File:Popov coherer radio receiver.jpg

English: One of the first radio receivers, built by Russian physicist Alexander Popov as a lightning detector in 1894. It is not clear from the source whether this is the original or a recreation.
It uses a coherer (top), a glass tube containing metal filings between two electrodes, invented by Edouard Branley in 1890. Radio waves are received by the wire dipole antenna (top) and applied to the coherer electrodes. The radio waves cause the metal powder to stick together (cohere) and decrease the resistance between the coherer electrodes, turning on a DC circuit to ring a bell. Popov improved the coherer by adding a relay, and a mechanical vibrator (electric bell, center) which disturbed the filings, resetting the coherer to its receptive state after each signal had been received. On 25 April 1895 Popov demonstrated the receiver to a meeting of the Russian Physico-Chemical Society at Petersburg University. In Russia Popov is regarded as the inventor of radio, and 25 April is celebrated as Radio Day..