De Forest Audion, unknown maker, 1906-1915.
In October 1906 American engineer Lee de Forest announced the invention of the audion at a meeting of the American Institute of Electrical Engineers. De Forest had added a ‘grid’ to the thermionic valve which John Ambrose Fleming had invented and shared with the Marconi Company in 1904. De Forest’s innovation enabled the device to not only detect radio signals, but also amplify them. This was a crucial step in the development of radio engineering. A bitter battle over patents and credit for the devices followed between Fleming and de Forest.