CathodeK

Control gridG

Plate (anode)P

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— Emitted 0/s Returned 0/s To plate 0/s

Controls

Heater power75%

Hotter cathode, stronger thermionic emission (≈ T² e^(−W/kT))

Grid voltage V_g−4.0 V

A negative grid repels electrons — this is the "valve"

Plate voltage V_p180 V

The plate's high positive voltage attracts the electron stream

AC grid signal — amplification demo Electric-field arrows Electrode labels Auto-orbit (sweep around the viewing window)

Plate current

Oscilloscope

Grid voltage (input) Plate current (output)

A few volts of swing on the grid produce milliamp-scale changes in plate current; across a load resistor that becomes an amplified, inverted voltage output — this is amplification. The jitter on the cyan trace is genuine shot noise: electrons arrive one at a time.

How it works

Thermionic emission. The heater warms the cathode until electrons gain enough energy to escape its surface into the vacuum.

Space charge. The escaped electrons form a negative cloud near the cathode that repels latecomers — a self-regulating reservoir of electrons.

Plate attraction. The plate's high positive voltage pulls electrons out of the cloud and across the vacuum. Current flows one way only — remove the grid and you have a diode.

Grid control. The sparse helical grid sits directly in the electrons' path. A small negative bias is enough to throttle the current, cutting it off entirely near V_g = −V_p/μ. Tiny input, large output — this is how the triode amplifies.

I_p ≈ K·(V_g + V_p/μ)^3/2 — the Child–Langmuir three-halves-power law; here μ = 18

About Triode Vacuum Tube

Before the transistor there was a heated wire in a glass bottle. Boil electrons off a glowing cathode, attract them to a positively charged anode, and you have a one-way valve for current. Slip a wire mesh — the grid — between the two, and a small voltage on that mesh throttles a large current through the tube. That is amplification, and for half a century it was how every radio, telephone line, and computer worked.

This exhibit is a particle-level simulation of a coaxial triode, the same geometry as the real thing: cathode at the center, grid wound around it, anode cylinder outside. Swing the grid voltage and watch the electron cloud pinch off or pour through; the tube’s characteristic curves are not painted on — they emerge from the simulated particles.

Lee de Forest added the grid to Fleming’s diode in 1906 and called the result the Audion. Cleaned up into the vacuum triode, it became the founding component of electronics — and the direct ancestor of the logic gates you can build in DigiSim today.

Specifications

Invented: 1906 — Lee de Forest’s Audion triode
Structure: Coaxial: cathode, control grid, anode
Principle: Thermionic emission, grid-controlled current
Simulated: Per-electron particle dynamics in the tube field

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Controls

Plate current

Oscilloscope

How it works

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