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Fleming's Original Thermionic Valves
From: Science Museum
| By:
Eryl Davies |
EDITOR'S INTRODUCTION |
The tale of the inventions of John Ambrose Fleming is sad. Despite his incredible discovery and his role as an instigator of early electronics, Fleming's thermionic valve brought him little respect or financial reward. Instead it created great competition and complex litigation with the American Lee de Forest, who used the valve to generate and amplify speech and radio signals. |
  istress and disappointment hardly seem an appropriate reward for the holder of one of the most significant patents in the history of electronics. Yet those were the misfortunes that befell John Ambrose Fleming (1849-1945) after registering his 'Improvements in Instruments for Detecting and Measuring Alternating Currents' in 1904. This title, and the ensuing unpleasantness, obscure the fact that Fleming's invention was the first practical thermionic device for detecting radio signals. Thermionic devices are those where the source of current is a heated negative electrode (the cathode), and the current flows through a vacuum to the positive electrode (the anode) via other electrodes which can be used to control the current. |
Fleming's valve was the first of a line of devices which were to be the mainstay of electronics for its first half of the twentieth century and well into the 'solid-state' era, when transistors and integrated circuits inspired the miniaturisation which we now expect. Even now, thermionic devices are not extinct. Cathode-ray tubes in television and computer screens and high-power valves in transmitters are examples; perversely, a few hi-fi purists eschew the predictable and consistent performance of solid-state amplifiers, preferring bulky ones with glowing valves. |
John Ambrose Fleming became professor of electrical engineering at University College London in 1885. He was knighted in 1929. Although an academic, he was the instigator of what might be called applied electronics. In 1882 Fleming had become 'electrician' to the Edison Electric Light Company and was soon absorbed with the task of improving carbon-filament light bulbs, which had a tendency to darken. Edison himself had already tried, unsuccessfully, to prolong bulb life by placing an extra electrode alongside the filament; he noticed that, with an extra electrode connected to the positive end of the filament, a small but measurable electric current flowed between them. This 'Edison effect' caused widespread curiosity and Edison patented his modified lamp as a novel 'electrical indicator'. |
Fleming commenced more serious research into the Edison effect in 1889. He had experimental lamps made at the Edison and Swan (later Ediswan) Lamp Works. The results obtained with these, reported in 1890, led to further work in 1895 and 1896. By then, he had investigated electrical conduction in a vacuum between glowing filaments and a second electrode in a variety of shapes and sizes of tubes. |
His painstaking account of this work is now regarded as a classic. But in a world which was more excited by Röntgen's revelations on X-rays and J.J. Thompson's identification of the electron, Fleming's work was over shadowed.
But his moment was yet to come. Wireless telegraphy was expanding rapidly. Fleming became a technical adviser to the Marconi Company in 1899, and helped design the transmitter at Poldhu, Cornwall, for the 1901 transatlantic transmissions. Receiving weak wireless signals, especially over such long distances, was a significant problem, and Fleming, inspired in 1904 by 'a sudden very happy thought', turned to one of his earliest 1889 experimental valves for a solution. He linked it up to a simple circuit containing a battery, a meter, and suitable coupling to an aerial. He found the system worked well. Thus the thermionic valve was born. |
 A patent was granted in 1905, but any benefits it might have brought Fleming became the property of the Marconi Company. 'Cat's whisker' detectors appeared a year later, and were in fact less cumbersome and equally efficient. And in 1907, the American Lee de Forest patented a detecting circuit using a valve with a third electrode. Fleming was already aware of de Forest's activities and had disputed the American's claims to originality through a sometimes acrimonious correspondence with him in the technical press in 1906. De Forest persisted, and a lifelong enmity between the pair ensued. However, it was not until 1912 that the full potential of the extra wire in de Forest's valve was realised, when it was incorporated into circuits which could generate and amplify speech and radio signals--a development which took communications firmly into the electronic era. But without Fleming's persistent work this transition would have been delayed--or credited elsewhere. |
A final sad twist to the story was that enmity between the Fleming and de Forest factions in the United States spilled over into patent litigation, and in 1943 the US Supreme Court declared that Fleming's American patent was invalid. |
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