It was the invention of the Thermionic Valve, by Flemming and De Forest, in 1906 that allowed Marconi to eventually produce a reliable 'Carrier Wave' that could be 'modulated' so that voice and music could be transmitted, rather than the spark-gap transmitters that had been used to produce the dots and dashes used by the Morse Code signals that had hitherto been employed. Using this new method - the carrier wave - enabled Marconi to transmit speech and music from his experimental station 2MT in 1920. Marconi sited this experimental station, 2MT, in an ex army hut at Writtle, Essex. The station was initially allowed to transmit its test transmissions for only half an hour a week. Dame Nellie Melba made one of the first broadcasts from 2MT at 7.10 pm on 15th June 1920. Consisting of a concert of opera music to entertain the listeners, the broadcast opened with a recital of Home Sweet Home and finishing with the national anthem. Those early wireless listeners - the early radio amateurs - heard the broadcasts from Chelmsford to Paris, Madrid and Berlin. Two years later, on 14th February 1922, 2MT in Writtle would commence broadcast daily half hour programmes of news and entertainment which lasted for a period of nearly three years. Listeners could tune in using crystal sets, the simplest form of radio receiver that required no external power or batteries. All that powers the headphones of a crystal set is the energy collected from its aerial which is derived from the radio waves sent by the transmitter of the radio station to which the set is tuned.