Sir John Ambrose Fleming led an active scientific life. His career covered the time from James Clerk Maxwell to the advent of electronic television. Fleming has been described as the scientific and technical link between Maxwell and Guglielmo Marconi. He was an outstanding teacher and highly successful popular scientific lecturer. He published more than 100 important papers on his discoveries. Fleming is best known for the thermionic vacuum tube or valve, the first electron tube that could change alternating current, such as a radio wave, to pulsating, one-way flow direct current. Fleming's diode improved radio reception and was a forerunner of the triode tube developed by Lee de Forest. Although the transistor eventually replaced Fleming's valve, his valve remained an important component of radios for nearly three decades and was used in the early days of computers and television.

Fleming began his study of electricity and mathematics under James Clerk Maxwell in the new Cavendish Laboratory at St. John's College in Cambridge. During his studies there, Fleming worked on improving the Carey Foster Bridge, a method for measuring the difference between two nearly equal resistances in electrical conduction. Fleming's improvement made the measuring device faster and more accurate. Maxwell labeled the device "Fleming's banjo" because of the measuring wire's circular shape.

After receiving his doctor of science degree in 1880, Fleming worked as a consultant for private industry. His consulting work resulted in many new methods and instruments for measuring high-frequency currents and new transformer designs. Fleming was a primary contributor to the development of electrical generator stations and distribution networks for several companies, including the London National Company, the Edison Telephone and Electric Light Companies, and the Swan Lamp Factory. For both the Swan and Edison companies, Fleming lent his expertise to photometry and helped develop the large-bulb incandescent lamp that used an aged filament as the light source.

In 1899 the Marconi Wireless Telegraph Company hired Fleming as a scientific adviser to help design the Poldhu wire- less station in Cornwall, England. This was the largest wireless station in England and the source of the first transatlantic wireless telegraph transmission in 1901. Fleming's most important contribution to electrical engineering was his vacuum tube, widely used in both radiotelegraphy and radiotelephony. In 1888 Thomas Edison announced his "Edison effect," which described how electronic particles were emitted from a hot electric lamp filament. Fleming had repeated Edison's findings in 1899 but had found little practical use for Edison's discovery.

In 1904, however, while searching for a more efficient and reliable detector of weak electrical currents, Fleming was inspired to make a new lamp, or valve, that would have a hot filament and an insulated plate sealed inside a high vacuum tube. When a current was passed through the carbon or metal filament, the rarefied air between the hot filament and the cold plate filled with electrons and became a conductor of electricity. He found that the electrons would travel only when the plate was attached to the positive terminal of a generator and that the plate would attract the negatively charged electrons. Fleming also noticed that this flow of electrons was in only one direction, from the hot filament to the cold metal plate, and not in the reverse direction. Alternating current would enter the device, but direct current would leave. Fleming had converted alternating-current radio signals into weak direct-current signals that could be heard with a telephone receiver. This was a major advance in radio technology. Fleming called his discovery a thermionic valve or tube, because it acted much like a check valve, which allows fluids to flow in only one direction. Eventually, the device was labeled "Fleming's valve." It provided the first truly reliable method to measure high-frequency radio waves. Fleming patented his valve in 1904. This discovery revolutionized radio telegraphy communication technology. The vacuum tube was the foundation of electronics until the 1960s, when solid-state technology was developed, replacing vacuum tubes in most electronic devices.

In 1906 Lee de Forest added a third element to Fleming's diode valve, thus effectively separating the high-frequency circuit from that of the filament, making amplification of radio signals possible. Litigation of the de Forest and Fleming patents continued for years. Court decisions in 1916 tied most companies into knots. As the United States entered World War I, the navy offered to indemnify all manufacturers of radio apparatus for the armed forces against any resulting patent infringement suits. This pooling of all patents enabled manufacturers to produce modern equipment without fear of lawsuits. Patent disputes between de Forest and Fleming were not fully resolved until after American Telephone and Telegraph (AT&T) bought de Forest's Audion patent, the Radio Corporation of America (RCA) acquired rights to the Fleming valve, and AT&T and RCA entered into a cross-licensing agreement in 1920.

Throughout his long career, Fleming lectured often at University College, the Royal Institution, and the Royal Society of Arts. He published extensively and presented several important research papers at learned societies' conferences. After Fleming read a paper on the need for an authoritative body for electrical standards for the burgeoning electric lighting industry, the Board of Trade Laboratory and eventually the National Physical Laboratory were established in Great Britain. Fleming is credited with developing a direct-reading potentiometer, set to read current and potential directly in amperes and volts, and with encouraging R.E.B. Crompton to put it on the market in a practical form.

Fleming placed his long scientific career into perspective when he-wrote that in comparing the last half of the 19th century and the first third of the 20th century, there was an enormous increase in practical technical achievement, despite the diminished confidence we now have in the validity of our theoretical explanations of natural phenomena. His peers regarded Fleming very highly because of his extraordinary devotion to his work. He never lost sight of the potential for wireless. He wrote that

radiotelegraphy has not only given to mankind a super latively beneficial means of communication, but has also opened up for discussion physical and cosmical problems of profound interest We are only at the very beginning of this evolution, yet it has already completely revolutionized the practical side of wireless telegraphy, as well as telephony (Fleming, 1921).