Nov. 16, 1904: John Ambrose Fleming Patents the Vacuum Tube

Just one month after he conceived of the idea, British engineer John Ambrose Fleming patented the vacuum tube. Although the device would bring him some grief, its ability to convert alternating current into direct current would revolutionize communication and broadcasting, kicking off the electronics era.

“Just as the double helix inaugurated the age of molecular biology, Fleming’s vacuum tube inaugurated the age of electronics — and dominated it until the advent of the transistor,” said Fred Dylla and Steven Corneliussen in a 2005 paper in the Journal of Vacuum Science & Technology A.

Although the design came together quickly, the physical concepts it exploited had been swirling around Fleming’s brain for years. As a teenager, he’d displayed an aptitude for math and science and a fascination with electromagnetism. He studied under James Clerk Maxwell at Cambridge University and worked in the newly established Cavendish Laboratory, benefiting from what he later called Maxwell’s “supreme genius” for two years before Maxwell’s death in 1879.

After receiving a Ph.D., Fleming became the science advisor of the Edison Electric Light Company’s London branch. Thomas Edison’s incandescent lamps — consisting of a carbon filament enclosed in an evacuated glass bulb — were lighting up the United States, and the new branch was tasked with operating his lighting systems and generators in Great Britain.

This positioned Fleming “to investigate carefully some of the problems connected with the physics of the incandescent lamp,” he wrote in his book The Thermionic Valve and its Developments in Radiotelegraphy and Telephony.

In one of Edison’s experiments with early lamps, he had inserted a metal probe into an incandescent bulb. The probe was attached to a galvanometer, which measures electric current. Edison noticed that charge flowed from the glowing filament to the probe — but only when the probe was positively charged. Noting that the current varied with voltage, Edison patented a lamp-style voltage indicator, but his investigation didn’t go much deeper.

The current — dubbed the Edison effect — intrigued scientists, including Fleming. But no existing theory explained the phenomenon: It was the mid-1880s, and the discovery of electrons was more than a decade away. Scientists reasoned that the filament discharged negative carbon molecules.

Fleming began experimenting with Edison effect lamps, which consisted of an incandescent bulb containing an extra plate electrode with an external connection. Over the next several years, he repeated Edison’s experiments and conducted his own, testing an assortment of filament and bulb designs.

Fleming noted that the bulb’s negative leg was the “active agent” producing the Edison effect, and the space inside the bulb conducted only “negative electricity.” He also found that when the lamp was activated by an alternating current, a continuous current flowed through a galvanometer connected between the extra electrode and either terminal.

“The glow lamp and the electric arc have revolutionized our methods of artificial lighting,” he wrote in an 1890 paper for the Royal Society of London, “but they present themselves also as subjects of scientific study, by no means yet exhausted of all that they have to teach.”

Around the time Fleming started the experiments, he accepted an invitation to establish and chair an electrical engineering department at University College London, England’s first such department, where he lectured regularly and conducted research.

He began a new role around the turn of the century, this time as scientific advisor for Guglielmo Marconi’s Wireless Telegraphy Company. The young Marconi was earning a name for himself by demonstrating long-distance wireless communication using radio technology. He enlisted Fleming’s help for the first wireless transatlantic transmission, which took place in 1901. While proclaimed a success, the transmission underscored two difficulties plaguing radio communication: signal detection and amplification.

Marconi’s technology could transmit radio waves over long distances, but the instruments for detecting the oscillating signals and translating them into direct current were noisy, finicky, and often unreliable.

The coherer, which converted alternating current to direct current, was “about as exasperating a tool for the purpose of making quantitative measurements as one could well imagine,” John Turner MacGregor-Morris wrote in a 1955 article for the Royal Society’s Notes and Records.

While mulling over the problem in October 1904, Fleming was struck by a flash of insight. He could create an electronic rectifier by placing an open metal cylinder around the filament of an incandescent bulb with an outside connection. When inserted in the circuit, “this at once gave us a means for converting the feeble but rapid to-and-fro motions of electricity in an aerial wire . . . into a current of electricity all in the same direction,” Fleming recalled in a 1923 talk broadcast by the BBC. The vacuum tube was born.

Just a month later, the patent application was in. Fleming knew he was on to something, but his device came at a cost. “Few inventions can have brought their inventors so much distress, disappointment and trouble,” wrote Geroge Shiers in a 1969 Scientific American article.

In an effort to create a radio system that bypassed existing patents, U.S. physicist and radio pioneer Lee De Forest set out to develop his own technology. In 1905, he debuted a device similar to Fleming’s vacuum tube. When Fleming, who had already applied for a U.S. patent, pointed De Forest to his work, De Forest promptly dismissed it in a paper. Both patents were granted but, to his chagrin, Fleming’s was eventually ruled invalid.

“Small modifications of the original instruments have been christened, especially in the United States, by many strange and fanciful names,” Fleming wrote in his book. Uninitiated patent examiners may think devices are new when they are, in fact, “destitute of real novelty,” he said.

In the ensuing years, De Forest and others made key modifications that allowed vacuum tubes to function as detectors, amplifiers, and oscillators. That brought them to the forefront of not just radio technology but also telephones, televisions, and nearly all electronic devices prior to the invention of the transistor.

Despite the U.S. patent resolution, Fleming was widely recognized for his pioneering invention and expertise in electronics. He continued consulting, researching, and giving lectures for decades. He was a well-liked professor known for his organized, clear lectures and tendency to talk fast. In 1926, he retired from University College London after more than 40 years of chairing the electrical engineering department. Three years later, he was knighted for his service to science and industry.

Fleming stayed scientifically active after his retirement. In 1932, the Physical Society of London noted that Fleming had been presenting papers at its meetings for nearly 60 years. “Sir Ambrose Fleming has not only made history in this interval, but also still holds a place on the stage of contemporary events,” the article reads. Fleming would give his last paper there in 1939, at age 90. He had been president of the Television Society of London for 15 years when he died in 1945.