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Although Oliver Heaviside began his career as a telegraph operator, he spent much of his time thinking about the theory of telegraph circuits and cables. His earliest papers of 1872 and 1873, before his retirement in 1874, already reveal his ingenuity on these topics. His second paper introduced his duplex telegraph invention, which allowed telegraph signals to be sent both ways on a telegraph cable at the same time without interfering with each other, eliminating the need for two separate cables. The key to this invention was the application of his uncle's invention of the Wheatstone bridge to telegraph cables, which he used at both ends of the line. By properly arranging the resistances of the bridge circuits and the connecting telegraph cable, he could assure that the receivers placed in the bridge would be isolated from the sending signals.
A general transmission-line theory was developed by Lord Kelvin in 1855, using a diffusion model for the current in a submarine cable, including the effects of resistance and capacitance. Kelvin's model correctly predicted the poor performance of the first trans-Atlantic submarine cable, completed in 1858, which could transmit only about one-tenth of a word per minute and operated for just one month before technical mistakes led to its demise. A second improved cable was completed in 1866, but still was limited to about eight words per minute. This problem led to considerable distortion in telephone messages when they began to be used in the 1880's. Heaviside began to analyze this problem after deriving his telegrapher's equations in 1885 from James Clerk Maxwell's equations.
The telegrapher's equations generalized Kelvin's equation with the inclusion of inductance, thus going beyond mere diffusion effects and accounting for the kind of wave propagation involved at the higher frequencies of telephone signals. After a careful analysis, Heaviside published the conditions for distortion-free signaling in 1887 and showed that adding the right amount of inductance along the transmission line would reduce and equalize the attenuation of currents at all frequencies. Unfortunately, his suggestion of adding induction coils along the line was rejected by William Preece, engineer in chief of the British General Post Office. Thus, the development of trans-Atlantic telephony was delayed for nearly twenty years until Heaviside's ideas were revived in 1904 by Michael Pupin and AT&T.
The main impact of Heaviside's work came from the mathematical techniques he invented and their applications in electromagnetic theory. He used these techniques to simplify Maxwell's electromagnetic equations and applied them to a wide range of transmission-line problems. His independent invention of vector analysis, including the divergence and curl operators of vector calculus, made it possible to reduce the twenty equations of Maxwell's electromagnetic field theory to their modern form of just four compact and symmetric equations. It was this form of Maxwell's equations that led Heinrich Hertz to his 1887 discovery of radio waves. Heaviside's invention of operational calculus, similar to the method of Laplace transforms that largely replaced it after 1937, led him to many important solutions of Maxwell's equations. These mathematical techniques have had wide applications in many areas of engineering and physics since their invention.
Heaviside's work is especially important for his innovations in electrical engineering, including the complex number analysis of electric circuits and the invention of the telegrapher's equation. He introduced much of the modern terminology of alternating-current circuit analysis, including such terms as conductance, permeability, inductance, impedance, admittance, reactance, and reluctance. His derivation of the telegrapher's equation to analyze transmission lines led him to recommend adding induction coils, which made it possible to have distortion-free transmission over long-distance cables for a wide range of frequencies as required in modern telephone communications. One of his final contributions was the publication of his three-volume Electromagnetic Theory (1950), summarizing and extending much of his life's work.
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