January 2009 – History

Scanning the Past: A History of Electrical Engineering from the Past

Submitted by Bob Morrison, Editor

Copyright 1995 IEEE. Reprinted with permission from the IEEE publication, “Scanning the Past” which covers a reprint of an article appearing in the Proceedings of the IEEE Vol. 83, No. 9, September 1995.

Irving Langmuir and the Thermionic Vacuum Tube

Eighty years ago this month, the PROCEEDINGS OF THE INSTITUTE OF RADIO ENGINEERS (IRE) included a paper by Irving Langmuir on the theory of the thermionic vacuum tube and some of its radio applications. The author, a future IRE president and Nobel Prize recipient, was at the General Electric Research Laboratory (GERL) when his paper was published.

Langmuir was born in 1881 in Brooklyn, NY, and graduated in metallurgical engineering from the Columbia School of Mines in 1903. He went to Germany for graduate work where he earned a Ph.D. in physical chemistry at the University of Gottingen in 1906. He returned to the United States, where he taught for three years at the Stevens Institute of Technology in Hoboken, NJ, and where a heavy teaching load left him with little time for research. In 1909, he joined the research staff at the GERL where he spent the rest of his professional career.

Langmuir’s initial research at GERL concerned phenomena in electric lamps with tungsten filaments. Beginning in 1913, he investigated thermionic vacuum tubes and made important contributions to both theory and practice including improved vacuum pumps and pressure gauges. He found that even small traces of gases in a thermionic tube produced enormous changes in its performance. In his September 1915 paper, Langmuir explained the effect of space charge in the vicinity of the cathode and included theoretical equations for the current between electrodes as a function of tube geometry and applied voltage. He pointed out that “with the elimination of the gas effects, all the irregularities which had previously been thought inherent in vacuum discharges from hot cathodes were found to disappear.” He proposed the use of the term “kenotron” for two-element tubes that did not depend on gas for their operation and the term “pliotron” for vacuum triodes.

As examples of pliotron applications, Langmuir reported that small pliotrons were especially suitable for radio receivers where a cascade arrangement of tuned amplifiers had provided “a wonderfully high degree of selectivity.” He stated that a larger version of the pliotron could produce up to a kilowatt of power as an oscillator and that a 500-W radio telephone transmitter using pliotrons had been tested recently. Pliotrons were produced in large numbers by GE for military use during World War I and the availability of a 20-kW pliotron tube with water cooling was announced in 1922. Subsequently, GE produced pliotrons rated at 100 kW which were about 5 ft long and 6 in. in diameter.

Langmuir served as president of the IRE during 1923. During the 1920’s he investigated plasma physics and developed an ion sheath theory which proved important in the design of thyratrons and other gaseous electronic tubes. He also contributed to the theory of design of miniature vacuum tubes suitable for use at ultra-high frequencies. Langmuir received the Nobel Prize in chemistry in 1932 in recognition of his work on surface chemistry and surface films. An interest in weather phenomena led him to work on methods of producing smoke screens and deicing aircraft wings during World War II. After the War he engaged in somewhat controversial experiments of cloud seeding as part of an effort to modify hurricanes or to induce rain at periodic intervals over large areas. He enjoyed outdoor activities including mountain climbing, skiing, and skate sailing. Langmuir retired from GE in 1950, but he continued as a consultant until his death in 1957. Twelve volumes of his collected papers were published from 1960 to 1962.

James E. Brittain
School of History , Technology and Society
Georgia Institute of Technology