All early radio work used spark transmitters, which could only transmit the dots and dashes of Morse code. But, just as the telegraph had led to the telephone, various experimenters worked to develop radio transmitters which could transmit full audio. By 1914 the first vacuum-tube radio transmitters began to appear, a key technical development that would eventually lead to the introduction of widespread broadcasting. Both amateurs and commercial firms started to experiment with the new vacuum-tube transmitters. However, all of these fledgling broadcasting experiments came to an abrupt end on April 6, 1917, when the United States entered World War One. At that time, all radio stations not needed by the government were closed, and it became illegal, for the duration of the war, for the general population to listen to any radio transmissions, from any source. Broadcasting experimentation, in most cases using vacuum-tube transmitters, accelerated beginning in 1919, after the end of the wartime civilian radio restrictions. On April 26, 1921, weather broadcasts were heard for the first time on radio when WEW in St. Louis, Missouri, aired weather news. Weather forecasts continue to be the top reason why people listen to radio, rating higher than music, news, sports, or commercials.