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Tom Lewis

Empire of The Air: The Men Who Made Radio

 

By Tom Lewis, HarperCollins (C) 1991 Tom Lewis All rights reserved.

 

THE WILL TO SUCCEED

 

". . . By 1880, Edison had created a lamp that glowed brightly when direct current passed through its carbon filament in a vacuum. But he found that over time particles of the carbon were transferred to the glass. In experiments to correct the fault, the inventor learned that electric current could flow from the filament through the vacuum surrounding it to a positively charged metal plate, a process later dubbed the "Edison effect"meaning that no one could explain precisely how the process worked. Furthermore, the amount of current that flowed from the filament to the plate stood in direct proportion to the incandescence of the lamp. He noted his findings in a patent that showed how such a modified lamp might measure the flow of electrical current. But the date was 1883, half a decade before Hertz's experiments, and fourteen years before an English physicist named Joseph John Thomson discovered the existence of the electron. Besides, the untheoretical Edison believed inventive genius to be "one percent inspiration and ninety-nine percent perspiration." Decidedly uninspired at this point, he saw little commercial value for his discovery. Without further speculation, Edison proceeded with his quest to perfect the electric lamp.

 

John Ambrose Fleming, then an employee of the Edison Company in London, knew of the inventor's patent. His studies of the same carbon deposits led him to publish four papers on the subject to the Royal Society between 1883 and 1896. But then, diverted by other work, Fleming suspended his inquiry for nearly eight years. In 1904, when he had become scientific adviser for the Marconi Company, he was charged with the job of creating a new detector of wireless waves.

 

"Why not try the lamps?" Fleming remembered thinking years later. This time, working with the alternating current of wireless waves, he made a remarkable discovery: while the current flowing into the filament alternated between a positive and negative charge, the current leaving the lamp from the metal plate was direct. Fleming's bulb was acting as a valve that allowed only the negative electrons to pass. Indeed, he entitled his patent an "instrument for converting alternating electric currents into continuous currents," and he called his bulb an "oscillation valve." Fleming's valve stands as a dramatic achievement. The electrons liberated by Marconi's spark gap transformer imperceptibly traveled through the air at the speed of light. Now they could be captured and converted into direct current through the agency of a small filament and plate in a little glass bulb. From there the current could flow into an earphone and become a perceptible sound once again. Fleming had created a new detector of wireless waves, one that worked with a modified Edison effect lamp.

 

 

The Fleming valve. "The valve consists of an incandescent electric lamp comprising a filament (F) of carbon, tungsten, or other material which can be made incandescent by an electric current. Around the filament, but not touching it, is a cylinder of metal (C). The electrical connection to the cylinder is brought out through the side of the glass enclosure." J. Jenkins

 

 

The above description and diagram was from John Jenkins excellent site: http://www.halcyon.com/johnj/radios/FLEMING.HTM (Now a dead link)

 

(This illustration is not in the book.)

 

In the spring of 1905, Fleming published his discovery for the Royal Society, but the tube was a crude apparatus and needed more study to be practical. If he had had more encouragement, Fleming might possibly have developed-the potential of his tube, but in this he was thwarted by his employer. The Marconi Company, which held all rights to his patent, was more interested in developing galena crystal as a detector. Instead, two years later, Lee de Forest took the fame and some of the fortune for Fleming's work.

 

De Forest always avoided acknowledging Edison's and Fleming's obvious antecedents to his own work. Though he read voraciously in scientific periodicals at Talladega, at the Chittenden Library at Yale, at the John Crerar Library in Chicago, among others, and though he subscribed to technical periodicals, he steadfastly claimed ignorance of their discoveries.

 

Since 1900, de Forest had occasionally experimented with the possibility that heat from a gas burner created electrical vibrations. Early in 1905, after Fessenden had launched his suit and de Forest realized he might be forced to abandon his spade detector, he intensified his tests and took out patents on several "oscillation responsive" devices, which used a gas flame. No evidence suggests these inventions ever worked, but patent them he did. In the late summer of 1905, he read Fleming's article on his valve in the Proceedings of the Royal Society.

 

Late that fall, an assistant brought a bulb about the shape of a small pear to Henry W. McCandless at 67 Park Place in New York City and asked him to duplicate it. A manufacturer whose principal trade was making automobile lamps for Westinghouse and General Electric's Mazda [the original name for Edison's line of bulbs, ed.], McCandless had no difficulty meeting this special order. With a brass candelabra screw base and a carbon filament, the lamp resembled others available at the time. But there was one significant difference: beside the filament inside the bulb was a nickel plate. To that was attached a short wire that protruded through the top of the glass. The assistant explained that it was a Fleming valve. On December 9 that year, de Forest took out a patent on a "static valve for wireless telegraph systems." Five weeks later, he made another application for a similar tube and circuit; this time he ran wires from a small battery to both the filament and plate. This he called the "audion," and he claimed in a talk to a gathering of the American Institute of Electrical Engineers in New York on October 26, 1906, that his tube was "a new receiver for wireless telegraphy."

 

All that de Forest had developed thus far bore a remarkable resemblance to the valve Fleming had described to the Royal Society in 1905. He had introduced the use of a battery on the plate as well as the filament circuit, but that was all. Nor was de Forest's change necessarily an improvement, for the small positive charge of electrons flowing from the filament to the plate was no match for the positive charge of electrons flowing to the plate from the battery. What came next, however, was de Forest's idea alone, and without question will endure as the inventors greatest insight.

 

Click here for my drawings, history, and explanation of the vacuum tube

 

On November 25, 1906, after further experiments and several false starts, de Forest ordered another tube from McCandless. The specifications called for three elements: a filament a plate, and, interposed between the two, as close to the filament as possible, another nickel wire. As was the case with the other wires, it too was drawn out through the side of the lamp. When this wire was positively charged, de Forest found it would attract the stream of positive electrons flowing from the filament, accelerate them, and send them toward the plate, and the more positive the charge, the greater the charge on the plate circuit.

 

Detail from the constantly amended patent on the audion. (Illustration not in the book.)

 

On the suggestions of John Grogan, one of McCandless's assistants, de Forest decided to bend the wire zigzag fashion in order to create a greater surface to accelerate the electrons flowing from the filament. To this de Forest gave the name "grid." Now he could regulate the flow of electrons from the filament to the plate and amplify them. Precisely how the filament, grid, and plate worked, he was not sure. The theories he did propose about their action were in fact incorrect. But the sounds coming from his earphones showed that his audion did work. With the simple addition of a plate to Fleming's tube, modem electronics was born.

 

 

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