English: Model RJ5 Audion vacuum tube detector, produced by the De Forest Telephone and Telegraph Co., from an advertisement in the back of a radio magazine. Roughly 6 inches (16 cm) square. The Audion (visible on front), invented by American engineer Lee De Forest in 1906, was the first device that could amplify and the first triode, consisting of a partially evacuated glass bulb containing three electrodes: a electrical filament, a wire grid, and a flat plate. This device was designed to be used as the detector in an early radio receiver, rectifying the radio signal provided by a "tuner" section to DC to extract the audio modulation. The Audion required careful adjustment of the filament current and plate voltage during operation, accomplished by the potentiometer on the right side and the plate resistance switch on the front.

During the early years De Forest did not sell Audions alone, but only as part of these detector modules. The RJ5 was priced at $25. This was a fortune for the young experimenters who were the Audion's main customers. The other parts could be purchased anywhere; they wanted the bare Audions. When the tube burned out, in order to get a replacement De Forest required the purchaser to mail the grid and plate assembly back to the company as proof of purchase, to get a replacement. A lively trade developed in burned-out Audion parts in the back of electronics magazines. After it burned out owners would often break the tube up, sending one of the dual plates back to De Forest to get a replacement, and selling the other to another radio amateur so he could send it in to get his own Audion.