Salamis Tablet - Journal of Computer History


The Vacuum Tube in Computer History


Throughout the first half of the 20th century the vacuum tube was the preeminent device of the electronics age. The emergence of vacuum tube technology represented a major advance not only in our ability to better control the flow of electricity through complex circuits, but also in our understanding of the fundamental nature of electrons and their behavior. Beginning with the invention of the Fleming diode in 1904, vacuum tubes figured prominently in the development of radio, telephone networks, radar, industrial control, and computing.

The vacuum tube holds a particularly significant place in the evolution of electronic computing. This was the first device which could function as a fully electronic switch. Electromagnetic relays, which had been in use since the 1830s, enabled electronically controlled switching but still relied on a mechanical component for the switch itself. With the invention of the triode in 1907 and a flurry of subsequent improvements, current flowing through a circuit could be controlled by voltage applied to the tube with no moving parts. This switching functionality is fundamental in the design of circuits which use digital logic to perform computation.

Tube-based diodes, triodes and other components were ultimately replaced by silicon-based diodes and transistors as these faster, smaller and more reliable devices became available in the 1950s.

Thermionic Emission and the Invention of the Vacuum Tube

Vacuum tubes were derived from the incandescent lightbulb and it is therefore not surprising the earliest work in the field took place at Thomas Edison's Menlo Park laboratory. The basic configuration of Edison's lightbulb was a highly resistive carbon filament inside a glass bulb from which the air had been removed. Electricity passing through the filament would cause it to glow from the heat, while being sealed in a vacuum would prevent it from burning away.

In 1883, the lightbulbs were suffering from a tendency to have the glass become blackened with use. Edison, along with his science and technical advisor Francis Upton, began a series of trial and error experiments in search of a solution to the problem. One such experiment involved sealing a second metal electrode in the bulb. Edison and Upton observed that when the electrode was positively charged an electric current would flow from the glowing filament to the electrode, across the vacuum and without direct contact. When the electrode was negatively charged, no current would flow.

The phenomenon they observed would come to be known as thermionic emission, or more colloquially the Edison Effect. In thermionic emission, heating the filament results in electrons being cast-off. In the vacuum of the bulb, the free electrons are attracted to a positively charged metal plate called the anode, effectively creating a circuit.

Edison secured a patent on the vacuum tube as a measuring device. The patent, and subsequent devices, focused on the ability to use variations in the current flowing through the filament to produce variations in the current flowing to anode. In the next few years, this characteristic would be employed in volume control circuits.

It was English physicist John Ambrose Fleming who more than 20 years later first exploited the fact that in thermionic emission the current only flows in one direction - from the heated filament (the cathode) to the positively charged anode. He originally called his device an oscillation valve. It would later be called the Fleming valve or Fleming diode.

Fleming used the valve as a rectifier for a radio circuit. Being waves, radio signals are received as an alternating current (AC), oscillating rapidly between negative and positive voltage. When the signal is "rectified" the negative component of the signal is stripped out, and the positive fluctuations of the wave cause a speaker or earpiece to vibrate and produce sound.

In early radios rectification was often achieved using a small naturally occurring mineral crystal with semiconducting properties such as galena - the crystal of crystal radios - and a thin wisp of metal wire called a cat's whisker. These primitive semiconductor diodes were marginal at best. They were very sensitive to vibration and required frequent adjustment to keep the crystal and whisker properly aligned. They were better suited for hobbyists and experimenters than for commercial use. Fleming's oscillation valve was a tremendous improvement over previous methods for rectification earning him the moniker of The Father of Modern Electronics.

Lee de Forrest is credited with the invention of the triode in 1907, which in essence completed the basic groundwork for the use of vacuum tube devices as amplifiers and switches. De Forrest was looking for ways to improve the rectifying abilities of the diode and began experimenting with the addition of a metal grid as the third electrode in the tube between the filament and the anode. With no current applied to the grid, the thermionic effect was unaffected and electrons flowed freely to the anode. However, when a negative charge was placed on the grid current flowing through the device was reduced and when a positive charge was placed on the grid current flowing was increased. An extremely useful aspect of this behavior was that small changes in the grid voltage would create much larger fluctuations in the current flow.

Vacuum Tubes and Electronic Computers

As the crown jewel and driving force of the emerging electronics industry, vacuum tubes were the focus of tremendous research and development effort. Innovations in pumping enabled the manufacture of higher vacuum tubes and better materials were identified for the filaments, electrodes and other components. Numerous variations and special purpose devices were manufactured, many featuring additional electrodes (tetrode and pentode) as engineers sought to refine and improve performance.

It was against this backdrop of innovation that the age of modern, electronic computing emerged. In 1918 William Eccles and F.W. Jordan invented the flip flop circuit, originally called the Eccles-Jordan trigger circuit, based on two triodes with a feedback mechanism. The flip flop was the first binary circuit and became the foundation for digital memory technologies.

The vacuum tube as a switching device was not universally embraced by computing pioneers. It was seen by many as too unstable and unreliable. More than 30 years after the invention of the triode, such seminal machines as the Harvard Mark I and the Bell Labs computers used electromechanical relays for switching.

An early proponent of implementing digital logic circuitry using vacuum tubes was Colossus designer Tommy Flowers. In order to improve reliability in tube based systems, Flowers contributed multiple innovations, including running tubes at reduced current and developing modular designs that could better tolerate tube failure. Developed to assist in the World War II Allied codebreaking effort, Colossus Mark I, the first electronic, binary, programmable computer went into operation at Bletchley Park in February 1944.

ENIAC, the first general purpose programmable computer, went live at the University of Pennsylvania in 1946. ENIAC is perhaps the most famous of the vacuum tube computers. It was called the Giant Brain and was covered extensively in newspapers and magazines. As a fully electronic computer it was estimated to be 1,000 times faster than electromechanical machines.

Despite the clear advance over the electromagnetic relay, vacuum tube computers faced significant practical challenges. ENIAC was built from more than 17,000 vacuum tubes and had about 5 million soldered connections. The computer occupied 1,800 square feet and required a power supply of about 150kW. Also, engineers were never able to completely resolve reliability issues. ENIAC's longest run between failures was 116 hours.

With the invention of the transistor in 1947 the era of vacuum tube computing quickly drew to a close. In 1953 the first experimental transistor computer prototype was unveiled, followed quickly by full size implementations across academe and industry.


Ifrah, Georges (2001). The Universal History of Computing: From the Abacus to the Quantum Computer. John Wiley & Sons. ISBN 0-471-39671-0.

Reid, T. R. (2001). The Chip: How Two Americans Invented the Microchip and Launched a Revolution (Rev. ed.). Random House. ISBN 978-0-375-75828-7.

Electronics Notes. History of Vacuum Tube/Thermionic Valve.

Wikipedia. Vacuum Tube.

Other Articles in the Series:

The Antikythera Mechanism and the History of Clockwork

From Boole to Bits - Claude Shannon's Digital Revolution

George Stibitz and the Bell Laboratories Relay Computers

Grace Hopper - Matriarch of Programming

SCELBI, Altair and the Journey to Home Computing

The Commodore VIC-20 - The Friendly Computer

Computer History Timeline