The Vacuum Tube: A Unit Study

The Vacuum Tube: A Unit Study
Lee de Forest

On October 20, 1906, the American Lee De Forest demonstrated what is now called a triode, a type of vacuum tube. This device opened the way to practical radio communications and led to the invention of the transistor.

How the Triode Vacuum Tube Works

A triode is a device in a sealed glass tube or metal envelope. The tube is always completely evacuated of air. The actual amplifying device inside the tube consists of a cathode, a grid and an anode, also called the plate. The purpose of the cathode is to emit electrons. To do this it must be hot, as hot metals emit electrons when the heat causes the electrons to vibrate enough so as to escape the metal.

The cathode can either be a plain filament, or a metal cylinder with a filament wrapped inside it. The stream of electrons thus emitted must, on their way to the plate, pass through a spiral of wire on which the signal to be amplified is applied. It is this spiral of wire, called the grid, that causes the power of amplification.

The Vacuum Tube: A Unit StudyBy applying a negative signal on this grid, an electrostatic field is created within the tube between the cathode and the plate. This field has the net result of greatly reducing the current flowing to the plate. Inversely, by putting a positive signal on the grid, the created electrostatic field greatly increases the current flowing to the plate. This increase and decrease in current can be easily translated into an increase and decrease in voltage. Hence, the final result is a much larger change in voltage on the plate of the tube than that on the grid, resulting in amplification.

The Transistor

In the late 1940’s, the transistor was invented. Similar in principle to the triode, the transistor did not need a filament; and, as the filament in a vacuum tube typically pulled at least five times more power than the rest of the tube, the result was an improvement in efficiency – indispensable for battery-powered devices. The transistor also wouldn’t burn out over time; it could last indefinitely. To top that off, the transistor operated on much lower voltages. To allow electrons to cross the vacuum, the tube needed high voltages on the plate, 250V being the average. As far as size goes, there was no comparison, the smallest vacuum tubes ever produced were still at least four times as large as a standard transistor.

Thus, the triode opened the door to longer distance radio communications because of its amplifying powers, and led to the invention of the transistor. The transistor led to modern day compact electronics, which owe their production to those bulky glass bulbs called vacuum tubes.

Parent-Teacher Resources


Further Investigation

A Yale Man
A look at a few of Lee de Forest’s notebooks.

Electronics At Work
1943 look inside vacuum tubes and how they were used by Westinghouse.
(You may want to install an ad blocker before viewing.)

The Vacuum Tube
Brief history from PBS.

The Transistor in a Century of Electronics
Easy-to-understand background.

From Tube to Chip: Early Computer History
Interesting slide presentation

Milestones in Radio Technology

How Vacuum Tubes Really Work
More technical information for the older student.

Vacuum Tubes
“Overgrown page” that “covers all kinds of vacuum-tube electron devices, especially receiving tubes. Many experiments are suggested, and much tube lore and curious circuits are presented.”



Vacuum Tube Diodes
Interactive from Florida State University that shows what happens when you increase current in a vacuum tube.

An interactive showing how the triode vacuum tube works.

One Tube Regenerative Radio Project
From 1935, but you can still find the tube used if you are adventurous.

Electron Tubes
Part of the free electronics course, Lessons in Electric CircuitsExperiment also available.



Experiments With Vacuum Tubes by Sir David L. Salomons
Public domain work for the enthusiast.


Printables and Notebooking Pages

Science Experiment Notebooking Pages
For documenting.

The Vacuum Tube Notebooking pages
Simple pages for copywork, narrations, or wrapping up.