Editing Loading coil (section)

==History==

===Oliver Heaviside===
[[File:Heaviside cropped.png|thumb|Oliver Heaviside]]
The origin of the loading coil can be found in the work of [[Oliver Heaviside]] on the theory of [[transmission line]]s. Heaviside (1881) represented the line as a network of infinitesimally small circuit elements. By applying his [[operational calculus]] to the analysis of this network he discovered (1887) what has become known as the [[Heaviside condition]].<ref>Heaviside, O, "Electromagnetic Induction and its propagation", ''The Electrician'', 3 June 1887.</ref><ref>Heaviside, O, ''Electrical Papers'', vol. 1, pp. 139-140, Boston, 1925.</ref> This is the condition that must be fulfilled in order for a transmission line to be free from [[distortion]]. The Heaviside condition is that the series [[electrical impedance|impedance]], Z, must be proportional to the shunt [[admittance]], Y, at all frequencies. In terms of the [[primary line coefficients]] the condition is:

:<math>\frac{R}{G}=\frac{L}{C}</math>

where:

:<math>R</math> is the series resistance of the line per unit length
:<math>L</math> is the series self-inductance of the line per unit length
:<math>G</math> is the shunt leakage [[Electrical resistance and conductance|conductance]] of the line insulator per unit length
:<math>C</math> is the shunt capacitance between the line conductors per unit length

Heaviside was aware that this condition was not met in the practical telegraph cables in use in his day. In general, a real cable would have,

:<math>\frac{R}{G} \gg \frac{L}{C}</math>

This is mainly due to the low value of leakage through the cable insulator, which is even more pronounced in modern cables which have better insulators than in Heaviside's day. In order to meet the condition, the choices are therefore to try to increase G or L or to decrease R or C. Decreasing R requires larger conductors. Copper was already in use in telegraph cables and this is the very best conductor available short of using silver. Decreasing R means using more copper and a more expensive cable. Decreasing C would also mean a larger cable (although not necessarily more copper). Increasing G is highly undesirable; while it would reduce distortion, it would at the same time increase the signal loss. Heaviside considered, but rejected, this possibility which left him with the strategy of increasing L as the way to reduce distortion.<ref>Brittain, pp. 39-40</ref>

Heaviside immediately (1887) proposed several methods of increasing the inductance, including spacing the conductors further apart and loading the insulator with iron dust. Finally, Heaviside made the proposal (1893) to use discrete inductors at intervals along the line.<ref>''The Electrician'', 1887 and reproduced (according to Brittain) in Heaviside, O, ''Electromagnetic Theory'', p. 112</ref> However, he never succeeded in persuading the British [[General Post Office|GPO]] to take up the idea. Brittain attributes this to Heaviside's failure to provide engineering details on the size and spacing of the coils for particular cable parameters. Heaviside's eccentric character and setting himself apart from the establishment may also have played a part in their ignoring of him.<ref>Brittain, p. 40</ref>

===John Stone===
[[John Stone Stone|John S. Stone]] worked for the [[American Telephone & Telegraph Company]] (AT&T) and was the first to attempt to apply Heaviside's ideas to real telecommunications. Stone's idea (1896) was to use a bimetallic iron-copper cable which he had patented.<ref>Stone, M S, ''Electric Circuit'', US patent 0 578 275, filed 10 September 1896, issued 2 March 1897.</ref> This cable of Stone's would increase the line inductance due to the iron content and had the potential to meet the Heaviside condition. However, Stone left the company in 1899 and the idea was never implemented.<ref>Brittain pp. 40-41</ref> Stone's cable was an example of continuous loading, a principle that was eventually put into practice in other forms, see for instance [[#Krarup cable|Krarup cable]] later in this article.

===George Campbell===
[[George Ashley Campbell|George Campbell]] was another AT&T engineer working in their Boston facility. Campbell was tasked with continuing the investigation into Stone's bimetallic cable, but soon abandoned it in favour of the loading coil. His was an independent discovery: Campbell was aware of Heaviside's work in discovering the Heaviside condition, but unaware of Heaviside's suggestion of using loading coils to enable a line to meet it. The motivation for the change of direction was Campbell's limited budget.

Campbell was struggling to set up a practical demonstration over a real telephone route with the budget he had been allocated. After considering that his artificial line simulators used [[lumped-element model|lumped]] components rather than the [[distributed-element model|distributed]] quantities found in a real line, he wondered if he could not insert the inductance with lumped components instead of using Stone's distributed line. When his calculations showed that the manholes on telephone routes were sufficiently close together to be able to insert the loading coils without the expense of either having to dig up the route or lay in new cables he changed to this new plan.<ref>Brittain, pp. 42-45</ref> The very first demonstration of loading coils on a telephone cable was on a 46-mile length of the so-called Pittsburgh cable (the test was actually in Boston, the cable had previously been used for testing in Pittsburgh) on 6 September 1899 carried out by Campbell himself and his assistant.<ref>Brittain, pp. 43-44</ref> The first telephone cable using loaded lines put into public service was between Jamaica Plain and West Newton in Boston on 18 May 1900.<ref>Brittain p. 45</ref>

Campbell's work on loading coils provided the theoretical basis for his subsequent work on filters which proved to be so important for [[frequency-division multiplexing]]. The cut-off phenomena of loading coils, an undesirable side-effect, can be exploited to produce a desirable filter frequency response.<ref>Campbell, G A, "Physical Theory of the Electric Wave-Filter", ''Bell System Tech J'', November 1922, vol. 1, no. 2, pp. 1-32.</ref><ref>Brittain, p. 56</ref>

