John Ambrose Fleming was in Lancaster, Lancashire, England, on 29 November 1849.
His father was Rev. James Fleming a Congregational minister, his mother, Mary Ann. John was the eldest of 7 children.
His family moved to North London when he was very young, and he spent most of the rest of his life there. John's father had only a slender stipend and had considerable difficulty in educating his son who didn't start school until about the age of ten, prior to which is mother tutored him and he had learned, virtually by heart, a book called the "Child's Guide to Knowledge", a popular book of the day. Even as an adult he would quote from it. Because of the lack of funds, his education was interrupted several times while John different jobs to survive. Even as a boy he wanted to become an engineer. At the age of 11 he had his own workshop where he built model boats and engines. He even built his own camera, the start of a lifelong interest in photography.
John enrolled at London's University College in 1867, having earlier been to the University College School, where he showed aptitude in science. He chiefly studied physics and mathematics during his undergraduate years, while working jobs with in the shipwright's drawing office of a Dublin shipbuilder and then as a stockbroker's clerk with a London company. He completed his degree in 1870. Between 1871 and 1880 he had alternate periods of school science teaching and further study, including working under James Clerk Maxwell from 1877 at the new Cavendish Laboratory in Cambridge. In 1881 he was appointed professor of mathematics and physics at University College, Nottingham. From 1882 to 1885 he worked as consultant to the Edison Electric Light Company in London. He was then appointed professor of electrical technology at University College, London, a post he held for 41 years. At University College Fleming gave special courses and experimented on wireless telegraphy, cooperating a great deal with Guglielmo Marconi.
In 1899 John became a consultant to the Marconi Company in addition to his duties at UCL. At this time wireless, as it was then known, was still in its infancy and Marconi was continually making an improvement in the distance that could be achieved. In 1901 he succeeded in sending a message across the Atlantic. John became quite absorbed in the subject. He even designed the transmitter that made the first transatlantic transmission. He was also somewhat eccentric and during his experiments with transmitters he would always use the letter V sent in Morse (...-) as the test letter. In fact he became so involved in this work that he would often be heard unconsciously humming the letter V or whistling it between his teeth.
In November 1904 he applied for a patent on the two-electrode vacuum-tube rectifier, which he called the oscillation valve. It was also called a thermionic valve, the term comes from the Greek thermos, meaning warm. John calls the device a valve because it allows electrical currents to pass only in one direction. It becomes known as a 'vacuum tube' in America. In his rectifying vacuum tube, electrons flow from the negatively charged cathode to the positively charged anode. As the current within the tube is moving from negative to positive, the oscillations of incoming signal are rectified into detectable direct current.
John made many adjustments to his valve over the next few years. Some of these adjustments included tungsten filaments and the addition of a shield within the tube to eliminate electrically charged bodies from affecting the valve. He special ordered several types of specially designed lamps from Edison's factory and continued his perfection of the valve. John applied for a patent for his improved device on January 25, 1908. The institution of the valve was almost immediate; being used in several electrical devices soon after the development. One of the first receivers to use the valves was the Marconi-Fleming Valve Receiver. This was the start of the wireless revolution.
In a famous letter to Marconi, John wrote of his discovery and added as an after-thought: "I have not mentioned this to anyone yet as it may become very useful." Three years later began one of the most famous litigations in scientific history, Fleming vs. de Forest. Dr. Lee de Forest in America had made the significant contribution of introducing a grid between the filament and the plate in the valve which allowed control of the current flowing. The valve was not a very versatile device at all until the grid was introduced because there was no means of controlling the current flowing in the valve other than by varying the amount of power supplied between the filament and the anode. The legal action centred around whether the addition of the grid to the valve - the addition of the third electrode, was an invention in its own right. The Marconi Company, to whom John was a consultant for some thirty years, asserted that it was not (rather naturally), whereas Dr. de Forest on his part took the opposite view, contending that what Fleming claimed as invention was already inherent in Edison's patent of 1883. It was not until 1920 that a settlement was found - in favour of John Fleming.
On 11 June 1887 he married Clara Ripley, daughter of Walter Freake Pratt, a solicitor from Bath. She died in 1917 in Hampstead, London, the couple had no children.
John also invented the mnemonic to remember the rules for the relative directions of the field, current, and force in electrical machines. The left hand refers to motors, the right hand to generators. If the forefinger, second finger, and thumb of the left hand are extended at right angles to each other, the forefinger indicates the direction of the field, the second finger the direction of the current, and the thumb the direction of the force. If the right hand is used the digits indicate these directions in a generator. see the picture for examples.
There had been much controversy regarding the wavelength used to flash the first transatlantic signal, John was asked in 1935 what wave was used. He replied: "The wavelength of the electric waves sent out from Poldhu Marconi station in 1901 was not measured because I did not invent my cymometer or wavemeter until October 1904. The height of the original aerial (1901) was 200 feet, but then there was a coil of a transformer or "jiggeroo" as we called it in series with it. My estimate was that the original wavelength must have been not less than about 3,000 feet, but it was considerably lengthened later on. I knew at that time that the diffraction or bending of the rays around the earth would be increased by increasing the wavelength and after the first success I was continually urging Marconi to lengthen the wavelength, and that was done when commercial transmission began. I remember I designed special cymometers to measure up to 20,000 feet or so".
John remained at University College until 1926, retiring to the quiet town of Sidmouth in Devon. Two years after his retirement he was knighted for the many advances he had made to electrical and electronic engineering. During his retirement, John still took an active interest in many new developments in the electronics world. For fifteen years he was president of the Television Society, often travelling to London for their meetings.
At the age of eighty-four he married for the second time, the popular young singer Olive May Franks of Bristol, daughter of George Franks, a Cardiff businessman. The couple had no children.
John had many interests outside his work, a keen interest in photography and he loved walking. He was a devout Christian, and would often preach at various churches. With his advancing age John became increasingly deaf, however he remained active until his death on April 18 1945, in Sidmouth, at the great age of 95. Having had no children, he bequeathed much of his estate to Christian charities, especially those that helped the poor. But of his many achievements his fame rests on turning a light bulb into the first electronic valve.
Vacuum Tubes have now been almost entirely replaced by transistors, which are cheaper, smaller, and more reliable. Vacuum Tubes still play an important role in certain applications, however, such as in power stages in radio and television transmitters, and military equipment that must resist the electro magnetic pulse (EMP) induced by an atmospheric nuclear explosion that destroys transistors and integrated circuits.