Editing Loading coil (section)

===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>