I. Cathode Rays & the Discovery of the Electron
Although many of the pioneers of 19th Century physics, including Faraday, were convinced on the basis of chemistry and the phenomena observed in electrolysis that electric current consisted of the flow of particles of charge, the nature of these charges was not understood. Even the basic question of whether the charge of the particles was positive or negative remained undetermined. The answers to these questions, and to the basic structure of matter, were resolved by experiments that began with the study of electric discharges in evacuated tubes. Along the way a series of discoveries were made which led to the technological revolution of the 20th Century.
II. Wireless Telegraphy
Maxwell's 1865 publication of a theory which unified electrodynamics, magnetodynamics, and optics had seemingly little impact in Britain where it was not widely accepted. Surprisingly, during the remaining fourteen years of his life, Maxwell, who was a skillful experimentalist, did not attempt to verify the existence of the electromagnetic waves that his theory predicted. However, the leading German scientist of the period, von Helmholtz, believed the Maxwell theory and he set his pupil Hertz on the track of producing and detecting electromagnetic radiation, opening the path to wireless communication.
The Edison effect, the appearance of an electric current flowing between a heated cathode and an anode in an evacuated tube, was a mysterious phenomenon when it was discovered in 1882; it was not understood how electric current could pass through a vacuum. Thomson's identification of cathode rays as streams of electrons resolved the mystery and led to the invention of the thermionic diode by Fleming. The diode, intended to serve as a rectifier to detect radiotelegraphic signals, had little impact as the coherer, invented by Branly and Lodge, and crystal and magnetic detectors continued to be used. The invention of the triode by DeForest, however, did revolutionize radio communication.
Speech transmission using a spark transmitter was demonstrated by Fessenden in 1900 but was too noisy; in 1906 he broadcast the first program of speech and music using 50 KHz generated by an alternator. Fessenden also discovered the heterodyne principle of mixing a low frequency signal with the high frequency carrier. The 1913 discovery by De Forest and Armstrong of regenerative feedback and how to use the triode as an oscillator made commercial radio possible. Armstrong’s invention of the superheterodyne receiver in 1917 and FM in 1933 brought radio into the modern era. However, the technical triumphs were marred by years of bitter patent suits between all the participants and led to great personal tragedies.
The pioneers of television were the Russians, Nipkow who invented a mechanical revolving scanning disk in 1884 and Rosing who used a cathode ray tube in 1907 to display images from a mechanical transmitter. In Britain in 1923, John Logie Baird began to demonstrate television transmission using Nipkow disks. In America, Rosing’s student, Vladimir Zworykin, filed a patent for an electronic television system in 1923, but the project was dropped by Westinghouse and Zworykin had to wait for RCA to restart the project in 1930. Meanwhile, an Idaho schoolboy, Philo Farnsworth, invented an electronic system in 1922, and by 1927 had transmitted television images. The development of the kinescope and its successor, the image orthicon tube, at RCA, plus a licensing agreement between RCA and Farnsworth led to the first appearance of commercial TV in April,1939 at the RCA pavilion at the New York World Fair.
In the period before World War II, all the major powers were developing radio location systems. The British concentrated on aircraft detection and location while the Germans developed aircraft navigation systems. These devices operated at meter wave lengths. The invention of the multicavity magnetron by Randall and Root in Britain in 1939 provided the impetus to the development of the centimeter wavelength systems required for modern radar. The disclosure of the device to the U.S. in 1940 was followed by the founding of the Radiation Laboratory at MIT. The Radiation Laboratory technical staff grew to more than 1300 engineers and scientists, including ten future Nobel Laureates, and developed more than one hundred models of radar, including early warning systems, anti-aircraft gun-laying radars, anti-submarine radars, ground approach systems, and bomber targeting radars. Other radars were developed at Bell Labs and elsewhere. Nearly one million radar sets were produced in the U.S. as the war progressed! The Germans and the Japanese also produced a variety of radar systems. However, the Germans never produced the short wavelength systems available to the Allies and were caught in a losing game of technical catch-up. The Japanese, who had independently invented the magnetron, were hampered by bureaucratic entanglements, military secrecy and personnel shortages as engineers were regardlessly drafted into the army.
VII. Electrons and Waves
The revolution in physics which was introduced by the emergence of the quantum theory of matter led to the invention of the devices of modern electronics. The roots of the quantum theory lie in unanswered questions of 19th Century Physics, which were resolved in 1927 by the Schrödinger theory which encompassed the wave/particle duality of radiation and matter discovered by Planck and DeBroglie. The quantum theory provided the foundation for the theories of conduction in metals and semiconductors which form the basis for solid state electronics. The physicists who created this scientific revolution were, for the most part, Europeans whose lives and work were disrupted in the 1930’s as many became refugees from Hitler.
The analogy between the diode and the solid state devices such as the copper oxide rectifier and the crystal detectors of early radio was obvious early on. During the 1920’s, several inventors attempted devices that were intended to control the current in solid state diodes and convert them into triodes. Success, however, had to wait until after World War II, during which the attempt to improve silicon and germanium crystals for use as radar detectors led to improvements both in fabrication and in the theoretical understanding of the quantum mechanical states of carriers in semiconductors and after which the scientists who had been diverted to radar development returned to solid state device development.