English engineer who made numerous contributions to electronics, photometry, electric measurements, and wireless telegraphy. He is best remembered as the inventor of the two-electrode radio rectifier, which he called the thermionic valve; it is also known as the vacuum diode, kenotron, thermionic tube, and Fleming valve. It was patented in 1904.
His work with the thermionic valve in 1904 and 1905 was important to the development of radio. He also contributed to the science of photometry, the measurement of the intensity of light. J.A. Flemingbecame a consultant to the Edison Electric Light Company and a popular teacher at University College. He was knighted in 1929 for the many advances he had made to electrical and electronic engineering. John Ambrose Fleming, was born on 29th November 1849, the eldest son of seven children born to a Congregational minister, Rev. James Fleming.
Although born in Lancaster his family moved to North London where he spent most of his early life. John’s father had only a slender stipend and had considerable difficulty in educating his son. Fleming, in fact, however, showed early signs of prodigy and delivered his first lecture on electromagnetic phenomena at the age of thirteen. John waseducated mainly at University College School on Gower Street in the West End of London. He matriculated at the age of sixteen.
From here he moved on to take his degree also at University College. Graduating with a Bachelor of Science degree in 1870, he was for a brief period science master at Rossall School, returning to London to the Royal School of Mines to study Chemistry under supervision of the eminent chemist, Sir Edward Frankland.
Although he had to take up a daytime teaching job to finance himself he was still able to gain a first class degree. During this time he came across some of Maxwell’s work. This fascinated him and he decided to further his career in this direction. So in 1877 Fleming started to study electricity and magnetism at Cambridge under professor James Clerk Maxwell. Here he was particularly successful gaining his D.Sc. and then a year later he was elected a fellow by his college.
In 1881, when electric lighting began to attract public attention, Flemingwas appointed electrician to the Edison Electric Light Company of London, a position which he occupied for the ensuing ten years. His great practical knowledge qualified him to practice as a consulting electrical engineer, and he became adviser to many city corporations on their electric lighting plans and problems. He worked with high-voltage alternating currents, and designed some of the first electric lighting for ships. He also made many important contributions to the field of electrical machinery.
Fleming’s long term aim was to be able to return to more scientifically meaningful job. At the time there were no positions in the new and developing science of electrical engineering. Instead the nearest subject was physics. However Fleming was invited to give a series of lectures on electrical engineering at University College London (UCL), the premier college of London University.
Then in 1885 he was asked to set up a new department for electrical engineering for which he would be professor. This was the first department of its sort in the country and it reflected the forward thinking of the College. Fleming held his position at UCL for more than forty years. His abilities as a lecturer and teacher brought him many invitations to speak before audiences of the Royal Institution and the Royal Society of Arts. His treatise on electric – wave telegraphy was for many years a standard book on the subject.
Fleming greatly enjoyed his time at UCL. He was able to spend time lecturing, he was able to undertake his research and in addition to this he was in London where he wanted to be. He devoted much of his time to work on a variety of aspects of AC machines and he became a leading authority on transformers as well as performing much valuable work on improving the accuracy of AC measurements.
In 1899 Fleming 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. Fleming became quite absorbed in the subject. He even designed the transmitter that made the first transatlantic transmission.
In 1900 Marconi had conceived the idea of trying to establish wireless communication between England and the continent of America. Despite the gloomy prognostications of the theoreticians who said that this could not be done because of the inability of wireless waves to follow the earth’s curvature, Marconi persisted, and, as we well know, his experiments were successful, against all the odds, because of the existence of the, up till that time unexpected, ionosphere.
Fleming had acted very much as Marconi’s adviser in the design of his transmitter, although he did not use thermionic valves as Fleming had not yet invented them. The fact that Marconi required a 25-h.p. oil engine to drive his transmitter merely serves to underline the sophistication which our subject has achieved in that, for example, we can now transmit from an artificial satellite some 22,000 miles high to earth using only the relatively minute energy provided by a battery of solar cells surrounding the body of the satellite itself.
Since there had been much controversy regarding the wavelength used to flash the first transatlantic signal, Fleming 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”.
