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American Telephone & Telegraph: The Art of Telephony, 1916

Loading Coils

AT&T's long-distance network reached Denver, Colorado in 1911, helped along by the development of the loading coil. Loading coils, when inserted periodically into the twisted pairs of wires transmitting speech from one long-distance caller to another, work in conjunction with line capacitance and resistance. Prior to the introduction of loading coils, high frequency noise caused degradation of communication signals and speech transmittals over long distances. To resolve this problem, loading coils were inserted in order to create low-pass filtering. Loading coils worked with telephone wires to effectively cut out the high frequencies causing interference by creating an electrical filter which allowed only low frequencies to pass along the telephone wires. Less higher frequency interference brought AT&T miles closer to its goal of bridging the distance between the East and West Coast.