Saturday, July 1, 2000
F A C T   F I L E

Jonas Edward Salk
June 24, 2000
Richard Evelyn Byrd
June 18, 2000
William Faulkner
June 10, 2000
Sir John Hunt
June 3, 2000

Lee De Forest
By Illa Vij
Vacuum tubes

THE startlingly simple audion tube upon which many inventions were based was the invention of Lee De Forest. The world remains ever so thankful to him for giving it the wonderful world of electronics. The audion tube led to the invention of the radio and television transmission radar, computers, space vehicles and many other electric devices. He’s rightfully called one of the authentic parents of the electronic age.

De Forest was born in 1873, in Council Bluffs, Iowa. He grew up in Talladega, Alabana. His father, who was a minister, was sent there to reorganise an African-American college. Lee was lonely here, as he didn’t have many friends. Lee’s father was very keen to steer him towards the ministry, but Lee had other ideas. He showed a tremendous inclination towards science. His talent was further honed at Yale’s Sheffield Scientific School. He obtained his doctorate in 1899. As the new century dawned, he left for Chicago. Here he worked in a dynamo factory of the Western Electric Company. Later he worked in a telephone lab and edited an electrical magazine. He invented the responder and founded the American De Forest Wireless Telegraph Company. In 1908, De Forest got married and took his wife to Paris for their honeymoon. But work went along with them. He installed a telephone transmitter on top of Eiffel Tower. In 1910, an audience of music patrons were enthralled as De Forest transmitted the opera star’s voice in the first broadcast of its kind. The singer was the famous Enrico Caruso.

  Soon Lee was flooded with requests for his inventions. He built aerials on roofs of skyscrapers and installed his electronic sound amplification equipment (microphone) in theatres and the Metropolitan Opera House. One invention led to another. He invented a special surgical knife, train communication systems, radio signalling and receiving systems, photoelectric cell, television and colour television apparatus. Of all his inventions, the most important was the audion tube. He gave the world this remarkable gift in 1906.

Lee’s entire life was devoted to inventions, which transformed the world. He died in 1961 at the age of 88, his mind alert and active till the last.


Vacuum tubes

VACUUM tubes are electronic devices, consisting of a glass or steel vacuum envelope and two or more electrodes between which electrons can move freely. The vacuum-tube diode was first developed by English physicist John Ambrose Fleming. It contains two electrodes: the cathode, a heated filament or a small, heated, metal tube that emits electrons through thermionic emission; and the anode, or plate, which is the electron-collecting element. In diodes, the electrons emitted by the cathode are attracted to the plate only when the latter is positive with respect to the cathode. When the plate is negatively charged, no current flows through the tube. If an alternating potential is applied to the plate, the tube passes current only during the positive halves of the cycle and thus acts as a rectifier. Diodes are used extensively in the rectification of alternating current.

The introduction of a third electrode, called a grid, interposed between the cathode and the anode, forms the triode, which for many years was the basic tube used for amplifying current. (The triode was invented in 1906 by American engineer Lee De Forest.) The function of the grid is to control the current flow. At a certain negative potential, the grid, because it repels electrons, can impede the flow of electrons between the cathode and the anode. At lower negative potentials, the electron flow depends on the grid potential. The grid usually consists of a network of fine wire surrounding the cathode. The capacity of the triode to amplify depends on the small changes in the voltage between the grid and the cathode causing large changes in the number of electrons reaching the anode.

Through the years more complex tubes with additional grids have been developed to provide greater amplification and to perform specialised functions. Tetrodes have an additional grid, closer to the anode, that forms an electrostatic shield between the anode and the grid to prevent feedback to the grid in high-frequency applications. The pentode has three grids between the cathode and the anode; the third grid, close to the anode, reflects electrons that are emitted by the anode as it is heated by electron impact when the electron current in the tube is high. Tubes with even more grids, called hexodes, heptodes, and octodes, find applications as frequency converters and mixers in radio receivers Vacuum 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 transmitters, and in military equipment that must resist the voltage pulse (which destroys transistors) induced by an atmospheric nuclear explosion.