Radio Receivers, Early. Edison Effect (1883)

While early magnetic detectors such as coherers provided an effective means of receiving wireless telegraph (code) signals, the continuous waves used to transmit wireless telephony (voice, music) required both improved means of reception and effective signal amplification. The crucial and related inventions that allowed this appeared within two years of each other early in the twentieth century, and led to considerable legal wrangling over patent control.

Edison Effect (1883). While experimenting with his new incandescent electric light, prolific American inventor Thomas A. Edison noted that the glowing (because it was heated) lamp carbon filament somehow created conditions that slowly blackened the insides of light bulbs, eventually rendering them useless. Preoccupied with perfecting his breakthrough invention, he sought to understand and limit the causes (he presumed carbon particles had been thrown off), filed a patent on a related device in late 1883 (now considered to be the first electronics patent), but did not pursue practical applications of his finding.

What became known as the ‘‘Edison effect’’ soon fascinated researchers on both sides of the Atlantic, one of whom was John Ambrose Fleming, a London-based electrician for the British branch of Edison’s company. Edison demonstrated the effect to Fleming while the latter visited the U.S. Fleming and others tried to determine what caused the effect and whether it had any potential use, and published brief reports on their limited findings. All noted that a narrow strip of glass was not blackened, where one leg of the light filament shielded the other.

By the late 1890s, building on a growing understanding of both conduction through gasses and emissions from hot elements within vacuums, researchers finally understood that streams of electrons, not carbon particles, were responsible for the Edison effect.

Fleming Valve (1904). Within a few years, devices to improve wireless technology were often the focus of experimental electrical work in many countries. Ambrose Fleming was one of many researchers who began to seek whether newly discovered electrons could be effectively harnessed to detect wireless signals. He had a particular reason for seeking an improved detector that could display its results on an electrical meter—he was partially deaf and often missed weak signals.

Fleming and others determined that electrons were negatively charged, and thus a positively charged plate could be used to attract them. Put another way, electrical current would ‘‘flow’’ only when a plate was positively charged. Inserting such a plate into a small glass vacuum tube along with a filament conductor created what looked very much like an incandescent light, but was in reality a oneway gate or ‘‘valve’’ which could change (rectify) alternating current (such as wireless or radio waves) to pulsating direct current. Using such a two-element (filament and plate) valve or ‘‘diode,’’ an operator could detect the presence of wireless telegraphy signals.

Fleming (by then a scientific advisor for the Marconi Company in London) was granted British, American and German patents for his “oscillating valve’’ device in 1904-1905. Though his valve worked as well as the earlier magnetic detectors, and was somewhat more stable in operation, it demonstrated no greater sensitivity to weak signals. It was simply an alternative approach to wireless signal detecting. The Marconi Company made wide use of the device in many of its radio stations up to World War I. Fleming was knighted in 1929. Over time, his device (indeed all that followed it) became better known as ‘‘thermionic’’ valves as they involved the use of incandescent (heated) electrons.

De Forest Audion (1906). In America, experimenter Lee de Forest (1873-1961) was also seeking improved means of wireless signal detection, especially those that might avoid further patent infringement problems that already plagued his operations. He had laboriously developed a series of detectors (one even used small gas flames) and soon turned to variations on the Fleming valve as a promising approach. However, he lacked a scientific understanding of its principals, as over the space of several months he sought to improve its operation by experimenting with added elements, soon focusing on placing an additional electrode inside the tube. This took the form of a second plate or ‘‘wing,’’ and was patented in 1906.

The resulting device was little better than Fleming’s valve. Indeed, Fleming became bitter over de Forest’s ignoring of his prior work (de Forest claimed not to know of the Fleming valve patent before 1906) as well as his own lack of any royalties on the diode (as the Marconi Company owned the patent rights).

De Forest’s added tube element soon changed, taking the form of a tiny inserted wire in the form of a grid, and the true three-element tube or ‘‘triode’’ was born, dubbed the ‘‘Audion’’ by one of his assistants, and patented in 1907 (confusingly, the term Audion was applied by de Forest to many diode and triode devices). Even this latest version was initially perceived as just another detector among many available options.

The new Audion tube would, however, turn out to be a far more capable device—indeed it can be argued that it lay the foundation for electronics development until the era of solid-state devices; but realization of the full potential of the Audion took time, and much of the effort to make that so came from others.

The improved tube’s ability to amplify weak signals came to the attention of the American Telephone & Telegraph Company (AT&T), which for years had sought an amplifier for long-distance wired telephone circuits. AT&T purchased partial rights to the Audion in 1909 and using a modified Audion device, opened transcontinental telephone service in 1914.

Meanwhile the man who would become de Forest’s primary radio rival, Edwin Howard Armstrong (1890-1954), had discovered the ‘‘feedback’’ potential of the Audion as a student at Columbia University. When an Audion’s output was fed back into its own input, the circuit could become a generator of electrical signals—effectively a radio transmitter. While virtually all engineers credited Armstrong’s genius, the U.S. Supreme Court terminated two decades of patent battles in 1934 when it held that de Forest deserved credit.