Although it was possible to communicate with our transatlantic neighbours, we could not talk with them, for communication was achieved only by means of an old-fashioned kind of spark transmitter and, at that period, we had no effective means of detecting the signals which were received and thus turning them back into speech. Various devices were tried but very few of them were very efficient, and it is ironic that, at that period when such tremendous powers were necessary to establish communication at all, there lay in a cupboard a device which was quite capable of having carried out simply and relatively efficiently all that was needed.
In 1883 Edison had been concerned with the blackening of electric lamps by a carbon deposit thrown off from the filament and had experimented with a metal plate interposed between the glass envelope and the filament, in an effort to minimise it. In the course of this work he had connected the plate to the positive terminal of the filamentsupply and had noted that a small current passed in the plate circuit. He also observed that when the plate was connected to the negative terminal no current flowed.
This phenomenon and others like it had excited Fleming’s interest. In a paper given to the Royal Society in 1890 he appeared to have solved the mystery. More significantly, however, he had also observed that by feeding the lamp from an alternating current supply rectification (the process of converting alternating to direct current) had occurred.
Fleming, however, was a very busy man and also perhaps one who did not always see the immediate applications of the things on which he had worked. Fourteen years therefore elapsed before he filed his momentous 1904 patent with which the thermionic valve found its first public announcement.
The actual detector on which he performed the experiments which were the subject of the 1904 patent was one of these earlier mentioned valves made in 1883 which he had stored away over the intervening period. He observed that the direct current which passed through the valve was related to the amount of radio frequency power applied to it and hence was born the rectifier principle which is still the basis of the whole electronics industry – even today.
Telling the story of how he came to invent the valve detector, Fleming said:”In 1882, as electrical adviser of the Edison Electric Light Company of London, I was brought into close touch with the many problems of incandescent lamps and I began to study the physical phenomena with all the scientific means at my disposal. Like everyone else, I noticed that the filaments broke easily at the slightest shock, and when the lamps burned out the glass bulbs became discolored.
This discoloration of the glass was generally accepted as a matter of course. It seemed too trifling to notice. But in science it is the trifles that count. The little things of today may develop into the great things of tomorrow. Wondering why the glass bulb grew dark, I started to investigate the matter, and discovered that in many burned-out lamps there was a line of glass that was not discolored.
It was as though someone took a smoked glass, drew a finger down it, and left a perfectly clean line behind. I found that the lamps with these strange, sharply-defined clean spaces were covered elsewhere with a deposit of carbon or metal, and that the clean line was immediately in the plane of the hairpin-shaped carbon filament and on the side of the loop opposite to the burned-out point of the filament.
It was obvious to me that the unbroken part of the filament acted as a screen to that particular line of clear glass, and that the discharge from the overheated point on the filament bombarded the remainder of the bulb with molecules of carbon or vaporized metal shot out in straight lines. My experiments at the end of 1882 and early in 1883 proved that I was right.”
To find a better detector he tried to develop chemical rectifiers, until one day the thought occurred to him:”Why not try the lamps?” First he constructed an oscillatory circuit, with two Leyden jars, a wired wooden frame and an induction coil. He then made another circuit, in which one of the lamps and a galvanometer were inserted. Both circuits were tuned to the same frequency. Later Fleming was recalling this experiment:
“It was about 5 o’clock in the evening when the apparatus was completed. I was, of course, most anxious to test it without further loss of time. We set the two circuits some distance apart in the laboratory, and I started the oscillations in the primary circuit.
To my delight I saw that the needle of the galvanometer indicated a steady direct current passing through, and found that we had in this peculiar kind of electric lamp a solution of the problem of rectifying high-frequency wireless currents. The missing link in wireless was”found”and it was an electric lamp! I saw at once that the metal plate should be replaced by a metal cylinder enclosing the whole filament, so as to collect all the electrons projected from it.
I accordingly had many carbon filament lamps made with metal cylinders and used them for rectifying the high-frequency currents of wireless telegraphy. This instrument I named an oscillation valve.
It was at once found to be of value in wireless telegraphy, the mirror galvanometer that I used being replaced by an ordinary telephone, a replacement that could be made with advantage in those days when the spark system of wireless telegraphy was employed. In this form my valve was somewhat extensively used by Marconi’s Telegraph Company as a detector of wireless waves. I applied for a patent in Great Britain on November 16, 1904.”
