4.4 Signals Along a Transmission Line
When Electrical Ignorance Became a Prescription for Failure
Heaviside’s starting point for working out the Telegrapher’s Equation was Lord Kelvin’s diffusion theory of signal propagation, worked out in 1854 when Kelvin was still merely William Thomson (1824–1907) [[ii]]. Thomson’s work was prompted by the audacious plan to lay a 2000-mile-long telegraph cable across the Atlantic [[iii], [iv]]. In support of this effort, Thomson further invented a “mirror galvanometer” in which he attached a mirror to the needle of a sensitive galvanometer to amplify the movement in response to minute currents. A beam of light reflecting on the mirror yields an optical lever to translate the smallest twist into a substantial deflection at a distance.
Long telegraph cables exhibited time dispersion in which a crisp dot or dash would be smeared out, blurring and bleeding into adjacent impulses. In part, this could be overcome by slowing down the transmission rate. Thomson’s theory predicted that the Atlantic cable, ten times longer than currently employed cables, would exhibit a hundred times more delay.
“Nonsense,” insisted Dr. Edward Orange Wildman Whitehouse (1816–1890) physician turned electrical engineer who became the Chief Engineer for the first Atlantic cable [[v]]. And yet, the cable when finally laid at great effort and expense showed exactly the dispersion Thomson had predicted. Signals could be copied only at a very slow rate, about 15 characters per minute.
Dr. Whitehouse’s prescription? More voltage! He applied two kilovolts to better force the signals through the stubborn cable. The high voltage broke down the insulation, destroying the cable. “He sent a stroke of lightning over the cable, which required only a spark,” commented one observer [[vii]].
Ever the entrepreneur, Cyrus West Field (1819–1892), co-founder of the Atlantic Telegraph Company, mitigated the losses by selling his excess cable to Tiffany & Co., the famed New York City jewelers, who cut it into short lengths, affixed brass endcaps and a collar reading “Atlantic Telegraph Cable Guaranteed by Tiffany & Co. Broadway New York 1858,” and resold the souvenirs to the public at 50 cents each [[viii]].
This didn’t begin to offset the investment of $500,000 (~$62.5M in present-day dollars) ruined after only a few hundred messages in all had been relayed across the ocean. At $10/word (~$1250/word today), however, the business case was clear. Governments and news organization would pay dearly for rapid communications [[ix]]. The first transatlantic telegraph cable with sustained success began operation in 1866, a year after the 1865 publication of Maxwell’s theory.
How do transmission lines work? How do electric and magnetic fields guide the flow of energy in a cable? How was thinking in terms of energy flow important to Heaviside’s discovery? And what other physical processes are at play?
Next time: 4.4.1 How Do Transmission Lines Work?
A New Feature!
Full Table of Contents [click here]
Chapter 4 Electromagnetism Comes of Age
4.5 An Introduction to Electromagnetic Models
4.5.1 Potentials and Actions at a Distance
4.5.2 Jefimenko & Lorentz
4.5.3 A Synthesis
4.6 Hertz & Radiation Fields
4.7 How Does Radiation Work?
4.8 Summary & Conclusions
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References
[[i]] Thompson, Silvanus P., Elementary Lessons in Electricity and Magnetism, London: Macmillan, 1881, p.169, fig.91. See: https://commons.wikimedia.org/wiki/File:Thompson_mirror_galvanometer_use.png
“When a galvanometer of great delicacy is needed, the moving parts must be made very light and small To watch the movements of a very small needle an index of some kind must be used ; indeed, in the tangent galvanometer it is usual to fasten to the short stout needle a delicate stiff pointer of aluminium. A far better method is to fasten to the needle a very light mirror of silvered glass, by means of which a beam of light can be reflected on to a scale so that every slightest motion of the needle is magnified and made apparent.”
[ii] Nahin, Paul J., Oliver Heaviside: Sage in Solitude, New York: IEEE Press, 1988, p. 29.
[iii] Bright, Charles, The Story of the Atlantic Cable, New York: Appleton and Company, 1903, p. 52.
[iv] Field, Henry M., The Story of the Atlantic Telegraph, New York: Charles Scribner’s Sons, 1893.
[[v]] Nahin, Paul J., Oliver Heaviside: The Life Work and Times of an Electrical Genius of the Victorian Age, Baltimore: The Johns Hopkins University Press, 2002, p. 34. Whitehouse’s exact words: “In all honesty, I am bound to answer, that I believe nature knows no such application of that law; and I can only regard it as a fiction of the schools, a forced and violent adaptation of a principle in Physics, good and true under other circumstances, but misapplied here.”
[[vi]] Bright, Charles, The Story of the Atlantic Cable, New York: Appleton and Company, 1903, p. 52.
[[vii]] McDonald, P.B., Saga of the Seas, The Story of Cyrus W. Field and the Laying of the First Atlantic Cable, New York: Wilson-Erickson, 1937, p.83. McDonald also notes “… after the 1866 cable was laid, an experiment was made in which a message was sent successfully from Newfoundland to Ireland by using …a small copper percussion-cup and a tiny strip of zinc, activated by a drop of acidified water – a truly pygmy battery.”
[[viii]] Klara, Robert, “To Make Tiffany & Co. a Household Name, the Luxury Brand’s Founder Cashed in on the Trans-Atlantic Telegraph Craze,” Smithsonian Magazine, February 15, 2024. See: https://www.smithsonianmag.com/history/to-make-tiffany-co-a-household-name-the-luxury-brands-founder-cashed-in-on-the-trans-atlantic-telegraph-craze-180983782/.
[[ix]] Hugill, Peter J., Global Communications Since 1844: Geopolitics and Technology, Baltimore: Johns Hopkins, 1999, pp. 30-31. The inflation calculator I used insisted $10 in 1856 was worth $370 today. But that $10 was redeemable for a half-ounce of gold which currently costs $1250 ($2500/oz). I used the latter conversion.
I would not have thought that they would have to lay the full 2,000 miles of cable to find out that it would not work?