducting mineral to make a contact-junction rectifier, was used as a receiver in early crystal radios from about 1906 to the 1940s; it was the first semiconductor electronic device.
