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History
The Vacuum-Tube Diode
The light bulb becomes a one-way electronic valve
The incandescent lamp gave engineers a hot filament inside an evacuated glass envelope. Edison's experiments had already shown that electrical charge could cross the apparently empty space between that hot filament and another electrode. The effect was interesting, but it had not yet become a practical electronic component.
put two useful electrodes in a vacuum, heat one of them, and make current flow mainly one way.
Thermionic emission
When a metal is heated strongly enough, some electrons gain enough energy to escape from its surface. This is called thermionic emission. In the early tube, the hot filament itself served as the electron-emitting electrode. Later tubes often used a separate heated cathode.
If a second electrode — the plate or anode — is made positive relative to the hot cathode, the emitted electrons are attracted across the vacuum toward it. If the plate is made negative, it repels the electrons and very little current crosses the gap.
negative plate → electrons are repelled → current is greatly reduced
John Ambrose Fleming — 1904
The British physicist and electrical engineer John Ambrose Fleming had worked with both incandescent lamps and early radio equipment. In 1904 he used the Edison effect deliberately to make a two-electrode vacuum device that conducted much more readily in one direction than the other.
It became known as the Fleming valve. Today we would call it a vacuum-tube diode: di- because it has two active electrodes.
- a heated cathode or filament that emits electrons;
- a plate/anode that collects electrons when it is positive;
- a vacuum so electrons can travel without colliding constantly with gas molecules;
- and electrical connections brought through the glass envelope.
Why one-way current matters
An alternating voltage repeatedly reverses polarity. A diode responds differently to the two halves of that cycle. During the half-cycle that makes the plate positive, it can conduct. During the opposite half-cycle, conduction is strongly suppressed.
AC in → one polarity passes much more readily than the other → pulsating DC out
That process is called rectification. The same basic idea later appears in semiconductor diodes, power supplies, signal detectors, protection circuits, and countless other electronic systems.
A detector for early radio
Early radio signals were high-frequency alternating currents that carried information in their changing amplitude. A detector had to recover the slower variation that represented the transmitted sound or code. A one-way device could help separate that useful variation from the rapidly alternating radio-frequency signal.
Fleming's valve therefore became useful as a radio detector. It did not amplify the signal; it mainly provided rectification. That limitation would soon become the reason for adding one more electrode.
Measurement trail 22 — a one-way circuit
Suppose an alternating source swings equally positive and negative around zero. An idealized diode can be thought of as passing one polarity while blocking the other.
What changes?
Before the diode, the current reverses direction every half-cycle. After simple half-wave rectification, the load receives pulses all in the same direction.
Real vacuum diodes are not ideal switches. Their current depends on heater temperature, plate voltage, electrode geometry, and the tube's ratings. But the ball-park behavior is easy to remember: one polarity conducts much more readily than the other.
Something is still missing
The diode can detect and rectify, but it cannot use a weak signal to control a much larger electrical current. Radio, telephone repeaters, sound systems, and long-distance communications badly needed amplification.
In 1906, Lee de Forest placed another electrode between the cathode and plate. This third electrode — a wire grid — could control the electron stream without having to carry the full plate current itself.
triode: three electrodes → control and amplification