1907

Audion

De Forest's two Audion patents of January 1907

The document filed in January was not called "Audion" or "Amplifier" — it was titled "Space Telegraphy," and it claimed an oscillation detector.

Two patents, two weeks apart

The United States issued Lee de Forest Patent No. 841,387, "Device for Amplifying Feeble Electrical Currents," on 15 January 1907. The application had been filed on 25 October 1906 and described an evacuated vessel with three electrodes, one of them heated.

That was not the clearest patent for the familiar grid Audion. De Forest filed a separate application on 29 January 1907 — the one describing a conducting grid between the heated filament and the plate — which became Patent No. 879,532 on 18 February 1908.

Why the year, not a day

"Invented in 1907" folds three different milestones together. The MIT Museum dates the Audion's invention to the latter half of 1906 and treats 15 January 1907 as the patent milestone.

The two January dates are both real and both partial — a grant and a filing — and the grid patent's grant belongs to 1908. Neither can stand alone as the day the triode was invented.

What the device could actually do

The January filing presented the device chiefly as a more sensitive detector of wireless oscillations, and its stated aim involved a gaseous medium — the opposite of the high-vacuum tubes that later made electronics reliable.

The early Audion was conceived as a detector or amplifier for weak signals, but in its original gas-filled, low-vacuum form it remained crude and unreliable.

Calling it the first practical electronic amplifier is a retrospective description of the improved tube: later engineering, including Harold Arnold's work on the vacuum, was needed before it became robust.

Sources

Researched 23 Aug 2026 7 sources 2 audit passes Date disputed

How this was checked

Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →

Date disputed. Sources disagree about when this happened.

What the sources leave uncertain

The listed year is conventional but does not identify one clean moment of invention. Smithsonian and MIT descriptions place development of the three-element Audion in 1906, while relevant patents were issued in both 1907 and 1908.
Patent No. 841,387 contains three-electrode claims but depicts multiple experimental arrangements. Patent No. 879,532 more clearly describes the grid between the heated filament and plate.
Whether the primitive Audion itself should be called the first “practical” amplifier depends on whether credit is assigned to de Forest’s concept or to the later high-vacuum engineering that made it reliable.