John Fleming patented his device (803,684 on 13th November 1905) which he called the Fleming valve. The term comes from the Greek thermos, meaning warm. Fleming 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.
Fleming 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.
Fleming applied for a patent on January 25, 1908. The institution of the valve was almost immediate; being employed 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 he 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 – for it was Dr. Lee de Forest in America who 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.
Really the valve could scarcely be considered as 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, that is – was an invention in its own right.
The Marconi Company, to whom Fleming 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 Fleming.
For that invention the Royal Society of Arts, London, in 1921 awarded Fleming its highest distinction: the Gold Albert Medal. His honors were many, including the Kelvin Medal, the Faraday Medal of the Institution of Electrical Engineers and the Franklin Medal of Franklin Institute, Philadelphia. In March, 1929, he received the honor of knighthood for his”valuable service in science and industry”.
The invention of the diode was a revolutionary idea, and put down the foundations for many further inventions. However it had very little impact at first.”Valves” were expensive to make and on top of this other ideas were overtaking him. In less than two years the cat’s whisker was produced. This was a very crude form of semiconductor rectifier that consisted of a thin wire positioned on a lump of suitable material (even coal) to produce a point contact rectifier.
This was far more convenient than Fleming’s diode and it soon caught on. Vaccum tubes have now been almost entirely replaced by transistors, which are cheaper, smaller, and more reliable. Tubes still play an important role in certain applications, however, such as in power stages in radio and television transmittors, and in military equipment that must resist the voltage pulse (which destroys transistors) induced by an atmospheric nuclear explosion.
The diode scheme
This is the simplest type of valve, having just two electrodes – anode and cathode (filament in the case of battery valves, as shown in the diagram). The electrodes are enclosed within an evacuated envelope – bulb – of glass, the connections to the electrodes passing through this envelope via airtight seals.
The hot filament or cathode generates an invisible cloud of electrons in the space around it. A positive potential on the anode attracts these and a current flows from cathode to anode. The air is removed in order to allow free movement of the electrons as they pass from cathode (filament) to anode.
The DIODE as a RECTIFIER
Under no conditions can current flow from ANODE to CATHODE in any diode. The device is a”one-way VALVE”. Increasing the positive potential will increase the flow of electrons from cathode to anode but if the anode is made negative, all current flow will cease. You can see from this that the positive-going section of the AC sine-wave will cause current flow, but the negative-going half (shown as a broken line) will stop all current flow.
As current only flows in the one direction, the result is a pulsing but direct current output. The addition of a reservoir capacitor across the output helps”fill in” the gaps between the pulses by charging on the pulses and discharging in the gaps between them. This is improved further by either a choke or a resistor in series with an additional capacitor, called the”smoothing” capacitor.
The choke/resistor – capacitor circuit forms a”time-constant” that filters even more of the residual AC ripple. Choke is best, having a low resistance at DC, unlike the resistor which tends to waste power, but the resistor is often used because it is cheaper. Note: although called on the diagram – rather quaintly – a”half-way” rectifier, this is a spelling error and should read”half-wave” rectifier.
The rectifying properties of the Fleming’s diode was certainly different from the ideal”single – way” electrical rectifier, but it was already sufficiently good for practical electronic applications.
Fleming was the author of more than a hundred scientific papers and books, including the influential”The Alternate Current Transformer” (1889, 1892),”The Principles of Electric Wave Telegraphy” (1906),”The Propagation of Electric Currents in Telephone and Telegraph Conductors” (1911) and”Memoirs of a Scientific Life” (1934).
Fleming’s rules: Rules to assist in remembering 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. The mnemonic was invented by Sir John Ambrose Fleming.
Fleming 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, Fleming 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.
The Fleming bust
At the age of eighty-four he married for the second time. He also had many interests outside his work. He had a keen interest in photography and loved walking. He was also a devout Christian, and he often preached at various churches as well as once being asked to St Martin’s in the Fields. With his advancing age Fleming became increasingly deaf, however he remained active until his death in 1945 at the great age of 95. During his life he made achieved a tremendous amount, but it is certain that he will be chiefly remembered for the invention of the thermionic valve.