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Campbell's delay in filing was due to the slow internal machinations of AT&T. To reduce losses due to high capacitance on long-distance bulk power transmission lines, inductance can be introduced to the circuit with a flexible AC transmission system (FACTS), a static VAR compensator, or a static synchronous series compensator. With loading coils, signal attenuation of a circuit remains low for signals within the passband of the transmission line but increases rapidly for frequencies above the audio cutoff frequency. This is the condition that must be fulfilled in order for a transmission line to be free from distortion. By applying his operational calculus to the analysis of this network he discovered (1887) what has become known as the Heaviside condition. Heaviside never patented his idea; indeed, he took no commercial advantage of any of his work. Details of how this would work were sketchy. 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. Brittain, p. 46, quoting a contemporary criticism in Electrical Review and experiments by the GPO showing that the scheme does not work. To add to the confusion, one variant of the capacitor scheme proposed by Pupin does indeed have coils.

The capacitively loaded antennas used at low frequencies have extremely narrow bandwidths, and therefore if the frequency is changed the loading coil must be adjustable to tune the antenna to resonance with the new transmitter frequency. To be "naturally" resonant, the antenna must have a physical length of one quarter of the wavelength of the radio waves used (or a multiple of that length, with odd multiples usually preferred). 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. Lumped loading with coils is cheaper but has the disadvantages of difficult seals and a definite cutoff frequency. With loading coils of exactly the right inductance, neither capacitance nor inductance dominate: the response is flat, waveforms are undistorted and the characteristic impedance is resistive up to the cutoff frequency.

With loading methods using continuous distributed inductance there is no cutoff. Previous telegraph lines were overland or shorter and hence had less delay, and the need for extra inductance was not as great. It was this problem on the first transatlantic telegraph cable that motivated Heaviside to study the problem and find the solution. With powerful multi-fx units such as those offered by Line 6 and Boss, you’ll find a variety of inputs/output on their rear panels, including their own external loop sections. Campbell arrived at this expression by analogy with a mechanical line periodically loaded with weights described by Charles Godfrey in 1898 who obtained a similar result. Heaviside, who began it all, came away with nothing. Western Union Telegraph Co. Western Union were in competition with AT&T and the Western Electric Company who were using permalloy. The patent for permalloy was held by Western Electric which prevented Western Union from using it. Since Pupin's patent contained a (less accurate) formula, AT&T was open to claims of incomplete disclosure.

AppleTalk's DDP corresponds closely to the Network layer of the Open Systems Interconnection (OSI) communication model. This is the first time in a decade that Cablevision has owned systems outside the New York area (although Bresnan, like Cablevision, had its headquarters in the New York City suburbs). From 1998 until April 29, 2013, Cablevision owned New York-area cinema chain Clearview Cinemas. Permalloy cable enabled signalling speed on submarine telegraph cables to be increased to 400 words/min at a time when 40 words/min was considered good. There are a number of difficulties using loading coils with heavy submarine cables. In industrial applications cables may be laid in cable trays. If the telephone line is subsequently reused to support applications that require higher frequencies, such as in analog or digital carrier systems or digital subscriber line (DSL), loading coils must be removed or replaced. 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. 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".

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