English: Diagram of a spark-gap transmitter, the first widely used type of radio transmitter, used to send Morse code signals during the "wireless telegraphy" era between 1900 and 1920. This example, from a 1917 boy's handcrafts book, shows a typical low power set that was homemade by radio amateurs and hobbyists of the time. Alterations to image: removed caption and added label to telegraph key.