4.3.3 Cross Coupling

Action at a Distance, Like It Or Not!

Pioneering musician and physicist, Thaddeus Cahill (1867–1934), patented an electronic music generation and distribution system in 1897 [[i]]. His “Telharmonium” was the first commercial electronic musical instrument, and his business model was to broadcast music to restaurants, hotels, and homes via telephone wires. In 1906, he launched his service in New York City. Telephone customers complained their conversations were interrupted by Rossini overtures. The New York Telephone Company prematurely terminated Cahill’s contract because of the complaints, and without access to phone lines to reach a larger audience, the venture ultimately collapsed [[ii]].

Figure 4.36: Thaddeus Cahill’s Telharmonium [[iv]].

Cahill’s problem was cross coupling – a long-standing problem in telegraphy and telephony [[iii]]. Although the telephone wires he used were not directly connected to those of other users, the lines running parallel to his tended to cross couple and pick off a portion of the music signal. The fields from one line attached themselves to adjacent lines. This was a common problem, because in the early days of telephone, many users’ telephone lines would be strung adjacent to those of other users for long distances back to the central telephone office for an area.

Figure 4.37: Telephone lines, New York City, 1891 [[v]].

Somehow, signals aren’t just confined to a wire. Electricity isn’t just a fluid. Electrical signals can reach out and couple to nearby wires. Let’s take a closer look at how signals propagate on a transmission line.

Next time: 4.4 Signals Along a Transmission Line: When Electrical Ignorance is a Prescription for Failure

A New Feature!

Full Table of Contents [click here]

Chapter 4 Electromagnetism Comes of Age

• 4.1 Heaviside, Poynting, & Energy Flow

• 4.2 Early Models of Electromagnetism

  • 4.2.1 Ancient Views

  • 4.2.2 Fluid Theories of Electricity

• 4.3 Actions at a Distance: Fluids or Fields?

  • 4.3.1 Skin Depth and Litz Wire

  • 4.3.2 Inductive Telegraphy at a Distance

  • 4.3.3 Cross Coupling

• 4.4 Signals Along a Transmission Line

  • 4.4.1 How Do Transmission Lines Work?

  • 4.4.2 The Catt Question

• 4.5 An Introduction to Electromagnetic Models

• 4.6 Hertz & Radiation Fields

• 4.7 How Does Radiation Work?

• 4.8 Summary & Conclusions

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References

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[i] Cahill, Thaddeus, “Art of and Apparatus for Generating and Distributing Music Electrically,” US Patent 580,035, April 6, 1897.

[ii] Chadabe, Joel, Electric Sound: The Past and Promise of Electronic Music, Upper Saddle River, NJ: Prentice Hall, 1997, pp. 3-8.

[iii] Hughes, D.E., “Experimental Researches Into Means of Preventing Induction Upon Lateral Wires,” Journal of the Society of Telegraph Engineers, Vol. 8, March 12, 1879, pp. 163-176. See: https://www.google.com/books/edition/Journal_of_the_Society_of_Telegraph_Engi/A4A3AQAAMAAJ?hl=en&gbpv=1&dq=journal+society+telegraph+engineers+lateral+induction+1879+hughes&pg=RA1-PA163&printsec=frontcover .

[iv] Telharmonium by Thaddeus Cahill. See: https://fr.wiktionary.org/wiki/telharmonium

[v] Gardiner, H. Dearborn, “The Curve on the “Long-Distance” Pole Line at Sixty-Fifth Street and Tenth Avenue. As printed in Webb, Herbert Laws, “The Telephone in New York,” Harper’s Weekly, vol. XXXV, no. 1815, October 3, 1891, p. 751.