English: Inventor Lee De Forest (left) and another man experimenting with an early AM radio transmitter using his audion vacuum tube. The Audion tube (inset, top), the first triode, invented by de Forest in 1906, was the first electron tube that could amplify. Its amplifying properties were only discovered around 1910-1912; this picture is from a 1916 Popular Science magazine article which describes its newly-discovered uses in radio transmitters and receivers, as an audio amplifier, and as an electronic oscillator to produce electronic music. The caption gives a simplified description of how the tube works in a radio receiver: "In appearance the audion closely resembles an electric light bulb. Built into the bulb are two metal electrodes which are connected in such a way that a perfect electrical balance is maintained between them. When a wireless wave disturbs this balance, the disturbance is heard in the telephone receivers." The article doesn't explain what the apparatus in the photo is, but the same picture appearing in the article Lee De Forest, A Review of Radio in Broadcast Radio magazine (Doubleday, Page, and Co.), Vol. 1, No. 4, August 1922, p.340 is captioned: "Dr. De Forest measuring the current in circuits employing a large-sized vacuum tube for wireless telephone transmitting."