===Michael Pupin===
[[File:Pupin coil.png|thumb|left|Pupin's design of loading coil]]
[[Mihajlo Idvorski Pupin|Michael Pupin]], inventor and [[Serbia]]n immigrant to the US, also played a part in the story of loading coils. Pupin filed a rival patent to the one of Campbell's.<ref>Pupin, M, ''Art of Reducing Attenuation of Electrical Waves and Apparatus Therefor'', US patent 0 652 230, filed 14 December 1899, issued 19 June 1900.</ref> This patent of Pupin's dates from 1899. There is an earlier patent<ref>Pupin, M, ''Apparatus for Telegraphic of Telephonic Transmission'', US patent 0 519 346, filed 14 December 1893, issued 8 May 1894.</ref> (1894, filed December 1893) which is sometimes cited as Pupin's loading coil patent but is, in fact, something different. The confusion is easy to understand, Pupin himself claims that he first thought of the idea of loading coils while climbing a mountain in 1894,<ref>Pupin, M I, ''From Immigrant to Inventor'', pp. 330-331, Charles Schribner & Sons, 1924.</ref> although there is nothing from him published at that time.<ref>Brittain, p. 46</ref>

Pupin's 1894 patent "loads" the line with capacitors rather than inductors, a scheme that has been criticised as being theoretically flawed<ref>Brittain, p. 46, quoting a contemporary criticism in ''Electrical Review'' and experiments by the [[General Post Office|GPO]] showing that the scheme does not work.</ref> and never put into practice. To add to the confusion, one variant of the capacitor scheme proposed by Pupin does indeed have coils. However, these are not intended to compensate the line in any way. They are there merely to restore DC continuity to the line so that it may be tested with standard equipment. Pupin states that the inductance is to be so large that it blocks all AC signals above 50&nbsp;Hz.<ref>Pupin, 1894, p. 5 lines 75-83</ref> Consequently, only the capacitor is adding any significant impedance to the line and "the coils will not exercise any material influence on the results before noted".<ref>Pupin, 1894, p. 5 lines 123-125</ref>
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===Legal battle===
Heaviside never patented his idea; indeed, he took no commercial advantage of any of his work.<ref>Bray, p. 53</ref> Despite the legal disputes surrounding this invention, it is unquestionable that Campbell was the first to actually construct a telephone circuit using loading coils.<ref>Brittain p. 56</ref> There also can be little doubt that Heaviside was the first to publish and many would dispute Pupin's priority.<ref>Brittain, pp. 36, 48-50<br/>Behrend to Searle, in letter quoted by Brittain, p37<br/>Searle to Behrend, 1931, in letter quoted by Brittain, p37<br/>Nahin, p276</ref>

AT&T fought a legal battle with Pupin over his claim. Pupin was first to patent but Campbell had already conducted practical demonstrations before Pupin had even filed his patent (December 1899).<ref>Pupin, M I, ''Art of Reducing Attenuation of Electrical Waves and Apparatus Therefore'', US patent 0 652 230, filed 14 December 1899, issued 19 June 1900.</ref> Campbell's delay in filing was due to the slow internal machinations of AT&T.<ref>Brittain, p. 44</ref>

However, AT&T foolishly deleted from Campbell's proposed patent application all the tables and graphs detailing the exact value of inductance that would be required before the patent was submitted.<ref>Brittain p. 44-45</ref> Since Pupin's patent contained a (less accurate) formula, AT&T was open to claims of incomplete disclosure. Fearing that there was a risk that the battle would end with the invention being declared unpatentable due to Heaviside's prior publication, they decided to desist from the challenge and buy an option on Pupin's patent for a yearly fee so that AT&T would control both patents. By January 1901 Pupin had been paid $200,000 ($13 million in 2011<ref name=Worth>Samuel H. Williamson, "Seven Ways to Compute the Relative Value of a U.S. Dollar Amount, 1774 to present" (Contemporary Standard of Living measure) ''MeasuringWorth'', April 2013.</ref>) and by 1917, when the AT&T monopoly ended and payments ceased, he had received a total of $455,000 ($25 million in 2011<ref name=Worth/>).<ref>Brittain, pp. 54, 55 (footnote), 57</ref>

===Benefit to AT&T===
The invention was of enormous value to AT&T. Telephone cables could now be used to twice the distance previously possible, or alternatively, a cable of half the previous quality (and cost) could be used over the same distance. When considering whether to allow Campbell to go ahead with the demonstration, their engineers had estimated that they stood to save $700,000 in new installation costs in New York and New Jersey alone.<ref>Brittain, p. 45</ref> It has been estimated that AT&T saved $100 million in the first quarter of the 20th century.<ref>Brittain, p. 36</ref><ref>Shaw, T & Fondiller, W, "Developments and Applications of Loading for Telephone Circuits", ''Transactions of the American Institute of Electrical Engineers'', vol. 45, pp. 291-292, 1926.</ref> Heaviside, who began it all, came away with nothing. He was offered a token payment but would not accept, wanting the credit for his work. He remarked ironically that if his prior publication had been admitted it would "interfere ... with the flow of dollars in the proper direction ...".<ref>Brittain quoting Heaviside letter to Behrend, 1918.</ref>

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