Editing Submarine communications cable (section)

===Bandwidth problems===
{{unreferenced section|date=December 2024}}
Early long-distance submarine telegraph cables exhibited formidable electrical problems. Unlike modern cables, the technology of the 19th century did not allow for in-line [[repeater]] [[amplifier]]s in the cable. Large [[voltage]]s were used to attempt to overcome the [[electrical resistance]] of their tremendous length but the cables' distributed [[capacitance]] and [[inductance]] combined to distort the telegraph pulses in the line, reducing the cable's [[Bandwidth (signal processing)|bandwidth]], severely limiting the [[Bit rate|data rate]] for telegraph operation to 10–12 [[words per minute]].

As early as 1816, [[Francis Ronalds]] had observed that electric signals were slowed in passing through an insulated wire or core laid underground, and outlined the cause to be induction, using the analogy of a long [[Leyden jar]].<ref>{{Cite book|title=Sir Francis Ronalds: Father of the Electric Telegraph|last=Ronalds|first=B.F.|publisher=Imperial College Press|year=2016|isbn=978-1-78326-917-4|location = London}}</ref><ref>{{Cite journal|last=Ronalds|first=B.F. |date=Feb 2016|title=The Bicentennial of Francis Ronalds's Electric Telegraph| journal=Physics Today|volume=69 |issue=2|pages=26–31 |doi= 10.1063/PT.3.3079|bibcode=2016PhT....69b..26R |doi-access=free}}</ref> The same effect was noticed by [[Latimer Clark]] (1853) on cores immersed in water, and particularly on the lengthy cable between England and The Hague. [[Michael Faraday]] showed that the effect was caused by capacitance between the wire and the [[ground (electricity)|earth]] (or water) surrounding it. Faraday had noticed that when a wire is charged from a battery (for example when pressing a telegraph key), the [[electric charge]] in the wire induces an opposite charge in the water as it travels along. In 1831, Faraday described this effect in what is now referred to as [[Faraday's law of induction]]. As the two charges attract each other, the exciting charge is retarded. The core acts as a [[capacitor]] distributed along the length of the cable which, coupled with the resistance and [[inductance]] of the cable, limits the speed at which a [[signal]] travels through the [[electrical conduction|conductor]] of the cable.

Early cable designs failed to analyse these effects correctly. Famously, [[E.O.W. Whitehouse]] had dismissed the problems and insisted that a transatlantic cable was feasible. When he subsequently became chief electrician of the [[Atlantic Telegraph Company]], he became involved in a public dispute with [[William Thomson, 1st Baron Kelvin|William Thomson]]. Whitehouse believed that, with enough voltage, any cable could be driven. Thomson believed that his [[law of squares]] showed that retardation could not be overcome by a higher voltage. His recommendation was a larger cable. Because of the excessive voltages recommended by Whitehouse, Cyrus West Field's first transatlantic cable never worked reliably, and eventually [[short circuit]]ed to the ocean when Whitehouse increased the voltage beyond the cable design limit.

Thomson designed a complex electric-field generator that minimized current by [[resonance|resonating]] the cable, and a sensitive light-beam [[mirror galvanometer]] for detecting the faint telegraph signals. Thomson became wealthy on the royalties of these, and several related inventions. Thomson was elevated to [[Lord Kelvin]] for his contributions in this area, chiefly an accurate [[mathematical model]] of the cable, which permitted design of the equipment for accurate telegraphy. The effects of [[atmospheric electricity]] and the [[geomagnetic field]] on submarine cables also motivated many of the [[International Geophysical Year|early polar expeditions]].

Thomson had produced a mathematical analysis of propagation of electrical signals into telegraph cables based on their capacitance and resistance, but since long submarine cables operated at slow rates, he did not include the effects of inductance. By the 1890s, [[Oliver Heaviside]] had produced the modern general form of the [[telegrapher's equations]], which included the effects of inductance and which were essential to extending the theory of [[transmission line]]s to the higher [[frequencies]] required for high-speed data and voice.